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CVE-2026-69205 – HTTP Request Smuggling – http4s Ember Server (versions ≤ 0.23.34 and 1.0.0-M1–1.0.0

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Summary

CVE-2026-69205 is a high-severity (CVSS 8.7) HTTP request smuggling vulnerability in the Ember server backend of http4s, a Scala HTTP library. The flaw arises from Ember’s HTTP/1.1 header parser using case-sensitive substring matching (hValue.contains("chunked")) to detect chunked transfer encoding, rather than the RFC 9112-mandated case-insensitive token comparison. Attackers can craft requests using non-standard casing (e.g., Transfer-Encoding: Chunked) or otherwise malformed Transfer-Encoding values that cause Ember and front-end intermediaries to disagree on request framing, enabling TE.CL/TE.0-style smuggling.

Technical details

  • Root cause: Ember’s Transfer-Encoding header parser performs a case-sensitive substring check for the literal "chunked" instead of parsing and comparing transfer-coding tokens case-insensitively as required by RFC 9112 §7, and also relies on platform-default charset decoding, which can be abused via Unicode case-folding tricks.
  • Trigger conditions: An attacker sends an HTTP request with a Transfer-Encoding header value that varies in case or is otherwise malformed in a way that a compliant intermediary (proxy, load balancer, CDN) interprets differently than Ember does — for example, treating the request as chunked-encoded on one side and as a Content-Length or non-chunked request on the other.
  • Attack vector: Network-based (AV:N), requires no authentication or user interaction, but has high attack complexity (AC:H) since it depends on the specific configuration and behavior of an intermediary sitting in front of the Ember server.
  • Impact: Desynchronization between Ember and front-end proxies/intermediaries can allow attackers to bypass front-end access controls, smuggle requests to hijack other users’ sessions (exposing cookies, authorization headers, or other sensitive data), and poison shared web caches. When http4s Ember acts as a client/gateway to an untrusted upstream, response smuggling is also possible.

Affected software

  • http4s-ember-core (Scala 2.12/2.13/3) versions up to and including 0.23.34
  • http4s-ember-core (Scala 2.13/3) versions 1.0.0-M1 through 1.0.0-M46

Severity

  • CVSS v3.1 Base Score: 8.7 (High)
  • Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:N

Mitigation and recommended actions

  • Immediate: Upgrade http4s Ember to version 0.23.35 or 1.0.0-M47, both of which fix the Transfer-Encoding parsing logic to perform RFC-compliant, case-insensitive comparison.
  • If patching is not immediately possible:
    • Configure any intermediaries/proxies in front of Ember to fully buffer and re-encode request bodies rather than passing through raw framing.
    • Normalize Transfer-Encoding header values (e.g., force lowercase, reject non-canonical tokens) before forwarding requests to Ember.
    • Disable HTTP keep-alive/connection reuse between intermediaries and Ember backends to limit the blast radius of any successful smuggling attempt.

References

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