{"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:\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. For example, SameSite=NoneOfYourBusiness is parsed as None (the most permissive setting), and 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\nPatches:\nUpgrade to undici v6.26.0, v7.28.0 or v8.5.0.\n\nWorkarounds:\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":"2026-08-12T03:51:42.811775599Z","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":0.14791,"asOf":"2026-09-07"},"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"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:42.811775599Z"}}