{
  "affected": [
    {
      "ranges": [
        {
          "database_specific": {
            "extracted_events": [
              {
                "introduced": "libssh2"
              },
              {
                "fixed": "1.11.1"
              }
            ],
            "source": [
              "DESCRIPTION",
              "REFERENCES"
            ]
          },
          "events": [
            {
              "introduced": "9896c291e46787352151daa0fabf8be81d84cd09"
            },
            {
              "fixed": "a312b43325e3383c865a87bb1d26cb52e3292641"
            },
            {
              "fixed": "42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4"
            }
          ],
          "repo": "https://github.com/libssh2/libssh2",
          "type": "GIT"
        }
      ]
    }
  ],
  "database_specific": {
    "cna_assigner": "VulnCheck",
    "cwe_ids": [
      "CWE-122"
    ],
    "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/66xxx/CVE-2026-66035.json"
  },
  "details": "libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer overflow vulnerability that allows a malicious SSH server to corrupt heap metadata in any connecting client by sending a packet with a packet_length smaller than the cipher's block size during Encrypt-then-MAC cipher negotiation. In the fullpacket() function in src/transport.c, the ETM path allocates a buffer of packet_length bytes but copies blocksize minus one bytes via memcpy, causing an overflow that on 32-bit glibc writes attacker-controlled bytes into an adjacent chunk's SIZE field, enabling tcache bin confusion, overlapping live objects, and function pointer overwrite during the session handshake before authentication.",
  "id": "CVE-2026-66035",
  "modified": "2026-07-28T03:55:45.412215906Z",
  "published": "2026-07-24T16:45:24.405Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/66xxx/CVE-2026-66035.json"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-66035"
    },
    {
      "type": "ADVISORY",
      "url": "https://www.vulncheck.com/advisories/libssh2-heap-buffer-overflow-via-etm-cipher-negotiation"
    },
    {
      "type": "FIX",
      "url": "https://github.com/libssh2/libssh2/commit/42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4"
    },
    {
      "type": "FIX",
      "url": "https://github.com/libssh2/libssh2/pull/2198"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/libssh2/libssh2"
    }
  ],
  "schema_version": "1.8.0",
  "severity": [
    {
      "score": "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "libssh2 Heap Buffer Overflow via ETM Cipher Negotiation"
}