CATS , REST API fuzzer and negative testing tool. Run thousands of self-healing API tests within minutes with no coding effort!

  • Comprehensive: tests are generated automatically based on a large number scenarios and cover every field and header
  • Intelligent: tests are generated based on data types and constraints; each Fuzzer has specific expectations depending on the scenario under test
  • Highly Configurable: high amount of customization: you can filter specific Fuzzers, HTTP response codes, HTTP methods, request paths, provide business context and a lot more
  • Self-Healing: as tests are generated, any OpenAPI spec change is picked up automatically
  • Simple to Learn: flat learning curve, with intuitive configuration and syntax
  • Fast: automatic process for write, run and report tests which covers thousands of scenarios within minutes

Overview

By using a simple and minimal syntax, with a flat learning curve, CATS (Contract Auto-generated Tests for Swagger) enables you to generate thousands of API tests within minutes with no coding effort. All tests are generated, run and reported automatically based on a pre-defined set of 93 Fuzzers. The Fuzzers cover a wide range of boundary testing and negative scenarios from fully random large Unicode values to well crafted, context dependant values based on the request data types and constraints. Even more, you can leverage the fact that CATS generates request payloads dynamically and write simple end-to-end functional tests.

Installation

Homebrew

brew tap endava/tap
brew install cats

Manual

CATS is bundled both as an executable JAR or a native binary. The native binaries do not need Java installed.

After downloading your OS native binary, you can add it in PATH so that you can execute it as any other command line tool:

sudo cp cats /usr/local/bin/cats

You can also get autocomplete by downloading the cats_autocomplete script and do:

source cats_autocomplete

To get persistent autocomplete, add the above line in ~/.zshrc or ./bashrc, but make sure you put the fully qualified path for the cats_autocomplete script.

You can also check the cats_autocomplete source for alternative setup.

There is no native binary for Windows, but you can use the uberjar version. This requires Java 17+ to be installed.

You can run it as java -jar cats.jar.

Head to the releases page to download the latest version: https://github.com/Endava/cats/releases.

Build

You can build CATS from sources on you local box. You need Java 17+. Maven is already bundled.

Before running the first build, please make sure you do a ./mvnw clean. CATS uses a fork ok OKHttpClient which will install locally under the 4.10.0-CATS version, so don’t worry about overriding the official versions.

You can use the following Maven command to build the project:

./mvnw package -Dquarkus.package.type=uber-jar

You will end up with a cats-runner.jar in the target folder. You can run it wih java -jar cats-runner.jar ....

You can also build native images using a GraalVM Java version.

./mvnw package -Pnative

Note: You will need to configure Maven with a Github PAT with read-packages scope to get some dependencies for the build.

Notes on Unit Tests

You may see some ERROR log messages while running the Unit Tests. Those are expected behaviour for testing the negative scenarios of the Fuzzers.

Running CATS

Blackbox mode

Blackbox mode means that CATS doesn’t need any specific context. You just need to provide the service URL, the OpenAPI spec and most probably authentication headers.

cats –contract=openapi.yaml –server=http://localhost:8080 –headers=headers.yml –blackbox

In blackbox mode CATS will only report ERRORs if the received HTTP response code is a 5XX. Any other mismatch between what the Fuzzer expects vs what the service returns (for example service returns 400 and CATS expects 200) will be ignored.

The blackbox mode is similar to a smoke test. It will quickly tell you if the application has major bugs that must be addressed immediately.

Context mode

The real power of CATS relies on running it in a non-blackbox mode also called context mode. Each Fuzzer has an expected HTTP response code based on the scenario under test and will also check if the response is matching the schema defined in the OpenAPI spec specific to that response code. This will allow you to tweak either your OpenAPI spec or service behaviour in order to create good quality APIs and documentation and also to avoid possible serious bugs.

Running CATS in context mode usually implies providing it a –refData file with resource identifiers specific to the business logic. CATS cannot create data on its own (yet), so it’s important that any request field or query param that requires pre-existence of those entities/resources to be created in advance and added to the reference data file.

cats –contract=openapi.yaml –server=http://localhost:8080 –headers=headers.yml –refData=referenceData.yml

Notes on skipped Tests

You may notice a significant number of tests marked as skipped. CATS will try to apply all Fuzzers to all fields, but this is not always possible. For example the BooleanFieldsFuzzer cannot be applied to String fields. This is why that test attempt will be marked as skipped. It was an intentional decision to also report the skipped tests in order to show that CATS actually tries all the Fuzzers on all the fields/paths/endpoints.

Notes on console output

CATS produces a significant amount of logging. If the output is redirected to a file please make sure you do proper cleanup when not needing it. A simple run can produce tens of MB of data. You can control the logging level using --log "error" to only log errors.

Additionally, CATS support a lot more arguments that allows you to restrict the number of fuzzers, provide timeouts, limit the number of requests per minute and so on.

Understanding how CATS works and reports results

CATS generates tests based on configured Fuzzers. Each Fuzzer has a specific scenario and a specific expected result. The CATS engine will run the scenario, get the result from the service and match it with the Fuzzer expected result. Depending on the matching outcome, CATS will report as follows:

  • INFO/SUCCESS is expected and documented behaviour. No need for action.
  • WARN is expected but undocumented behaviour or some misalignment between the contract and the service. This will ideally be actioned.
  • ERROR is abnormal/unexpected behaviour. This must be actioned.

CATS will iterate through all endpoints, all HTTP methods and all the associated requests bodies and parameters (including multiple combinations when dealing with oneOf/anyOf elements) and fuzz their values considering their defined data type and constraints. The actual fuzzing depends on the specific Fuzzer executed. Please see the list of fuzzers and their behaviour. There are also differences on how the fuzzing works depending on the HTTP method:

  • for methods with request bodies like POST, PUT, PATCH the fuzzing will be applied at the request body data models level
  • for methods without request bodies like GET, DELETE the fuzzing will be applied at the URL parameters level

This means that for methods with request bodies (POST,PUT) that have also URL/path parameters, you need to supply the path parameters via urlParams or the referenceData file as failure to do so will result in Illegal character in path at index ... errors.

