GHSA-g8m3-5g58-fq7m — undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching

Affected: npm: undici

## 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.

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GHSA-g8m3-5g58-fq7m — undici vulnerable to Set-Cookie SameSite attribute downgrade via permissive substring matching