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CVE-2026-48693 Pavel Odintsov CVE debrief

FastNetMon Community Edition through 1.2.9 contains a local symlink attack vulnerability stemming from insecure temporary file handling. The application defaults to writing statistics data to a predictable path at /tmp/fastnetmon.dat, which it opens with truncation semantics without symlink validation or O_NOFOLLOW protections. A secondary implementation error causes chmod operations to target a fixed path regardless of which file_path parameter was passed, while the daemonization process sets umask to 0, resulting in world-writable file creation. These conditions enable a local attacker with unprivileged access to create a symlink at the predictable path and redirect file operations to arbitrary target files writable by the FastNetMon process user, which typically runs as root. The vulnerability is classified as CWE-59 (Improper Link Resolution Before File Access).

Vendor
Pavel Odintsov
Product
Fastnetmon
CVSS
MEDIUM 5.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-26
Original CVE updated
2026-05-27
Advisory published
2026-05-26
Advisory updated
2026-05-27

Who should care

System administrators running FastNetMon Community Edition on multi-user systems or with root privileges; security teams monitoring for local privilege escalation vectors; DevOps engineers deploying FastNetMon in containerized or shared environments where /tmp isolation may be insufficient

Technical summary

The vulnerability exists in FastNetMon Community Edition's statistics output functionality. The default configuration writes to /tmp/fastnetmon.dat, a predictable path in a world-writable directory. The implementation uses std::ios::trunc for file operations without O_NOFOLLOW or symlink checks, creating a time-of-check to time-of-use (TOCTOU) race condition. A chmod implementation bug applies permissions to a hardcoded path rather than the passed parameter, and umask 0 during daemonization causes all created files to be world-writable. A local attacker can exploit this by placing a symlink at the predictable path before the FastNetMon process writes statistics, causing arbitrary file overwrite with attacker-controlled content as the process user.

Defensive priority

medium

Recommended defensive actions

  • Upgrade FastNetMon Community Edition to a version newer than 1.2.9 when available, or apply vendor-provided patches addressing symlink handling in temporary file operations
  • Configure FastNetMon to use a non-predictable statistics file path outside of world-writable directories such as /tmp, preferably in a dedicated application directory with restricted permissions
  • Implement filesystem-level protections by mounting /tmp with nosuid,nodev,noexec options and consider using private tmp directories via systemd or equivalent isolation mechanisms
  • Review and restrict local user access to systems running FastNetMon, as exploitation requires local access to create symlinks
  • Monitor for anomalous file creation or modification activity in /tmp and unexpected chmod operations on system files
  • If running FastNetMon as root is required, consider using Linux capabilities or seccomp profiles to restrict file operation syscalls and reduce attack surface
  • Audit existing FastNetMon installations for world-writable files created due to umask 0 configuration and remediate inappropriate permissions

Evidence notes

Vulnerability description identifies specific source file locations: src/fastnetmon.cpp line 159 (default statistics path), src/fastnetmon_logic.cpp line 2186 (print_screen_contents_into_file function with std::ios::trunc), and line 2190 (chmod bug). Umask 0 set during daemonization at src/fastnetmon.cpp line 1821. CVSS 3.1 vector AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N indicates local attack vector with low complexity, low privileges required, and high integrity impact.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-48693 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-48693

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-48693 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-48693

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.