Interpreting Results

HTML_JS

HTML_JS is the default report produced by CATS. The execution report in placed a folder called cats-report/TIMESTAMP or cats-report depending on the --timestampReports argument. The folder will be created inside the current folder (if it doesn’t exist) and for each run a new subfolder will be created with the TIMESTAMP value when the run started. This allows you to have a history of the runs. The report itself is in the index.html file, where you can:

  • filter test runs based on the result: All, Success, Warn and Error
  • filter based on the Fuzzer so that you can only see the runs for that specific Fuzzer
  • see summary with all the tests with their corresponding path against they were run, and the result
  • have ability to click on any tests and get details about the Scenario being executed, Expected Result, Actual result as well as request/response details

Along with the summary from index.html each individual test will have a specific TestXXX.html page with more details, as well as a json version of the test which can be later replayed using > cats replay TestXXX.

Understanding the Result Reason values:

  • Unexpected Exception – reported as error; this might indicate a possible bug in the service or a corner case that is not handled correctly by CATS
  • Not Matching Response Schema – reported as a warn; this indicates that the service returns an expected response code and a response body, but the response body does not match the schema defined in the contract
  • Undocumented Response Code – reported as a warn; this indicates that the service returns an expected response code, but the response code is not documented in the contract
  • Unexpected Response Code – reported as an error; this indicates a possible bug in the service – the response code is documented, but is not expected for this scenario
  • Unexpected Behaviour – reported as an error; this indicates a possible bug in the service – the response code is neither documented nor expected for this scenario
  • Not Found – reported as an error in order to force providing more context; this indicates that CATS needs additional business context in order to run successfully – you can do this using the --refData and/or --urlParams arguments

HTML_ONLY

This format is similar with HTML_JS, but you cannot do any filtering or sorting.

JUNIT

CATS also supports JUNIT output. The output will be a single testsuite that will incorporate all tests grouped by Fuzzer name. As the JUNIT format does not have the concept of warning the following mapping is used:

  • CATS error is reported as JUNIT error
  • JUNIT failure is not used at all
  • CATS warn is reported as JUNIT skipped
  • CATS skipped is reported as JUNIT disabled

The JUNIT report is written as junit.xml in the cats-report folder. Individual tests, both as .html and .json will also be created.

Slicing Strategies for Running Cats

CATS has a significant number of Fuzzers. Currently, 89 and growing. Some of the Fuzzers are executing multiple tests for every given field within the request. For example the ControlCharsOnlyInFieldsFuzzer has 63 control chars values that will be tried for each request field. If a request has 15 fields for example, this will result in 1020 tests. Considering that there are additional Fuzzers with the same magnitude of tests being generated, you can easily get to 20k tests being executed on a typical run. This will result in huge reports and long run times (i.e. minutes, rather than seconds).

Below are some recommended strategies on how you can separate the tests in chunks which can be executed as stages in a deployment pipeline, one after the other.

Split by Endpoints

You can use the --paths=PATH argument to run CATS sequentially for each path.

Split by Fuzzer Category

You can use the --checkXXX arguments to run CATS only with specific Fuzzers like: --checkHttp, -checkFields, etc.

Split by Fuzzer Type

You can use various arguments like --fuzzers=Fuzzer1,Fuzzer2 or -skipFuzzers=Fuzzer1,Fuzzer2 to either include or exclude specific Fuzzers. For example, you can run all Fuzzers except for the ControlChars and Whitespaces ones like this: --skipFuzzers=ControlChars,Whitesspaces. This will skip all Fuzzers containing these strings in their name. After, you can create an additional run only with these Fuzzers: --fuzzers=ControlChars,Whitespaces.

These are just some recommendations on how you can split the types of tests cases. Depending on how complex your API is, you might go with a combination of the above or with even more granular splits.

Please note that due to the fact that ControlChars, Emojis and Whitespaces generate huge number of tests even for small OpenAPI contracts, they are disabled by default. You can enable them using the --includeControlChars, --includeWhitespaces and/or --includeEmojis arguments. The recommendation is to run them in separate runs so that you get manageable reports and optimal running times.

Ignoring Specific HTTP Responses

By default, CATS will report WARNs and ERRORs according to the specific behaviour of each Fuzzer. There are cases though when you might want to focus only on critical bugs. You can use the --ignoreResponseXXX arguments to supply a list of response codes, response sizes, word counts, line counts or response body regexes that should be ignored as issues (overriding the Fuzzer behaviour) and report those cases as success instead or WARN or ERROR. For example, if you want CATS to report ERRORs only when there is an Exception or the service returns a 500, you can use this: --ignoreResultCodes="2xx,4xx".

Ignoring Undocumented Response Code Checks

You can also choose to ignore checks done by the Fuzzers. By default, each Fuzzer has an expected response code, based on the scenario under test and will report and WARN the service returns the expected response code, but the response code is not documented inside the contract. You can make CATS ignore the undocumented response code checks (i.e. checking expected response code inside the contract) using the --ignoreResponseCodeUndocumentedCheck argument. CATS with now report these cases as SUCCESS instead of WARN.

Ignoring Response Body Checks

Additionally, you can also choose to ignore the response body checks. By default, on top of checking the expected response code, each Fuzzer will check if the response body matches what is defined in the contract and will report an WARN if not matching. You can make CATS ignore the response body checks using the --ingoreResponseBodyCheck argument. CATS with now report these cases as SUCCESS instead of WARN.

Replaying Tests

When CATS runs, for each test, it will export both an HTML file that will be linked in the final report and individual JSON files. The JSON files can be used to replay that test. When replaying a test (or a list of tests), CATS won’t produce any report. The output will be solely available in the console. This is useful when you want to see the exact behaviour of the specific test or attach it in a bug report for example.

The syntax for replaying tests is the following:

cats replay “Test1,Test233,Test15.json,dir/Test19.json”

Some notes on the above example:

  • test names can be separated by comma ,
  • if you provide a json extension to a test name, that file will be search as a path i.e. it will search for Test15.json in the current folder and Test19.json in the dir folder
  • if you don’t provide a json extension to a test name, it will search for that test in the cats-report folder i.e. cats-report/Test1.json and cats-report/Test233.json

You can also use environment variables for headers when replaying tests using $$varible. This is useful for authentication headers.

