Low [CVE-2026-11525] Weakening of cookie SameSite policy due to incorrect parsing of Set-Cookie header

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. 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.