CVE-2026-69204 Http4s: Ember accepts Transfer-Encoding combined with Content-Length (CL.TE request smuggling)

What is CVE-2026-69204?

CVE-2026-69204 is a critical vulnerability in http4s, published on 15 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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.

Weakness: CWE-444

Which versions are affected by CVE-2026-69204?

10 open source packages ship an affected version, including org.http4s:http4s-ember-core_2.13, org.http4s:http4s-ember-core_2.12 and org.http4s:http4s-ember-core_3.

How do I fix CVE-2026-69204?

Upgrade org.http4s:http4s-ember-core_2.13 to a fixed version for your release line: 0.23.35 or 1.0.0-M47.

Is CVE-2026-69204 being exploited?

There is no known exploitation of CVE-2026-69204 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-8h4c-x2wg-6xp8.