Available Commands

To list all available commands, run:

cats -h

All available subcommands are listed below:

  • > cats help or cats -h will list all available options
  • > cats list --fuzzers will list all the existing fuzzers, grouped by categories
  • > cats list --fieldsFuzzingStrategy will list all the available fields fuzzing strategies
  • > cats list --paths --contract=CONTRACT will list all the paths available within the contract
  • > cats replay "test1,test2" will replay the given tests test1 and test2
  • > cats fuzz will fuzz based on a given request template, rather than an OpenAPI contract
  • > cats run will run functional and targeted security tests written in the CATS YAML format
  • > cats lint will run OpenAPI contract linters, also called ContractInfoFuzzers

Available Arguments

  • --contract=LOCATION_OF_THE_CONTRACT supplies the location of the OpenApi or Swagger contract.
  • --server=URL supplies the URL of the service implementing the contract.
  • --basicauth=USR:PWD supplies a username:password pair, in case the service uses basic auth.
  • --fuzzers=LIST_OF_FUZZERS supplies a comma separated list of fuzzers. The supplied list of Fuzzers can be partial names, not full Fuzzer names. CATS which check for all Fuzzers containing the supplied strings. If the argument is not supplied, all fuzzers will be run.
  • --log=PACKAGE:LEVEL can configure custom log level for a given package. You can provide a comma separated list of packages and levels. This is helpful when you want to see full HTTP traffic: --log=org.apache.http.wire:debug or suppress CATS logging: --log=com.endava.cats:warn
  • --paths=PATH_LIST supplies a comma separated list of OpenApi paths to be tested. If no path is supplied, all paths will be considered.
  • --skipPaths=PATH_LIST a comma separated list of paths to ignore. If no path is supplied, no path will be ignored
  • --fieldsFuzzingStrategy=STRATEGY specifies which strategy will be used for field fuzzing. Available strategies are ONEBYONE, SIZE and POWERSET. More information on field fuzzing can be found in the sections below.
  • --maxFieldsToRemove=NUMBER specifies the maximum number of fields to be removed when using the SIZE fields fuzzing strategy.
  • --refData=FILE specifies the file containing static reference data which must be fixed in order to have valid business requests. This is a YAML file. It is explained further in the sections below.
  • --headers=FILE specifies a file containing headers that will be added when sending payloads to the endpoints. You can use this option to add oauth/JWT tokens for example.
  • --edgeSpacesStrategy=STRATEGY specifies how to expect the server to behave when sending trailing and prefix spaces within fields. Possible values are trimAndValidate and validateAndTrim.
  • --sanitizationStrategy=STRATEGY specifies how to expect the server to behave when sending Unicode Control Chars and Unicode Other Symbols within the fields. Possible values are sanitizeAndValidate and validateAndSanitize
  • --urlParams A comma separated list of ‘name:value’ pairs of parameters to be replaced inside the URLs. This is useful when you have static parameters in URLs (like ‘version’ for example).
  • --functionalFuzzerFile a file used by the FunctionalFuzzer that will be used to create user-supplied payloads.
  • --skipFuzzers=LIST_OF_FIZZERs a comma separated list of fuzzers that will be skipped for all paths. You can either provide full Fuzzer names (for example: --skippedFuzzers=VeryLargeStringsFuzzer) or partial Fuzzer names (for example: --skipFuzzers=VeryLarge). CATS will check if the Fuzzer names contains the string you provide in the arguments value.
  • --skipFields=field1,field2#subField1 a comma separated list of fields that will be skipped by replacement Fuzzers like EmptyStringsInFields, NullValuesInFields, etc.
  • --httpMethods=PUT,POST,etc a comma separated list of HTTP methods that will be used to filter which http methods will be executed for each path within the contract
  • --securityFuzzerFile A file used by the SecurityFuzzer that will be used to inject special strings in order to exploit possible vulnerabilities
  • --printExecutionStatistics If supplied (no value needed), prints a summary of execution times for each endpoint and HTTP method. By default this will print a summary for each endpoint: max, min and average. If you want detailed reports you must supply --printExecutionStatistics=detailed
  • --timestampReports If supplied (no value needed), it will output the report still inside the cats-report folder, but in a sub-folder with the current timestamp
  • --reportFormat=FORMAT Specifies the format of the CATS report. Supported formats: HTML_ONLY, HTML_JS or JUNIT. You can use HTML_ONLY if you want the report to not contain any Javascript. This is useful in CI environments due to Javascript content security policies. Default is HTML_JS which includes some sorting and filtering capabilities.
  • --useExamples If true (default value when not supplied) then CATS will use examples supplied in the OpenAPI contact. If false CATS will rely only on generated values
  • --checkFields If supplied (no value needed), it will only run the Field Fuzzers
  • --checkHeaders If supplied (no value needed), it will only run the Header Fuzzers
  • --checkHttp If supplied (no value needed), it will only run the HTTP Fuzzers
  • --includeWhitespaces If supplied (no value needed), it will include the Whitespaces Fuzzers
  • --includeEmojis If supplied (no value needed), it will include the Emojis Fuzzers
  • --includeControlChars If supplied (no value needed), it will include the ControlChars Fuzzers
  • --includeContract If supplied (no value needed), it will include ContractInfoFuzzers
  • --sslKeystore Location of the JKS keystore holding certificates used when authenticating calls using one-way or two-way SSL
  • --sslKeystorePwd The password of the sslKeystore
  • --sslKeyPwd The password of the private key from the sslKeystore
  • --proxyHost The proxy server’s host name (if running behind proxy)
  • --proxyPort The proxy server’s port number (if running behind proxy)
  • --maxRequestsPerMinute Maximum number of requests per minute; this is useful when APIs have rate limiting implemented; default is 10000
  • --connectionTimeout Time period in seconds which CATS should establish a connection with the server; default is 10 seconds
  • --writeTimeout Maximum time of inactivity in seconds between two data packets when sending the request to the server; default is 10 seconds
  • --readTimeout Maximum time of inactivity in seconds between two data packets when waiting for the server’s response; default is 10 seconds
  • --dryRun If provided, it will simulate a run of the service with the supplied configuration. The run won’t produce a report, but will show how many tests will be generated and run for each OpenAPI endpoint
  • --ignoreResponseCodes HTTP_CODES_LIST a comma separated list of HTTP response codes that will be considered as SUCCESS, even if the Fuzzer will typically report it as WARN or ERROR. You can use response code families as 2xx, 4xx, etc. If provided, all Contract Fuzzers will be skipped.
  • --ignoreResponseSize SIZE_LIST a comma separated list of response sizes that will be considered as SUCCESS, even if the Fuzzer will typically report it as WARN or ERROR
  • --ignoreResponseWords COUNT_LIST a comma separated list of words count in the response that will be considered as SUCCESS, even if the Fuzzer will typically report it as WARN or ERROR
  • --ignoreResponseLines LINES_COUNT a comma separated list of lines count in the response that will be considered as SUCCESS, even if the Fuzzer will typically report it as WARN or ERROR
  • --ignoreResponseRegex a REGEX that will match against the response that will be considered as SUCCESS, even if the Fuzzer will typically report it as WARN or ERROR
  • --tests TESTS_LIST a comma separated list of executed tests in JSON format from the cats-report folder. If you supply the list without the .json extension CATS will search the test in the cats-report folder
  • --ignoreResponseCodeUndocumentedCheck If supplied (not value needed) it won’t check if the response code received from the service matches the value expected by the fuzzer and will return the test result as SUCCESS instead of WARN
  • --ignoreResponseBodyCheck If supplied (not value needed) it won’t check if the response body received from the service matches the schema supplied inside the contract and will return the test result as SUCCESS instead of WARN
  • --blackbox If supplied (no value needed) it will ignore all response codes except for 5XX which will be returned as ERROR. This is similar to --ignoreResponseCodes="2xx,4xx"
  • --contentType A custom mime type if the OpenAPI spec uses content type negotiation versioning.
  • --outoput The path where the CATS report will be written. Default is cats-report in the current directory
  • --skipReportingForIgnoredCodes Skip reporting entirely for the any ignored arguments provided in --ignoreResponseXXX

