Weakening of cookie SameSite policy due to incorrect parsing of Set-Cookie header

Summary

When 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). Affected 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. This was introduced in undici 5.15.0 when the cookies feature was added. Patches: Upgrade to undici v6.26.0, v7.28.0 or v8.5.0. Workarounds: After 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. A flaw was found in undici. When undici processes Set-Cookie headers, it incorrectly interprets the SameSite attribute, accepting partial matches instead of exact ones. This allows a malicious server to downgrade a cookie's SameSite policy to a less secure setting, potentially leading to unintended information disclosure or a weakening of security protections for the user. This Low impact flaw in undici, as used in various Red Hat products, arises from incorrect parsing of the `SameSite` cookie attribute. A malicious server could exploit this by sending a specially crafted `Set-Cookie` header, causing the `SameSite` policy to be downgraded to a less secure setting. This could lead to a minor weakening of security protections or unintended information disclosure in applications that process and rely on these cookie attributes. Red Hat severity: Low — CVSS 3.7 (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N). Weakness: CWE-1286. Fixed by RHSA-2026:35841, RHSA-2026:35842 — update the affected packages (`sudo dnf update`). Affected Red Hat products: Red Hat Enterprise Linux 10.