# libssh2 - Out-of-Bounds Write via Unchecked packet_length in transport.c

CVE record · `CVE-2026-55200`

- **Identifiers:** `CVE-2026-55200`
- **State:** Confirmed
- **Severity:** CVSS 9.2 Critical
- **Critical:** Yes: CVSS 9+
- **CNA:** VulnCheck
- **Weakness (CWE):** [CWE-680](https://cwe.mitre.org/data/definitions/680.html)
- **Tags:** [Memory corruption](https://agentprime.io/advisories/tag/memory-corruption.md)
- **Technologies:** [libssh2](https://agentprime.io/advisories/technology/libssh2.md)
- **Published:** 17 Jun 2026
- **Updated:** 22 Oct 2026
- **Reviewed by a person:** No

Published by the CVE Program (CNA: VulnCheck) at 17 Jun 2026, 19:03 UTC. agentprime has not analysed this yet, so whether it affects any app is not known. This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it.

## Critical

Critical, because:

- CVSS score 9.2: 9.0 or more.

How we decide what’s critical: <https://agentprime.io/advisories/critical.md>

## Affected products, as the CNA wrote them

- Vendor: libssh2. Product: libssh2.
  - affected: `1.11.1` (semver)
  - affected: `1c3f1b7da588f2652260285529ec3c1f1125eb4e to before 97acf3dfda80c91c3a8c9f2372546301d4a1a7a8` (git)
  - Other versions: unaffected
  - purl: `pkg:github/libssh2/libssh2`

## Not analysed yet

We haven’t published our own analysis of this CVE. If a person reviews it, this page will say what’s affected and what has to be true for it to matter.

## Severity and exploitation

- 9.2 Critical, CVSS 4.0: `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`, from CNA (VulnCheck) (used for this record)
- 8.1 High, CVSS 3.1: `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H`, from CNA (VulnCheck)

- **CISA KEV:** Not listed.
- **EPSS:** 0.039, the 90th percentile, on 6 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

This record came from the full history of the CVE Program, published 17 Jun 2026. New records show when each source confirmed them.

## Sources

- [CVE List: CVE-2026-55200](https://www.cve.org/CVERecord?id=CVE-2026-55200)
- [NVD: CVE-2026-55200](https://nvd.nist.gov/vuln/detail/CVE-2026-55200)

### References from the CNA

- <https://github.com/libssh2/libssh2/pull/2052> (issue-tracking)
- <https://github.com/libssh2/libssh2/commit/97acf3dfda80c91c3a8c9f2372546301d4a1a7a8> (patch)
- <https://www.vulncheck.com/advisories/libssh2-out-of-bounds-write-via-unchecked-packet-length-in-transport-c> (third-party-advisory)

Facts on this page come from the CVE List as the CNA published them, with CVSS from the sources named above, CISA KEV and FIRST EPSS. Vendors, products and versions are shown as the CNA wrote them. We don’t reproduce the CVE’s description.

## Check whether this affects you

This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it. Compare the affected versions above with what you run.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), `get_advisory` returns this record and `search_advisories` finds related ones. Setup: <https://agentprime.io/mcp.md>
- As JSON: <https://api.agentprime.io/v1/advisories/CVE-2026-55200>
- Everything that needs attention now: <https://agentprime.io/advisories/critical.md>

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The web page: <https://agentprime.io/advisories/CVE-2026-55200/>. Everything for agents: <https://agentprime.io/llms.txt>.