This will run CATS against http://localhost:8080 using my.yml as an API spec and will only run the HTTP headers Fuzzers.

Available Fuzzers

To get a list of fuzzers run cats list --fuzzers. A list of all available fuzzers will be returned, along with a short description for each.

There are multiple categories of Fuzzers available:

  • Field Fuzzers which target request body fields or path parameters
  • Header Fuzzers which target HTTP headers
  • HTTP Fuzzers which target just the interaction with the service (without fuzzing fields or headers)

Additional checks which are not actually using any fuzzing, but leverage the CATS internal model of running the tests as Fuzzers:

  • ContractInfo Fuzzers which checks the contract for API good practices
  • Special Fuzzers a special category which need further configuration and are focused on more complex activities like functional flow, security testing or supplying your own request templates, rather than OpenAPI specs

Field Fuzzers

CATS has currently 47 registered Field Fuzzers:

  • BooleanFieldsFuzzer – iterate through each Boolean field and send random strings in the targeted field
  • DecimalFieldsLeftBoundaryFuzzer – iterate through each Number field (either float or double) and send requests with outside the range values on the left side in the targeted field
  • DecimalFieldsRightBoundaryFuzzer – iterate through each Number field (either float or double) and send requests with outside the range values on the right side in the targeted field
  • DecimalValuesInIntegerFieldsFuzzer – iterate through each Integer field and send requests with decimal values in the targeted field
  • EmptyStringValuesInFieldsFuzzer – iterate through each field and send requests with empty String values in the targeted field
  • ExtremeNegativeValueDecimalFieldsFuzzer – iterate through each Number field and send requests with the lowest value possible (-999999999999999999999999999999999999999999.99999999999 for no format, -3.4028235E38 for float and -1.7976931348623157E308 for double) in the targeted field
  • ExtremeNegativeValueIntegerFieldsFuzzer – iterate through each Integer field and send requests with the lowest value possible (-9223372036854775808 for int32 and -18446744073709551616 for int64) in the targeted field
  • ExtremePositiveValueDecimalFieldsFuzzer – iterate through each Number field and send requests with the highest value possible (999999999999999999999999999999999999999999.99999999999 for no format, 3.4028235E38 for float and 1.7976931348623157E308 for double) in the targeted field
  • ExtremePositiveValueInIntegerFieldsFuzzer – iterate through each Integer field and send requests with the highest value possible (9223372036854775807 for int32 and 18446744073709551614 for int64) in the targeted field
  • IntegerFieldsLeftBoundaryFuzzer – iterate through each Integer field and send requests with outside the range values on the left side in the targeted field
  • IntegerFieldsRightBoundaryFuzzer – iterate through each Integer field and send requests with outside the range values on the right side in the targeted field
  • InvalidValuesInEnumsFieldsFuzzer – iterate through each ENUM field and send invalid values
  • LeadingWhitespacesInFieldsTrimValidateFuzzer – iterate through each field and send requests with Unicode whitespaces and invisible separators prefixing the current value in the targeted field
  • LeadingControlCharsInFieldsTrimValidateFuzzer – iterate through each field and send requests with Unicode control chars prefixing the current value in the targeted field
  • LeadingSingleCodePointEmojisInFieldsTrimValidateFuzzer – iterate through each field and send values prefixed with single code points emojis
  • LeadingMultiCodePointEmojisInFieldsTrimValidateFuzzer – iterate through each field and send values prefixed with multi code points emojis
  • MaxLengthExactValuesInStringFieldsFuzzer – iterate through each String fields that have maxLength declared and send requests with values matching the maxLength size/value in the targeted field
  • MaximumExactValuesInNumericFieldsFuzzer – iterate through each Number and Integer fields that have maximum declared and send requests with values matching the maximum size/value in the targeted field
  • MinLengthExactValuesInStringFieldsFuzzer – iterate through each String fields that have minLength declared and send requests with values matching the minLength size/value in the targeted field
  • MinimumExactValuesInNumericFieldsFuzzer – iterate through each Number and Integer fields that have minimum declared and send requests with values matching the minimum size/value in the targeted field
  • NewFieldsFuzzer – send a ‘happy’ flow request and add a new field inside the request called ‘catsFuzzyField’
  • NullValuesInFieldsFuzzer – iterate through each field and send requests with null values in the targeted field
  • OnlyControlCharsInFieldsTrimValidateFuzzer – iterate through each field and send values with control chars only
  • OnlyWhitespacesInFieldsTrimValidateFuzzer – iterate through each field and send values with unicode separators only
  • OnlySingleCodePointEmojisInFieldsTrimValidateFuzzer – iterate through each field and send values with single code point emojis only
  • OnlyMultiCodePointEmojisInFieldsTrimValidateFuzzer – iterate through each field and send values with multi code point emojis only
  • RemoveFieldsFuzzer – iterate through each request fields and remove certain fields according to the supplied ‘fieldsFuzzingStrategy’
  • StringFieldsLeftBoundaryFuzzer – iterate through each String field and send requests with outside the range values on the left side in the targeted field
  • StringFieldsRightBoundaryFuzzer – iterate through each String field and send requests with outside the range values on the right side in the targeted field
  • StringFormatAlmostValidValuesFuzzer – iterate through each String field and get its ‘format’ value (i.e. email, ip, uuid, date, datetime, etc); send requests with values which are almost valid (i.e. email@yhoo. for email, 888.1.1. for ip, etc) in the targeted field
  • StringFormatTotallyWrongValuesFuzzer – iterate through each String field and get its ‘format’ value (i.e. email, ip, uuid, date, datetime, etc); send requests with values which are totally wrong (i.e. abcd for email, 1244. for ip, etc) in the targeted field
  • StringsInNumericFieldsFuzzer – iterate through each Integer (int, long) and Number field (float, double) and send requests having the fuzz string value in the targeted field
  • TrailingWhitespacesInFieldsTrimValidateFuzzer – iterate through each field and send requests with trailing with Unicode whitespaces and invisible separators in the targeted field
  • TrailingControlCharsInFieldsTrimValidateFuzzer – iterate through each field and send requests with trailing with Unicode control chars in the targeted field
  • TrailingSingleCodePointEmojisInFieldsTrimValidateFuzzer – iterate through each field and send values trailed with single code point emojis
  • TrailingMultiCodePointEmojisInFieldsTrimValidateFuzzer – iterate through each field and send values trailed with multi code point emojis
  • VeryLargeStringsFuzzer – iterate through each String field and send requests with very large values (40000 characters) in the targeted field
  • VeryLargeDecimalsInNumericFieldsFuzzer – iterate through each numeric field and send requests with very large numbers (40000 characters) in the targeted field
  • VeryLargeIntegersInNumericFieldsFuzzer – iterate through each numeric field and send requests with very large numbers (40000 characters) in the targeted field
  • VeryLargeUnicodeStringsInFieldsFuzzer – iterate through each field and send requests with very large random unicode values in the targeted field
  • IterateThroughEnumValuesFieldsFuzzer – iterate through each enum field and send happy flow requests iterating through each possible enum values
  • ReplaceObjectsWithPrimitivesFieldsFuzzer – iterate through each non-primitive field and replace it with primitive values
  • WithinControlCharsInFieldsSanitizeValidateFuzzer – iterate through each field and send values containing unicode control chars
  • WithinSingleCodePointEmojisInFieldsTrimValidateFuzzer – iterate through each field and send values containing single code point emojis
  • WithinMultiCodePointEmojisInFieldsTrimValidateFuzzer – iterate through each field and send values containing multi code point emojis
  • ZalgoTextInStringFieldsValidateSanitizeFuzzer – iterate through each field and send values containing zalgo text

