{"id":"CVE-2026-11525","aliases":["GHSA-g8m3-5g58-fq7m"],"url":"https://o3.security/vulnerability/CVE-2026-11525","summary":"undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching","details":"## Impact\n\nWhen undici parses a `Set-Cookie` header, it accepts any `SameSite` attribute value that contains `Strict`, `Lax`, or `None` as a substring, rather than the case-insensitive exact match specified by RFC 6265. Non-spec values are silently mapped to one of the three standard tokens:\n\n- `SameSite=NoneOfYourBusiness` is parsed as `None`, the most permissive setting.\n- `SameSite=StrictLax` is parsed as `Lax`, a downgrade from `Strict`.\n\nAffected applications are those that consume `Set-Cookie` headers from server responses (for example via undici's `fetch` or proxy code paths) and then forward or rely on the parsed `sameSite` attribute. A malicious or non-compliant server can coerce the consumer's view of a cookie's SameSite policy to a weaker value, silently degrading the SameSite enforcement the cookie is supposed to provide.\n\nThis was introduced in undici 5.15.0 when the cookies feature was added.\n\n## Patches\n\nUpgrade to undici v6.27.0, v7.28.0 or v8.5.0.\n\n## Workarounds\n\nAfter parsing a `Set-Cookie` header, validate that the resulting `sameSite` attribute is one of `'Strict'`, `'Lax'`, or `'None'` (exact, case-insensitive) before forwarding or relying on it.","published":"2026-06-17T17:31:03.163Z","modified":"2026-10-21T02:30:33.775327614Z","cvss":{"score":3.7,"severity":"LOW","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N"},"epss":{"score":0.00238,"percentile":0.15089,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"undici","fixedVersion":"6.27.0"},{"ecosystem":"npm","name":"undici","fixedVersion":"7.28.0"},{"ecosystem":"npm","name":"undici","fixedVersion":"8.5.0"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://cna.openjsf.org/security-advisories.html"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/11xxx/CVE-2026-11525.json"},{"type":"ADVISORY","url":"https://github.com/nodejs/undici/security/advisories/GHSA-g8m3-5g58-fq7m"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-11525"},{"type":"PACKAGE","url":"https://github.com/nodejs/undici"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:30:33.775327614Z"}}