Security advisory

{
  "id": "26c8da557614b18b76591eb521d5217bfaab3ec63f825d24cdf8a6f915f2ce75",
  "url": "https://osv.dev/vulnerability/CVE-2026-34876",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "CVE-2026-34876",
    "summary": "An issue was discovered in Mbed TLS 3.x before 3.6.6. An out-of-bounds read vulnerability in mbedtls_ccm_finish() in library/ccm.c allows attackers to obtain adjacent CCM context data via invocation of the multipart CCM API with an oversized tag_len parameter. This is caused by missing validation of the tag_len parameter against the size of the internal 16-byte authentication buffer. The issue affects the public multipart CCM API in Mbed TLS 3.x, where mbedtls_ccm_finish() can be invoked directly by applications. In Mbed TLS 4.x versions prior to the fix, the same missing validation exists in the internal implementation; however, the function is not exposed as part of the public API. Exploitation requires application-level invocation of the multipart CCM API.",
    "evidence": "osv:CVE-2026-34876",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-34876",
      "aliases": [
        "CVE-2026-34876"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/mbed-tls/mbedtls",
              "type": "GIT",
              "events": [
                {
                  "introduced": "d65aeb37349ad1a50e0f6c9b694d4b5290d60e49"
                },
                {
                  "fixed": "0bebf8b8c7f07abe3571ded48a11aa907a1ffb20"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:trustedfirmware:mbed_tls:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "3.1.0"
                      },
                      {
                        "fixed": "3.6.6"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-34876.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:40.416550340Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/34xxx/CVE-2026-34876.json",
        "https://mbed-tls.readthedocs.io/en/latest/security-advisories/mbedtls-security-advisory-2026-03-ccm-finish-boundary-check/",
        "https://mbed-tls.readthedocs.io/en/latest/tech-updates/security-advisories/",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-34876"
      ],
      "provider_metadata": {
        "cna_assigner": "mitre",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/34xxx/CVE-2026-34876.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-34876",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 508187,
  "published_at": "2026-04-02T00:00:00+00:00"
}