zlib 1.3.1.2 through 1.3.2 Heap Buffer Overflow via gz_vacate

{
  "id": "cd6808e34e125f6b03f5c023516dd1efcbd593a05d138c23a355089c37eba3ad",
  "url": "https://osv.dev/vulnerability/CVE-2026-85091",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "zlib 1.3.1.2 through 1.3.2 Heap Buffer Overflow via gz_vacate",
    "summary": "zlib versions 1.3.1.2 through 1.3.2 contain a heap buffer overflow vulnerability in the gz_vacate() function when processing non-blocking gzwrite() operations with stale external buffer pointers. Attackers can trigger the overflow by calling gzprintf() or gzvprintf() after a write stall, causing an unchecked memmove() to write beyond the internal input buffer boundary.",
    "evidence": "osv:CVE-2026-85091",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-85091",
      "aliases": [
        "CVE-2026-85091"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/madler/zlib",
              "type": "GIT",
              "events": [
                {
                  "introduced": "570720b0c24f9686c33f35a1b3165c1f568b96be"
                },
                {
                  "last_affected": "da607da739fa6047df13e66a2af6b8bec7c2a498"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "1.3.1.2"
                      },
                      {
                        "last_affected": "1.3.2"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StandardRange"
                  },
                  {
                    "range": [
                      {
                        "introduced": "1.3.1.2"
                      },
                      {
                        "last_affected": "1.3.2"
                      }
                    ],
                    "source": "DESCRIPTION"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v1.3.2",
            "v1.3.1.2"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-85091.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:40.472495269Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/85xxx/CVE-2026-85091.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-85091",
        "https://www.vulncheck.com/advisories/zlib-1.3.1.2-through-1.3.2-heap-buffer-overflow-via-gz-vacate",
        "https://github.com/madler/zlib",
        "https://github.com/madler/zlib/blob/v1.3.2/gzwrite.c#L393",
        "https://gist.github.com/thesmartshadow/e0b9481792afb7c31e86fee1ff084490"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-787"
        ],
        "cna_assigner": "VulnCheck",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/85xxx/CVE-2026-85091.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-85091",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 508113,
  "published_at": "2026-09-03T01:04:44.46+00:00"
}