Impact
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:
SameSite=NoneOfYourBusiness is parsed as None, the most permissive setting.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 v6.27.0, 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.
UlisesGascon Remediation reviewer
KhafraDev Remediation reviewer
mcollina Remediation developer
Impact
When undici parses a
Set-Cookieheader, it accepts anySameSiteattribute value that containsStrict,Lax, orNoneas 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:SameSite=NoneOfYourBusinessis parsed asNone, the most permissive setting.SameSite=StrictLaxis parsed asLax, a downgrade fromStrict.Affected applications are those that consume
Set-Cookieheaders from server responses (for example via undici'sfetchor proxy code paths) and then forward or rely on the parsedsameSiteattribute. 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 v6.27.0, v7.28.0 or v8.5.0.
Workarounds
After parsing a
Set-Cookieheader, validate that the resultingsameSiteattribute is one of'Strict','Lax', or'None'(exact, case-insensitive) before forwarding or relying on it.