Wpgarlic : A Proof-Of-Concept WordPress Plugin Fuzzer

Wpgarlic is a proof-of-concept WordPress plugin fuzzer used in the research described in https://kazet.cc/2022/02/03/fuzzing-wordpress-plugins.html that helped to discover more than 140 vulnerablities in WordPress plugins installed on almost 15 million sites.

If you want to continue the research, start with less popular plugins – if a plugin achieved at least 10k active installs between October 2021 and January 2022, I have most probably looked at the fuzzer reports (and most focus has been put on plugins having at least 20k active installs). Because there is a lot of randomness in how fuzzer works, some vulnerabilities in these plugins remain undiscovered – but fewer ones.

Fuzzer reports contain a lot of false positives – most of them don’t indicate a vulnerability. After seeing a report, first analyze whether the behavior you’re observing is indeed a vulnerability or a false positive. Don’t spam WPScan/vendors with raw fuzzer reports – provide a PoC exploit instead.

Examples

For obvious reasons, the examples will contain only vulnerabilities that have already been fixed.

Arbitrary file read

Let’s assume you are fuzzing responsive-vector-maps in version 6.4.0:

./bin/fuzz_plugin responsive-vector-maps –version 6.4.0

(to fuzz the latest version, just skip --version).

After the fuzzing finishes (which would take 10-30 minutes for this plugin) you can call:

./bin/print_findings data/plugin_fuzz_results/

That means that the fuzzer detected executing fopen() on a known payload. Most of the payloads contain the word GARLIC in them to facilitate automatic detection in output. You may see or configure them in docker_image/magic_payloads.php.

Then, you may browse the source code and see that indeed the wp_ajax_rvm_import_markers endpoint uses the file content to render output, thus allowing you to read arbitrary files on the server: CVE-2021-24947.

What you see in white is a crash considered interesting (you may modify them or add new ones in crash_detectors.py). Green is the context. In blue you see the report file name (with plugin name), plugin popularity and endpoint name (here: the ajax action name).

The data in yellow are what payloads were injected into what variables.

Reflected XSS

Let’s assume you are fuzzing page-builder-add in version 1.4.9.4:

./bin/fuzz_plugin page-builder-add –version 1.4.9.4

Option update leading to stored XSS

./bin/fuzz_plugin duplicate-page-or-post –version 1.4.6

False positives

Unfortunately, for most of the plugins the fuzzer doesn’t find any interesting crashes, and for the rest, most of the reports are false positives. For example, if you see:

Call: wp_mail arguments={‘to’: ‘plugin@pluginvendor.com’, ‘subject’: ‘[Plugin contact] – http://GARLICGARLICGARLIC.example.com’}

This can mean, that wp_mail is indeed called, but you don’t control the recipient and most of the subject. If you want to be sure, look at the plugin source.

Additional tips for reading fuzzer reports

• If you see the message; May as well be equal: … and … – that means that the mechanism to pretend that a known payload is equal to any other string (described in https://kazet.cc/2022/02/03/fuzzing-wordpress-plugins.html#patched-equality) was triggered.
• If you see __GARLIC_ACCESSED__ _FILES[files] __ENDGARLIC__ – that means that an uploaded file access was detected. No further checks are currently made – to check whether this is a vulnerability, look at the code.
• Sometimes what the fuzzer injects into GET, POST etc. parameters is not reproducible in the real world: for example, you can’t inject anything into $_GET['page'] if you want a particular menu page to be displayed.

Usage cheatsheet

The first run of fuzzing or of the tests may take about an hour, because we need to build the Docker image with instrumented PHP and WordPress.

Fuzzing a plugin by name

./bin/fuzz_plugin PLUGIN_SLUG

Fuzzing a plugin from file

You can also install a plugin from a local zip file:

./bin/fuzz_plugin PLUGIN_FILE_NAME.zip

Printing findings

To print what the fuzzer found, use:

./bin/print_findings data/plugin_fuzz_results/

Running tests

To run the tests, use:

./bin/test

Manual testing environment

You may start a test environment with only one plugin installed using:

./bin/manual_testing PLUGIN_SLUG|PLUGIN_PATH.zip [version]

You can install a plugin using its slug or from a local zip file.

It will listen on http://127.0.0.1:8001/

There will be two test users in the database:

• username: admin, password: admin, privileges: administrator
• username: subscriber, password: subscriber, privileges: subscriber

Extending and configuring the fuzzer

This tool is a proof of concept – this section contains places where it can be improved to find more vulnerabilities.

You may want to edit filtering.py — it contains the rules that deem a particular crash important or not. If you change them, you may have more false positives, but find more vulnerabilities as well. For example, the only header that I consider interesting when emitted is the Location header, to detect Open Redirect vulnerabilities. That is just one idea and you may have others.

Another file that may be worth extending is docker_image/patch_wordpress.sh. It describes calls to which functions will be logged as interesting.

If you want to inject other payloads (or change the probabilities with which they are injected) edit docker_image/magic_payloads.php and docker_image/fuzz/config.py.

crash_detector.py contans regular expressions that find interesting crashes or interesting information (e.g. e-mails) being exposed.

Fuzzing files (i.e. executing each PHP file with injected payloads) has been disabled because it didn’t lead to many findings. Uncomment files in config.DEFAULT_ENABLED_FEATURES to change that.

Fuzzer internals

Blocklists

Some plugins need other ones (e.g. woocommerce) as a dependency. When fuzzing a plugin that has a dependency, we want to fuzz only the chosen plugin and skip the dependency AJAX actions, REST routes and menu pages. We want to fuzz woocommerce actions/routes/pages only when we picked woocommerce to fuzz.

List of dependency actions/routes/pages are called blocklists and are listed in docker_image/blocklists/. Files named common contain the WordPress core actions/routes/pages – we don’t want to fuzz these as well.

To update these blocklists, use ./bin/update_blocklists.