You can run only these Fuzzers by supplying the --checkFields argument.

Header Fuzzers

CATS has currently 28 registered Header Fuzzers:

  • AbugidasCharsInHeadersFuzzer – iterate through each header and send requests with abugidas chars in the targeted header
  • CheckSecurityHeadersFuzzer – check all responses for good practices around Security related headers like: [{name=Cache-Control, value=no-store}, {name=X-XSS-Protection, value=1; mode=block}, {name=X-Content-Type-Options, value=nosniff}, {name=X-Frame-Options, value=DENY}]
  • DummyAcceptHeadersFuzzer – send a request with a dummy Accept header and expect to get 406 code
  • DummyContentTypeHeadersFuzzer – send a request with a dummy Content-Type header and expect to get 415 code
  • DuplicateHeaderFuzzer – send a ‘happy’ flow request and duplicate an existing header
  • EmptyStringValuesInHeadersFuzzer – iterate through each header and send requests with empty String values in the targeted header
  • ExtraHeaderFuzzer – send a ‘happy’ flow request and add an extra field inside the request called ‘Cats-Fuzzy-Header’
  • LargeValuesInHeadersFuzzer – iterate through each header and send requests with large values in the targeted header
  • LeadingControlCharsInHeadersFuzzer – iterate through each header and prefix values with control chars
  • LeadingWhitespacesInHeadersFuzzer – iterate through each header and prefix value with unicode separators
  • LeadingSpacesInHeadersFuzzer – iterate through each header and send requests with spaces prefixing the value in the targeted header
  • RemoveHeadersFuzzer – iterate through each header and remove different combinations of them
  • OnlyControlCharsInHeadersFuzzer – iterate through each header and replace value with control chars
  • OnlySpacesInHeadersFuzzer – iterate through each header and replace value with spaces
  • OnlyWhitespacesInHeadersFuzzer – iterate through each header and replace value with unicode separators
  • TrailingSpacesInHeadersFuzzer – iterate through each header and send requests with trailing spaces in the targeted header \
  • TrailingControlCharsInHeadersFuzzer – iterate through each header and trail values with control chars
  • TrailingWhitespacesInHeadersFuzzer – iterate through each header and trail values with unicode separators
  • UnsupportedAcceptHeadersFuzzer – send a request with an unsupported Accept header and expect to get 406 code
  • UnsupportedContentTypesHeadersFuzzer – send a request with an unsupported Content-Type header and expect to get 415 code
  • ZalgoTextInHeadersFuzzer – iterate through each header and send requests with zalgo text in the targeted header

You can run only these Fuzzers by supplying the --checkHeaders argument.

HTTP Fuzzers

CATS has currently 6 registered HTTP Fuzzers:

  • BypassAuthenticationFuzzer – check if an authentication header is supplied; if yes try to make requests without it
  • DummyRequestFuzzer – send a dummy json request {‘cats’: ‘cats’}
  • HappyFuzzer – send a request with all fields and headers populated
  • HttpMethodsFuzzer – iterate through each undocumented HTTP method and send an empty request
  • MalformedJsonFuzzer – send a malformed json request which has the String ‘bla’ at the end
  • NonRestHttpMethodsFuzzer – iterate through a list of HTTP method specific to the WebDav protocol that are not expected to be implemented by REST APIs

You can run only these Fuzzers by supplying the --checkHttp argument.

