Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, Ember’s chunk decoder trims the chunk-size token and accepts leading plus or minus signs instead of requiring one or more hexadecimal digits followed by the required CRLF. When an intermediary forwards chunked data without re-encoding and interprets malformed chunk boundaries differently, an unauthenticated attacker can create TE.TE request smuggling that bypasses intermediary controls, poisons caches, or hijacks the request queue. The same response-path leniency can enable response smuggling against an ember-client used as a gateway when the upstream is malicious or compromised. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

Affected: Maven: org.http4s:http4s-ember-core_2.12, Maven: org.http4s:http4s-ember-core_2.13, Maven: org.http4s:http4s-ember-core_3

## 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 buffe…