Http4s: Ember chunk parser lenience (TE.TE request smuggling)

Summary

Ember's chunk decoder parses the size token leniently: it strips leading and trailing whitespace and accepts a leading + or - sign. RFC9112 §7.1 defines chunk-size = 1*HEXDIG. An intermediary that parses the chunk boundary differently (or rejects it) will disagree with Ember on request framing, enabling HTTP request smuggling (TE.TE).

Impact

Server

Request smuggling (TE.TE) when ember-server is an origin behind an intermediary that forwards the body verbatim but interprets the malformed chunk sizes differently

• Front-end security bypass: the smuggled request reaches paths the intermediary might have blocked, with attacker-chosen method and headers.
• Cache poisoning: the smuggled response is associated with the next request key in a caching proxy.
• Request queue hijack:

Client

ember-client has the same leniencies on the response path, which enables response smuggling when http4s is used as a gateway. This is less severe, as it requires a malicious or compromised upstream rather than an anonymous remote client.

Preconditions

• Unauthenticated remote attacker (server)
• ember-server as origin behind a keep-alive intermediary
• Malicious or compromised upstream (client)
• Intermediary or upstream forwards chunked bodies without re-encoding and disagrees with Ember on + prefix, - prefix, or leading and trailing octets rather than rejecting.

Workarounds

• Intermediary strictly rejects malformed chunk sizes
• Intermediary buffers and re-encodes request bodies

References

• RFC9112 §7.1: Chunked Transfer Coding