ContractInfo Fuzzers or OpenAPI Linters

Usually a good OpenAPI contract must follow several good practices in order to make it easy digestible by the service clients and act as much as possible as self-sufficient documentation:

  • follow good practices around naming the contract elements like paths, requests, responses
  • always use plural for the path names, separate paths words through hyphens/underscores, use camelCase or snake_case for any json types and properties
  • provide tags for all operations in order to avoid breaking code generation on some languages and have a logical grouping of the API operations
  • provide good description for all paths, methods and request/response elements
  • provide meaningful responses for POST, PATCH and PUT requests
  • provide examples for all requests/response elements
  • provide structural constraints for (ideally) all request/response properties (min, max, regex)
  • heaver some sort of CorrelationIds/TraceIds within headers
  • have at least a security schema in place
  • avoid having the API version part of the paths
  • document response codes for both “happy” and “unhappy” flows
  • avoid using xml payload unless there is a really good reason (like documenting an old API for example)
  • json types and properties do not use the same naming (like having a Pet with a property named pet)

CATS has currently 9 registered ContractInfo Fuzzers:

  • HttpStatusCodeInValidRangeFuzzer – verifies that all HTTP response codes are within the range of 100 to 599
  • NamingsContractInfoFuzzer – verifies that all OpenAPI contract elements follow REST API naming good practices
  • PathTagsContractInfoFuzzer – verifies that all OpenAPI paths contain tags elements and checks if the tags elements match the ones declared at the top level
  • RecommendedHeadersContractInfoFuzzer – verifies that all OpenAPI contract paths contain recommended headers like: CorrelationId/TraceId, etc.
  • RecommendedHttpCodesContractInfoFuzzer – verifies that the current path contains all recommended HTTP response codes for all operations
  • SecuritySchemesContractInfoFuzzer – verifies if the OpenApi contract contains valid security schemas for all paths, either globally configured or per path
  • TopLevelElementsContractInfoFuzzer – verifies that all OpenAPI contract level elements are present and provide meaningful information: API description, documentation, title, version, etc.
  • VersionsContractInfoFuzzer – verifies that a given path doesn’t contain versioning information
  • XmlContentTypeContractInfoFuzzer – verifies that all OpenAPI contract paths responses and requests does not offer application/xml as a Content-Type

You can run only these Fuzzers using > cats lint --contract=CONTRACT.

Special Fuzzers

FunctionalFuzzer

Writing Custom Tests

You can leverage CATS super-powers of self-healing and payload generation in order to write functional tests. This is achieved using the so called FunctionaFuzzer, which is not a Fuzzer per se, but was named as such for consistency. The functional tests are written in a YAML file using a simple DSL. The DSL supports adding identifiers, descriptions, assertions as well as passing variables between tests. The cool thing is that, by leveraging the fact that CATS generates valid payload, you only need to override values for specific fields. The rest of the information will be populated by CATS using valid data, just like a ‘happy’ flow request.

It’s important to note that reference data won’t get replaced when using the FunctionalFuzzer. So if there are reference data fields, you must also supply those in the FunctionalFuzzer.

The FunctionalFuzzer will only trigger if a valid functionalFuzzer.yml file is supplied. The file has the following syntax:

path:
testNumber:
description: Short description of the test
prop: value
prop#subprop: value
prop7:
– value1
– value2
– value3
oneOfSelection:
element#type: “Value”
expectedResponseCode: HTTP_CODE
httpMethod: HTTP_NETHOD

This is a description of the elements within the functionalFuzzer.yml file:

  • you can supply a description of the test. This will be set as the Scenario description. If you don’t supply a description the testNumber will be used instead.
  • you can have multiple tests under the same path: test1, test2, etc.
  • expectedResponseCode is mandatory, otherwise the Fuzzer will ignore this test. The expectedResponseCode tells CATS what to expect from the service when sending this test.
  • at most one of the properties can have multiple values. When this situation happens, that test will actually become a list of tests one for each of the values supplied. For example in the above example prop7 has 3 values. This will actually result in 3 tests, one for each value.
  • test within the file are executed in the declared order. This is why you can have outputs from one test act as inputs for the next one(s) (see the next section for details).
  • if the supplied httpMethod doesn’t exist in the OpenAPI given path, a warning will be issued and no test will be executed
  • if the supplied httpMethod is not a valid HTTP method, a warning will be issued and no test will be executed
  • if the request payload uses a oneOf element to allow multiple request types, you can control which of the possible types the FunctionalFuzzer will apply to using the oneOfSelection keyword. The value of the oneOfSelection keyword must match the fully qualified name of the discriminator.
  • if no oneOfSelection is supplied, and the request payload accepts multiple oneOf elements, than a custom test will be created for each type of payload
  • the file uses Json path syntax for all the properties you can supply; you can separate elements through # as in the example above instead of .

Dealing with oneOf, anyOf

When you have request payloads which can take multiple object types, you can use the oneOfSelection keyword to specify which of the possible object types is required by the FunctionalFuzzer. If you don’t provide this element, all combinations will be considered. If you supply a value, this must be exactly the one used in the discriminator.

Correlating Tests

As CATs mostly relies on generated data with small help from some reference data, testing complex business scenarios with the pre-defined Fuzzers is not possible. Suppose we have an endpoint that creates data (doing a POST), and we want to check its existence (via GET). We need a way to get some identifier from the POST call and send it to the GET call. This is now possible using the FunctionalFuzzer. The functionalFuzzerFile can have an output entry where you can state a variable name, and its fully qualified name from the response in order to set its value. You can then refer the variable using ${variable_name} from another test in order to use its value.

Here is an example:

pet:
test_1:
description: Create a Pet
httpMethod: POST
name: “My Pet”
expectedResponseCode: 200
output:
petId: pet#id
/pet/{id}:
test_2:
description: Get a Pet
id: ${petId}
expectedResponseCode: 200

When executing test_1 the value of the pet id will be stored in the petId variable (value 2). When executing test_2 the id parameter will be replaced with the petId variable (value 2) from the previous case.

