CVE-2025-51986
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedAn issue was discovered in the demo/LINUXTCP implementation of cwalter-at freemodbus v.2018-09-12 allowing attackers to reach an infinite loop via a crafted length value for a packet.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceA denial-of-service vulnerability exists in the LINUXTCP demo implementation of FreeMODBUS v.2018-09-12. Attackers can send a crafted packet with a malicious length value that causes the processing loop to never terminate, resulting in an infinite loop and service unavailability.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
-
Identify FreeMODBUS installation and versionSearch for FreeMODBUS libraries or demo applications on the system. Check version strings in binaries, source directories, or package metadata. Look for version tag '2018-09-12' or check version control history for this release date.Affected if The installed version is FreeMODBUS 2018-09-12 (or any version derived from this release)
-
Confirm Linux TCP demo is in useIdentify whether the deployed application uses the LINUXTCP demo component. Look for source files named 'linuxtcp.c' or 'mbtcp.c' in the TCP demo directory, or check for TCP server functionality in the FreeMODBUS stack.Affected if The system runs the FreeMODBUS Linux TCP demo/server component
-
Verify TCP server is actively listeningCheck running processes or network listeners for FreeMODBUS TCP ports (default 502). Use 'netstat -tlnp' or 'ss -tlnp' to list listening TCP servers.Affected if A FreeMODBUS TCP server process is running and accepting connections on any port
-
Inspect packet length validation in TCP receive codeExamine the TCP packet reception function in the LINUXTCP demo source. Look for the loop that processes incoming data and check if length fields are validated before entering or continuing the processing loop.Affected if The TCP receive handler lacks bounds checking on length fields before the processing loop, allowing a malicious length to cause infinite iteration
A system is affected if it runs FreeMODBUS version 2018-09-12 with the Linux TCP demo/server component active, where incoming packets are processed without validating length fields against buffer boundaries before entering the data processing loop.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataImplement proper input validation on packet length fields before entering processing loops to ensure the length value is within expected bounds and prevents infinite loop conditions.
- Consultation2.0 h
- Implementation4.0 h
- Testing4.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,328.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2025-51986 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2025-51986 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.
A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.
- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
- No spam, self-promotion, credentials, or personal data