# CVE-2026-66034: libssh2 Heap Out-of-Bounds Read via publickey subsystem

Status: published. Published: 2026-07-24. Updated: 2026-11-10.

libssh2 through 1.11.1, fixed in commit a13bb6c, contains a missing bounds check vulnerability that allows a malicious SSH server to trigger an arbitrary-length heap out-of-bounds read and a free of an uninitialized pointer via the publickey subsystem. In libssh2_publickey_list_fetch(), the version 1 response parser reads a server-controlled comment_len value and advances the parse pointer without verifying sufficient bytes remain in the buffer, causing the out-of-bounds read to leak heap pointers from adjacent allocations defeating ASLR, followed by heap allocator state corruption when the error cleanup path frees an uninitialized pointer from a non-zeroed realloc() region.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/libssh2/libssh2/pull/2202
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: VulnCheck) — https://github.com/libssh2/libssh2/commit/a13bb6c773f0d55ad1628cede57e99803cd898d9
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: disclosure@vulncheck.com) — https://github.com/libssh2/libssh2/commit/a13bb6c773f0d55ad1628cede57e99803cd898d9
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: disclosure@vulncheck.com) — https://github.com/libssh2/libssh2/pull/2202
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: disclosure@vulncheck.com) — https://www.vulncheck.com/advisories/libssh2-heap-out-of-bounds-read-via-publickey-subsystem

## Affected products and versions

- libssh2 / libssh2 (libssh2/libssh2): ≥ 0; ≤ 1.11.1 (affected, semver); a13bb6c773f0d55ad1628cede57e99803cd898d9 (unaffected, git). Source: CNA: VulnCheck
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0): ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0): ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0): ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0): ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0): ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP

## NVD configuration conditions

Affected CPE matches depend on the following AND/OR and negation conditions:

```json
[
  {
    "negate": false,
    "nodes": [
      {
        "cpeMatch": [
          {
            "criteria": "cpe:2.3:a:libssh2:libssh2:*:*:*:*:*:*:*:*",
            "versionEndIncluding": "1.11.1",
            "vulnerable": true
          }
        ],
        "negate": false,
        "operator": "OR"
      }
    ],
    "operator": "OR"
  }
]
```

## Severity and prioritization

- CVSS 4.0: 7.7 (high); CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N; source: CNA: VulnCheck

- CVSS 3.1: 7.5 (high); CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H; source: CNA: VulnCheck

- CVSS 4.0: 7.7 (high); CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X; source: NVD: disclosure@vulncheck.com

- CVSS 3.1: 7.5 (high); CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H; source: NVD: disclosure@vulncheck.com

- CISA KEV: No listing in collected sources

- EPSS: 0.40% as of 2026-11-09; percentile 34.77%

- SSVC: exploitation none; automatable no; technical impact total; source ADP: CISA-ADP

- Weaknesses: CWE-125, CWE-908

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/libssh2/libssh2/pull/2202 (CNA: VulnCheck; issue-tracking, patch)

- https://github.com/libssh2/libssh2/commit/a13bb6c773f0d55ad1628cede57e99803cd898d9 (CNA: VulnCheck; patch)

- https://www.vulncheck.com/advisories/libssh2-heap-out-of-bounds-read-via-publickey-subsystem (CNA: VulnCheck; third-party-advisory)

- https://cert-portal.siemens.com/productcert/html/ssa-457936.html (ADP: siemens-SADP)

- https://github.com/libssh2/libssh2/commit/a13bb6c773f0d55ad1628cede57e99803cd898d9 (disclosure@vulncheck.com; Patch)

- https://github.com/libssh2/libssh2/pull/2202 (disclosure@vulncheck.com; Issue Tracking, Patch)

- https://www.vulncheck.com/advisories/libssh2-heap-out-of-bounds-read-via-publickey-subsystem (disclosure@vulncheck.com; Patch, Third Party Advisory)

## Provenance

- CNA: VulnCheck: https://www.cve.org/CVERecord?id=CVE-2026-66034; retrieved 2026-11-10; source updated 2026-11-10

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66034; retrieved 2026-11-08; source updated 2026-11-08

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-66034; retrieved 2026-11-10; source updated Not reported

Revision: f6f3a50440bf9feed354175fb3ec1dd7d130f5511a7e48a576b5a0c0a3d24df9

Canonical: https://y2.dev/cve/CVE-2026-66034

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