Please note: variables are visible across all custom tests i.e. global variables; please be careful with the naming as they will get overridden.

Verifying responses

The FunctionalFuzzer can verify more than just the expectedResponseCode. This is achieved using the verify element. This is an extended version of the above functionalFuzzer.yml file.

/pet:
test_1:
description: Create a Pet
httpMethod: POST
name: “My Pet”
expectedResponseCode: 200
output:
petId: pet#id
verify:
pet#name: “Baby”
pet#id: “[0-9]+”
/pet/{id}:
test_2:
description: Get a Pet
id: ${petId}
expectedResponseCode: 200

Considering the above file:

  • the FunctionalFuzzer will check if the response has the 2 elements pet#name and pet#id
  • if the elements are found, it will check that the pet#name has the Baby value and that the pet#id is numeric

The following json response will pass test_1:

{
“pet”:
{
“id” : 2,
“name”: “Baby”
}
}

You can also refer to request fields in the verify section by using the ${request#..} qualifier. Using the above example, by having the following verify section:

/pet:
test_1:
description: Create a Pet
httpMethod: POST
name: “My Pet”
expectedResponseCode: 200
output:
petId: pet#id
verify:
pet#name: “${request#name}”
pet#id: “[0-9]+”

It will verify if the response contains a pet#name element and that its value equals My Pet as sent in the request.

Some notes:

  • verify parameters support Java regexes as values
  • you can supply more than one parameter to check (as seen above)
  • if at least one of the parameters is not present in the response, CATs will report an error
  • if all parameters are found and have valid values, but the response code is not matched, CATs will report a warning
  • if all the parameters are found and match their values, and the response code is as expected, CATs will report a success

Working with additionalProperties in FunctionalFuzzer

You can also set additionalProperties fields through the functionalFuzzerFile using the same syntax as for Setting additionalProperties in Reference Data.

FunctionalFuzzer Reserved keywords

The following keywords are reserved in FunctionalFuzzer tests: output, expectedResponseCode, httpMethod, description, oneOfSelection, verify, additionalProperties, topElement and mapValues.

Security Fuzzer

Although CATs is not a security testing tool, you can use it to test basic security scenarios by fuzzing specific fields with different sets of nasty strings. The behaviour is similar to the FunctionalFuzzer. You can use the exact same elements for output variables, test correlation, verify responses and so forth, with the addition that you must also specify a targetFields and/or targetFieldTypes and a stringsList element. A typical securityFuzzerFile will look like this:

/pet:
test_1:
description: Run XSS scenarios
name: “My Pet”
expectedResponseCode: 200
httpMethod: all
targetFields:
– pet#id
– pet#description
stringsFile: xss.txt

You can also supply output, httpMethod, oneOfSelection and/or verify (with the same behaviour as within the FunctionalFuzzer) if they are relevant to your case.

The file uses Json path syntax for all the properties you can supply; you can separate elements through # as in the example instead of ..

This is what the SecurityFuzzer will do after parsing the above securityFuzzerFile:

  • it will add the fixed value “My Pet” to all the request for the field name
  • for each field specified in the targetFields i.e. pet#id and pet#description it will create requests for each line from the xss.txt file and supply those values in each field
  • if you consider the xss.txt sample file included in the CATs repo, this means that it will send 21 requests targeting pet#id and 21 requests targeting pet#description i.e. a total of 42 tests
  • for each of these 42 tests, the SecurityFuzzer will expect a 200 response code. If another response code is returned, then CATs will report the test as error.

As an idea on how to create security tests, you can split the nasty strings into multiple files of interest in your particular context. You can have a sql_injection.txt, a xss.txt, a command_injection.txt and so on. For each of these files, you can create a test entry in the securityFuzzerFile where you include the fields you think are meaningful for these types of tests. (It was a deliberate choice (for now) to not include all fields by default.) The expectedResponseCode should be tweaked according to your particular context. Your service might sanitize data before validation, so might be perfectly valid to expect a 200 or might validate the fields directly, so might be perfectly valid to expect a 400. A 500 will usually mean something was not handled properly and might signal a possible bug.

Working with additionalProperties in SecurityFuzzer

You can also set additionalProperties fields through the functionalFuzzerFile using the same syntax as for Setting additionalProperties in Reference Data.

SecurityFuzzer Reserved keywords

The following keywords are reserved in SecurityFuzzer tests: output, expectedResponseCode, httpMethod, description, verify, oneOfSelection, targetFields, targetFieldTypes, stringsFile, additionalProperties, topElement and mapValues.

TemplateFuzzer

The TemplateFuzzer can be used to fuzz non-OpenAPI endpoints. If the target API does not have an OpenAPI spec available, you can use a request template to run a limited set of fuzzers. The syntax for running the TemplateFuzzer is as follows (very similar to curl:

cats fuzz -H header=value -X POST -d ‘{“field1″:”value1″,”field2″:”value2″,”field3″:”value3”}’ -t “field1,field2,header” -i “2XX,4XX” http://service-url

The command will:

  • send a POST request to http://service-url
  • use the {"field1":"value1","field2":"value2","field3":"value3"} as a template
  • replace one by one field1,field2,header with fuzz data and send each request to the service endpoint
  • ignore 2XX,4XX response codes and report an error when the received response code is not in this list

It was a deliberate choice to limit the fields for which the Fuzzer will run by supplying them using the -t argument. For nested objects, supply fully qualified names: field.subfield.

Headers can also be fuzzed using the same mechanism as the fields.

This Fuzzer will send the following type of data:

  • null values
  • empty values
  • zalgo text
  • abugidas characters
  • large random unicode data
  • very large strings (80k characters)
  • single and multi code point emojis
  • unicode control characters
  • unicode separators
  • unicode whitespaces

For a full list of options run > cats fuzz -h.

You can also supply your own dictionary of data using the -w file argument.

HTTP methods with bodies will only be fuzzed at the request payload and headers level.

HTTP methods without bodies will be fuzzed at path and query parameters and headers level. In this case you don’t need to supply a -d argument.

