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
References
ERobertGII Finder
rossabaker Analyst
morgen-peschke Remediation reviewer
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 defineschunk-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
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
+prefix,-prefix, or leading and trailing octets rather than rejecting.Workarounds
References
References