Pingora versions prior to 0.9.0 accepted HTTP/2 requests whose Host header differed from the `:authority` pseudo-header. Request filters, cache keys, and logs that read the request URI (`session.req_header().uri`) saw the `:authority` value, but the upstream received the client-supplied Host value. Pingora also accepted `:authority` values containing userinfo (for example, `user@example.com`). This violates RFC 9113 Section 8.3.1, which requires an intermediary to use the `:authority` value for Host and replace any existing Host field.
A remote, unauthenticated attacker can send a single HTTP/2 request that the application sees as one host and the upstream receives as another. Where request filters, access controls, or cache keys use the URI authority and the upstream routes by Host, this can lead to cache poisoning, where a response from another virtual host is stored under the victim host's cache key and served to other users, or host-based access control bypass, where a request approved for one host is served by a different virtual host on the upstream. Deployments that do not accept HTTP/2 from clients, or that do not use the request authority for security or caching decisions, are not affected.
This flaw was corrected in commits 915590a91811877795531f6ba0927a3138b5cb4b and ffab8302d08d4292d1924eb8fe84647fd5cb8e76. Pingora now returns 400 Bad Request for HTTP/2 requests whose Host differs from `:authority`, that contain more than one Host, or whose authority contains userinfo, and keeps the upstream Host or `:authority` consistent with the request authority. Users should upgrade to Pingora >= 0.9.0.
Note: Cloudflare CDN customers were unaffected, as internal Pingora services at Cloudflare do not have HTTP/2 clients.