Executive brief
Http4s Ember is a Java/Scala HTTP client and server library. The chunk decoder is too lenient in parsing chunk sizes, accepting leading/trailing whitespace and +/- signs, which allows attackers to craft malformed chunked requests that intermediaries may interpret differently. When an Ember-based server sits behind a proxy, this can allow request smuggling to bypass security controls, poison caches, or hijack request queues.
Technical details
The vulnerability is an HTTP request/response smuggling flaw (CWE-444) in Http4s Ember's chunk transfer coding decoder. The root cause is that the chunk size parser strips leading and trailing whitespace and accepts a leading + or - sign, deviating from RFC 9112 §7.1 which defines chunk-size as 1*HEXDIG (hex digits only). An intermediary (proxy, firewall) that interprets or rejects these non-conformant chunk sizes differently than Ember will misalign on request boundaries. Attack preconditions for the server path: (1) Ember-based server deployed behind a keep-alive intermediary, (2) intermediary forwards chunked bodies verbatim without re-encoding, (3) intermediary disagrees with Ember on the +/- prefix or whitespace handling. An unauthenticated remote attacker can smuggle a request through the intermediary to reach the origin server with attacker-chosen method and headers, bypassing front-end security controls, poisoning cached responses, or hijacking request queues. The client path (response smuggling) requires a malicious or compromised upstream server. Patches are available: 0.23.35 for the 0.23.x line and 1.0.0-M47 for the 1.0.0-M series.
Affected products
- Http4s http4s-ember-core_2.12 <= 0.23.34
- Http4s http4s-ember-core_2.13 <= 0.23.34, >= 1.0.0-M1 to <= 1.0.0-M46
- Http4s http4s-ember-core_3 <= 0.23.34, >= 1.0.0-M1 to <= 1.0.0-M46
Timeline
- 2026-09-15: disclosed: Vulnerability published to GitHub Advisory Database
- 2026-09-15: patched: Patches released: v0.23.35 and v1.0.0-M47