CVE-2026-84783: Software Security Vulnerability Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2pj2-4hfp-vxp8",
  "modified" : "2026-09-29T18:31:45Z",
  "published" : "2026-09-29T18:31:45Z",
  "aliases" : [ "CVE-2026-84783" ],
  "details" : "Issue summary: The first concurrent use of the same X.509 certificate by\nseveral threads may cause its cached extension data to be freed while\nanother thread is still using it.\n\nImpact summary: A remote, unauthenticated peer could crash a multi-threaded\nTLS client, or a multi-threaded TLS server that requests client\ncertificates, if the first certificate chains built to the same trusted CA\ncertificate are built by several connections at the same time. This is a\nuse-after-free read, which is likely to crash the process, resulting in a\nDenial of Service.\n\nCWE: CWE-416: Use After Free\n\nDescription: OpenSSL caches the decoded values of a certificate's X.509v3\nextensions inside the X509 object the first time they are needed. In\nOpenSSL 4.0 this cache is built in two phases: the extension values are\ncomputed while holding a read lock on the certificate, and the results are\nthen installed into the certificate under a write lock. Because a read lock\ndoes not exclude other readers, several threads can compute the cache for\nthe same certificate at the same time. Each thread that subsequently\nacquires the write lock installs its own results and frees the values\ninstalled by the thread before it, even though that earlier thread has\nalready marked the cache as complete and may have returned pointers into it\nto its caller. A caller still using those pointers then reads freed memory.\n\nAny certificate shared between threads is exposed the first time its\nextensions are decoded. In TLS the certificates at risk are the trusted CA\ncertificates supplied for chain verification, by whatever means, since these\nare shared by every connection and their extensions are decoded and cached\nthe first time a chain is built to them. Certificates sent by the peer are\ndecoded separately for each connection and are not shared, so they are not\naffected. In a TLS client verifying server certificates, or a TLS server\nthat requests and verifies client certificates, the use-after-free could\nonly occur if the first chains built to the same trusted CA are built by\nseveral connections at the same time.\n\nFIPS impact: no\nThe FIPS module is not affected as X.509 certificate handling is outside\nof the OpenSSL FIPS module boundary.\n\nOpenSSL 4.0 is vulnerable to this issue.\n\nOpenSSL 3.6, 3.5, 3.4, 3.0, 1.1.1 and 1.0.2 are not affected by this issue.\n\nOpenSSL 4.0 users should upgrade to OpenSSL 4.0.3.\n\nThis issue was reported on 27 August 2026 by Tim Becker (Xint.io) and\nindependently in a public report on 31 August 2026 by aydinmercan.\n\nThe fix has been developed by Bob Beck.\n\n-- cut (non-publishing metadata for internal use) --\nReported by: Tim Becker (Xint.io), aydinmercan\nFixed by: Bob Beck",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-84783"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/openssl/openssl/commit/de97a1a54f43edefd43b5084ecac54ecadb33081"
  }, {
    "type" : "WEB",
    "url" : "https://openssl-library.org/news/secadv/20260929.txt"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-416" ],
    "severity" : "HIGH",
    "github_reviewed" : false,
    "github_reviewed_at" : null,
    "nvd_published_at" : "2026-09-29T16:17:12Z"
  }
}