Summary
Before version 0.9.0, Pingora 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.
Root cause
HTTP/2 to HTTP/1.1: Host was generated from :authority only when the request had no Host header, so a conflicting client Host was forwarded unchanged.
HTTP/2 to HTTP/2: the client Host header was removed and used to overwrite the upstream :authority, after request filters had already run against the original :authority.
The HTTP/2 server did not compare Host with :authority and did not reject userinfo in the authority.
Impact
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: a response from another virtual host is stored under the victim hosts cache key and served to other users (this also requires the upstream to serve different content per Host).
Host-based access control bypass: 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.
Note that Cloudflare CDN customers were unaffected, as internal Pingora services at Cloudflare do not have HTTP/2 clients.
Mitigation:
Upgrade to pingora 0.9.0 or later. 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. Fix commits: https://github.com/cloudflare/pingora/commit/915590a91811877795531f6ba0927a3138b5cb4b , https://github.com/cloudflare/pingora/commit/ffab8302d08d4292d1924eb8fe84647fd5cb8e76
If you cannot upgrade: in request_filter, reject HTTP/2 requests that carry a Host header different from the URI authority, or whose authority contains @.
Weakness
The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
Extended Description
HTTP requests or responses (“messages”) can be malformed or unexpected in ways that cause web servers or clients to interpret the messages in different ways than intermediary HTTP agents such as load balancers, reverse proxies, web caching proxies, application firewalls, etc. For example, an adversary may be able to add duplicate or different header fields that a client or server might interpret as one set of messages, whereas the intermediary might interpret the same sequence of bytes as a different set of messages. For example, discrepancies can arise in how to handle duplicate headers like two Transfer-encoding (TE) or two Content-length (CL), or the malicious HTTP message will have different headers for TE and CL. The inconsistent parsing and interpretation of messages can allow the adversary to “smuggle” a message to the client/server without the intermediary being aware of it. This weakness is usually the result of the usage of outdated or incompatible HTTP protocol versions in the HTTP agents.