undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching

Summary

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. 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.

Timeline

1. Jun 17, 2026, 5:31 PM UTCCVE published
2. Jun 17, 2026, 5:53 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: partial

3. Jun 17, 2026, 5:54 PM UTCRecord last updated

Affected versions

• undici undici

  fixed in 6.26.0; From 7.0.0 · fixed in 7.28.0; From 8.0.0 · fixed in 8.5.0

• undicinpm

  fixed in 6.27.0

• undicinpm

  From 7.0.0 · fixed in 7.28.0

• undicinpm

  From 8.0.0 · fixed in 8.5.0

References 2