CVE-2026-107280: AsyncHttpClient: Cookie Domain attribute is not checked against the public suffix list, so a cookie can be set for co.uk

The cookie store decides whether a Domain attribute may be accepted using only the domain-matching rule of RFC 6265 Section 5.1.3, which asks whether the request host is the domain or ends with a dot followed by it. Section 5.3 step 5, which additionally requires rejecting a Domain that is a public suffix, is not implemented anywhere in the client.

So a host under a multi-label public suffix can set a cookie for the suffix itself, and the store then hands it to every other host under that suffix:

attacker.co.uk  ->  Set-Cookie: SID=attacker-value; Domain=co.uk; Path=/
bank.co.uk      ->  Cookie: SID=attacker-value

Domain=uk works the same way. The attacker needs only a site under the same suffix as the victim, which for suffixes such as co.uk, com.au, or github.io is trivially obtainable.

Depending on what the application does with the cookie, this is session fixation, or it overwrites a session the victim site set, or it lets the attacker plant a value the victim site trusts.