The setuid/setgid (SUID/SGID) bits allows the binary to run with the privileges of the user/group owner instead of those of the user executing it. They can be spotted with the s or S permission in the file user or group owner permissions (i.e. ---s--s---). When the file permissions features an uppercase S instead of a lowercase one, it means the corresponding user or group owner doesn't have execution rights.

Vulnerable programs with these permissions are often targeted by attacker to obtain the user (for setuid) or group (for setgid) privileges. There are many techniques that attackers can use to hijack these binaries and obtain the associated rights.

Identification

SUID

1. To hunt for all SUID binaries on the system

  • find / -type f -perm -4000 2>/dev/null
  • find / -type f -perm -u=s 2>/dev/null

SGID

1. You can also search for specific user SUID

  • find / -type f -perm -4000 -user root -ls 2>/dev/null
  • find / -type f -perm -u=s -user root -ls 2>/dev/null

Extra

1. You can search for both at the same time

  • find / \( -perm -4000 -o -perm -2000 \) -type f -exec ls -la {} \; 2> /dev/null
  • find / \( -perm -g=s -o -perm -u=s \) -type f -exec ls -la {} \; 2> /dev/null

LinPEAS

1. Using LinPEAS.sh we can enumerate SUID and SGID

  • Transfer the script into the target machine
  • Run it: ./LinPEAS.sh

Exploitation

1. Once you identify the list of SUID/SGID, you need to start searching for vulnerabilities related to the program. In order to start, you need to find the program information such a version number.

In this case, I will inspect exim4, program, so I start by looking at the version, first I take a look at the installed programs

  • dpkg -l | grep -i exim

Note, this time it only shows like dependencies, we can also search for the program dependencies

  • ldd /usr/sbin/exim4

2. Doing some research online, I found the command to print the exact version

  • exim4 --version

3. Knowing the version we can search on different sources for vulnerabilities & exploits

  • Google
  • Exploit DB
  • IBM Xforce
  • Github
  • Rapid7
  • CXSecurity
  • Vulnerability Lab
  • 0day
  • SecurityFocus
  • Packet Storm Security
  • Google Hacking Database
  • CVE Details
  • SecurityFocus

4. Searching in CVE Details for (exim 4.48) I found an interesting CVE, we need to investigate about it, and try to find an exploit.

4. We can search for this CVE in different sources like exploit DB or google as PoC, I found some exploits in Exploit-DB using searchsploit

4. Looking for this CVE in ExploitDB I found (https://www.exploit-db.com/exploits/39535)

5. I run it in the server, and since, this is running SUID, it executes and runs as root

Recommendations

Review and Minimize SUID/SGID Executables:

  • Identify and review all files with SUID/SGID permissions.
  • Evaluate the necessity of these permissions for each file. Remove SUID/SGID where unnecessary.

Use Least Privilege Principle:

  • Only grant SUID/SGID permissions where absolutely needed.
  • Assign permissions narrowly to limit potential exploitation.

Regularly Update and Patch:

  • Keep software and systems up to date to patch known vulnerabilities associated with SUID/SGID binaries.

Utilize Seccomp and AppArmor/SELinux:

  • Implement these security frameworks to confine processes, limiting the potential damage if SUID/SGID binaries are compromised.

Monitor and Audit:

  • Implement logging and auditing to monitor the usage and behavior of SUID/SGID binaries for suspicious activities.