CVE-2026-11525
LowCVSS 3.7Exploitation Probability (EPSS)
Low risk16th percentile — higher than 16% of all known CVEs
Summary
A vulnerability in undici allows the acceptance of incorrect SameSite attribute values in the Set-Cookie header, potentially leading to a weakening of the cookie's SameSite policy. Instead of the required case-insensitive exact match, values containing 'Strict', 'Lax', or 'None' as substrings are accepted.
Risk Assessment
Organizations may be exposed to attacks where a malicious server coerces the use of a weaker SameSite policy, potentially leading to unauthorized access to cookies. This weakening of security could result in serious privacy and data security breaches.
Recommendation
It is recommended to upgrade to undici v6.26.0, v7.28.0, or v8.5.0. Alternatively, after parsing a Set-Cookie header, validate that the resulting sameSite attribute is one of 'Strict', 'Lax', or 'None' before further use.
Original NVD description (English source)
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.