Reference Data File

There are often cases where some fields need to contain relevant business values in order for a request to succeed. You can provide such values using a reference data file specified by the --refData argument. The reference data file is a YAML-format file that contains specific fixed values for different paths in the request document. The file structure is as follows:

/path/0.1/auth:
prop#subprop: 12
prop2: 33
prop3#subprop1#subprop2: “test”
/path/0.1/cancel:
prop#test: 1

Setting additionalProperties

As additional properties are maps i.e. they don’t actually have a structure, CATS cannot currently generate valid values. If the elements within such a data structure are essential for a request, you can supply them via the refData file using the following syntax:

/path/0.1/auth:
address#street: “My Street”
name: “John”
additionalProperties:
topElement: metadata
mapValues:
test: “value1”
anotherTest: “value2”

The additionalProperties element must contain the actual key-value pairs to be sent within the requests and also a top element if needed. topElement is not mandatory. The above example will output the following json (considering also the above examples):

{
“address”: {
“phone”: “123”,
“postCode”: “408”,
“street”: “My Street”
},
“name”: “John”,
“metadata”: {
“test”: “value1”,
“anotherTest”: “value2”
}
}

RefData reserved keywords

The following keywords are reserved in a reference data file: additionalProperties, topElement and mapValues.

Sending ref data for ALL paths

You can also have the ability to send the same reference data for ALL paths (just like you do with the headers). You can achieve this by using all as a key in the refData file:

all:
address#zip: 123

This will try to replace address#zip in all requests (if the field is present).

Removing fields

There are (rare) cases when some fields may not make sense together. Something like: if you send firstName and lastName, you are not allowed to also send name. As OpenAPI does not have the capability to send request fields which are dependent on each other, you can use the refData file to instruct CATS to remove fields before sending a request to the service. You can achieve this by using the cats_remove_field as a value for the fields you want to remove. For the above case the refData field will look as follows:

all:
name: “cats_remove_field”

Creating a Ref Data file with the FunctionalFuzzer

You can leverage the fact that the FunctionalFuzzer can run functional flows in order to create dynamic --refData files which won’t need manual setting the reference data values. The --refData file must be created with variables ${variable} instead of fixed values and those variables must be output variables in the functionalFuzzer.yml file. In order for the FunctionalFuzzer to properly replace the variables names with their values you must supply the --refData file as an argument when the FunctionalFuzzer runs.

Headers File

This can be used to send custom fixed headers with each payload. It is useful when you have authentication tokens you want to use to authenticate the API calls. You can use path specific headers or common headers that will be added to each call using an all element. Specific paths will take precedence over the all element. Sample headers file:

all:
Accept: application/json
/path/0.1/auth:
jwt: XXXXXXXXXXXXX
/path/0.2/cancel:
jwt: YYYYYYYYYYYYY

This will add the Accept header to all calls and the jwt header to the specified paths. You can use environment (system) variables in a headers file using: $$VARIABLE_NAME. (notice double $$)

DELETE requests

DELETE is the only HTTP verb that is intended to remove resources and executing the same DELETE request twice will result in the second one to fail as the resource is no longer available. It will be pretty heavy to supply a large list of identifiers within the --refData file and this is why the recommendation was to skip the DELETE method when running CATS.

But starting with version 7.0.2 CATS has some intelligence in dealing with DELETE. In order to have enough valid entities CATS will save the corresponding POST requests in an internal Queue, and everytime a DELETE request it will be executed it will poll data from there. In order to have this actually working, your contract must comply with common sense conventions:

  • the DELETE path is actually the POST path plus an identifier: if POST is /pets, then DELETE is expected to be /pets/{petId}.
  • CATS will try to match the {petId} parameter within the body returned by the POST request while doing various combinations of the petId name. It will try to search for the following entries: petId, id, pet-id, pet_id with different cases.
  • If any of those entries is found within a stored POST result, it will replace the {petId} with that value.

Edge Spaces Strategy

There isn’t a consensus on how you should handle situations when you trail or prefix valid values with spaces. One strategy will be to have the service trimming spaces before doing the validation, while some other services will just validate them as they are. You can control how CATS should expect such cases to be handled by the service using the --edgeSpacesStrategy argument. You can set this to trimAndValidate or validateAndTrim depending on how you expect the service to behave:

  • trimAndValidate means that the service will first trim the spaces and after that run the validation
  • validateAndTrim means that the service runs the validation first without any trimming of spaces

This is a global setting i.e. configured when CATS starts and all Fuzzer expects a consistent behaviour from all the service endpoints.

URL Parameters

There are cases when certain parts of the request URL are parameterized. For example a case like: /{version}/pets. {version} is supposed to have the same value for all requests. This is why you can supply actual values to replace such parameters using the --urlParams argument. You can supply a ; separated list of name:value pairs to replace the name parameters with their corresponding value. For example supplying --urlParams=version:v1.0 will replace the version parameter from the above example with the value v1.0.

Dealing with AnyOf, AllOf and OneOf

CATS also supports schemas with oneOf, allOf and anyOf composition. CATS wil consider all possible combinations when creating the fuzzed payloads.

Dynamic values in configuration files

The following configuration files: securityFuzzerFile, functionalFuzzerFile, refData support setting dynamic values for the inner fields. For now the support only exists for java.time.* and org.apache.commons.lang3.*, but more types of elements will come in the near future.

Let’s suppose you have a date/date-time field, and you want to set it to 10 days from now. You can do this by setting this as a value T(java.time.OffsetDateTime).now().plusDays(10). This will return an ISO compliant time in UTC format.

Dealing with Authentication

HTTP header(s) based authentication

CATS supports any form of HTTP header(s) based authentication (basic auth, oauth, custom JWT, apiKey, etc) using the headers mechanism. You can supply the specific HTTP header name and value and apply to all endpoints. Additionally, basic auth is also supported using the --basicauth=USR:PWD argument.

One-Way or Two-Way SSL

By default, CATS trusts all server certificates and doesn’t perform hostname verification.

For two-way SSL you can specify a JKS file (Java Keystore) that holds the client’s private key using the following arguments:

  • --sslKeystore Location of the JKS keystore holding certificates used when authenticating calls using one-way or two-way SSL
  • --sslKeystorePwd The password of the sslKeystore
  • --sslKeyPwd The password of the private key within the sslKeystore

For details on how to load the certificate and private key into a Java Keystore you can use this guide: https://mrkandreev.name/blog/java-two-way-ssl/.

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