{"id":"CVE-2026-11525","title":"undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching","summary":"undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching","severity":"low","cvss":3.7,"cwe":["CWE-183"],"vendor":"undici","product":"undici","affected":["undici < 6.27.0","undici >= 7.0.0, < 7.28.0","undici >= 8.0.0, < 8.5.0"],"patched":["undici 6.27.0","undici 7.28.0","undici 8.5.0"],"published":"2026-06-19","updated":"2026-06-19","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-g8m3-5g58-fq7m","references":[{"url":"https://github.com/nodejs/undici/security/advisories/GHSA-g8m3-5g58-fq7m"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-11525"},{"url":"https://cna.openjsf.org/security-advisories.html"},{"url":"https://github.com/advisories/GHSA-g8m3-5g58-fq7m"}],"tags":["ghsa","npm"],"epss":0.00238,"epssPercentile":0.15172,"ingestedAt":"2026-06-22T15:59:08.194Z","ecosystem":"npm","slug":"CVE-2026-11525","body":"## Overview\n\n## 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.\n\n## Affected packages\n\n- `undici < 6.27.0`\n- `undici >= 7.0.0, < 7.28.0`\n- `undici >= 8.0.0, < 8.5.0`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `undici 6.27.0`\n- `undici 7.28.0`\n- `undici 8.5.0`","depth":"sunlit","depthScore":20,"depthScoreParts":{"impact":20.4,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}