CVE-2026-69204: HTTP4S

Critical severity, CVSS 9.2. EPSS: 0.6% chance of exploitation in the next 30 days.

Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, Ember HTTP/1.1 does not reject messages containing both Transfer-Encoding and Content-Length, so an intermediary and Ember can select different body framing rules. When ember-server is behind a keep-alive intermediary that forwards both headers and frames by Content-Length, an unauthenticated attacker can smuggle a second request, bypass intermediary access controls, poison caches, or cause a victim request to be joined to an attacker-controlled prefix. The shared response parser can also desynchronize an ember-client connection when a malicious or compromised upstream sends both headers. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

Affected products

• HTTP4S HTTP4S: before 0.23.35 (fixed in 0.23.35); from 1.0.0-M1, before 1.0.0-M47 (fixed in 1.0.0-M47)

Published 2026-09-15. Last modified 2026-09-16.