Listen to this Post
How the mentioned CVE works:
The vulnerability exists due to insecure handling of temporary file paths in FastNetMon Community Edition up to version 1.2.9. The statistics file is hardcoded to `/tmp/fastnetmon.dat` (src/fastnetmon.cpp line 159). The function `print_screen_contents_into_file()` (src/fastnetmon_logic.cpp line 2186) opens this path using `std::ios::trunc` without checking for symbolic links or using the `O_NOFOLLOW` flag. This allows a local attacker to create a symlink at `/tmp/fastnetmon.dat` pointing to any arbitrary file on the system. When the function writes data, it follows the symlink and overwrites the target file. Additionally, a bug on line 2190 causes `chmod()` to always operate on `cli_stats_file_path` instead of the passed `file_path` parameter, applying incorrect permissions. During daemonization (src/fastnetmon.cpp line 1821), the `umask` is set to 0, making all created files world-writable. The FastNetMon process typically runs as root, so an attacker can overwrite critical system files (e.g., /etc/passwd, /etc/shadow, binaries) leading to privilege escalation or denial of service. The attack is local and requires no user interaction.
dailycve form:
Platform: FastNetMon Community
Version: through 1.2.9
Vulnerability: Local symlink attack
Severity: Critical
date: 2026-05-26
Prediction: 2026-06-10
What Undercode Say:
Analytics:
Check if /tmp/fastnetmon.dat exists and is a symlink ls -la /tmp/fastnetmon.dat Monitor file creation and chmod calls strace -e trace=file,chmod -p $(pgrep fastnetmon) Test symlink vulnerability (as non-root user) ln -sf /etc/passwd /tmp/fastnetmon.dat Wait for FastNetMon to write stats Then check if /etc/passwd was overwritten
Exploit:
Attacker creates symlink to a critical file ln -sf /etc/crontab /tmp/fastnetmon.dat Trigger FastNetMon to write statistics (e.g., via CLI command) fastnetmon --print-statistics After overwrite, crontab is corrupted, allowing potential root access cat /etc/crontab
Protection from this CVE:
- Upgrade to patched version (if available) or apply vendor fix.
- Use `O_NOFOLLOW` and `O_EXCL` when opening temporary files.
- Fix `chmod()` bug to operate on correct file path.
- Set `umask` to 077 instead of 0 during daemonization.
- Monitor `/tmp` for suspicious symlinks using auditd.
- Run FastNetMon with least privilege (non-root) if possible.
Impact:
Arbitrary file overwrite as root, leading to root privilege escalation, denial of service (overwriting binaries or configs), system compromise, and persistence. Local attackers can gain full control over the host.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
Undercode
🎓 Live Courses & Certifications:
Join Undercode Academy for Verified Certifications
🚀 Request a Custom Project:
Secure, high-velocity infrastructure and disruptive technological engineering. Contact our engineering team for high-tier development and proprietary systems:
[email protected]
