# CVE-2026-69204: Http4s Ember HTTP request smuggling via Transfer-Encoding and Content-Length

Source: https://junglewise.ai/threats/cve-2026-69204-http4s-ember-http-request-smuggling-via-transfer-encoding-and  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-15. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-69204 · Severity: critical · CVSS 4 · Published 2026-09-15

Http4s Ember is an HTTP server and client library used in Scala applications. The library's HTTP/1.1 parser fails to reject requests containing both Transfer-Encoding and Content-Length headers, a violation of RFC 9112. When deployed behind a proxy that forwards both headers and prioritizes Content-Length for framing, an attacker can perform request smuggling attacks to bypass security controls, hijack user sessions, or poison cached responses.

Technologies: [Http4s-Ember-Core](https://junglewise.ai/threats/technologies/http4s-ember-core), [org.http4s:http4s-ember-core_2.12 (Maven)](https://junglewise.ai/threats/technologies/org-http4s-http4s-ember-core-2-12), [org.http4s:http4s-ember-core_2.13 (Maven)](https://junglewise.ai/threats/technologies/org-http4s-http4s-ember-core-2-13), [org.http4s:http4s-ember-core_3 (Maven)](https://junglewise.ai/threats/technologies/org-http4s-http4s-ember-core-3). Vendors: [Http4s](https://junglewise.ai/threats/vendors/http4s), [Maven](https://junglewise.ai/threats/vendors/maven).

## Quick answers

**Is CVE-2026-69204 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-69204 being exploited in the wild.

**How severe is CVE-2026-69204?** Critical severity, with a CVSS score of 4, published 15 September 2026.

**Which products and versions are affected?** Http4s-Ember-Core <= 0.23.34; org.http4s:http4s-ember-core_2.12 (Maven) ECOSYSTEM:introduced=0,fixed=0.23.35; org.http4s:http4s-ember-core_2.13 (Maven) ECOSYSTEM:introduced=0,fixed=0.23.35; org.http4s:http4s-ember-core_3 (Maven) ECOSYSTEM:introduced=0,fixed=0.23.35.

**Is there a fix for CVE-2026-69204?** Yes. org.http4s:http4s-ember-core_2.12 (Maven): upgrade to 0.23.35 or later; org.http4s:http4s-ember-core_2.13 (Maven): upgrade to 0.23.35 or later; org.http4s:http4s-ember-core_3 (Maven): upgrade to 0.23.35 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-69204 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-69204?** Check whether you run Http4s-Ember-Core, org.http4s:http4s-ember-core_2.12 (Maven), org.http4s:http4s-ember-core_2.13 (Maven) or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- org.http4s:http4s-ember-core_2.12 (Maven): upgrade to 0.23.35 or later

  ```sh
  # set org.http4s:http4s-ember-core_2.12 to 0.23.35 in your pom.xml or build.gradle
  ```

- org.http4s:http4s-ember-core_2.13 (Maven): upgrade to 0.23.35 or later

  ```sh
  # set org.http4s:http4s-ember-core_2.13 to 0.23.35 in your pom.xml or build.gradle
  ```

- org.http4s:http4s-ember-core_3 (Maven): upgrade to 0.23.35 or later

  ```sh
  # set org.http4s:http4s-ember-core_3 to 0.23.35 in your pom.xml or build.gradle
  ```


## Technical details

The vulnerability is an inconsistent HTTP request interpretation (CWE-444) in Http4s Ember's HTTP/1.1 parser. RFC 9112 §6.1 requires servers to reject—and close the connection on—any message carrying both a Transfer-Encoding and Content-Length header. Ember's parser fails to enforce this requirement, accepting both headers. When Ember is deployed behind a reverse proxy or HTTP intermediary that follows RFC guidance by stripping Transfer-Encoding and framing the message body according to Content-Length, the two systems interpret message boundaries differently. An attacker can send a crafted request with both headers to an intermediary; the intermediary frames the body by Content-Length (short), while Ember frames by chunked encoding (long), leaving extra attacker-controlled bytes in the connection buffer. On a keep-alive connection, this buffer overflow contaminates the next request, enabling request smuggling. Attack requires network access to the intermediary (unauthenticated) and a backend keep-alive connection to Ember. Patched versions (0.23.35, 1.0.0-M47, and later) reject messages with both headers. No patches for 0.23.34 or 1.0.0-M1–M46 are available; users must upgrade.

## 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

## Identifiers

- CVE-2026-69204
- GHSA-8h4c-x2wg-6xp8

## Timeline

- 2026-07-06: disclosed: Published to GitHub Advisory Database
- 2026-09-15: advisory: Reviewed and published to GitHub Advisory Database
- 2026-07-06: patched: Patched versions released: 0.23.35, 1.0.0-M47

## References

- [https://api.github.com/users/ERobertGII](https://api.github.com/users/ERobertGII)
- [https://github.com/ERobertGII](https://github.com/ERobertGII)
- [https://api.github.com/users/ERobertGII/gists%7B/gist_id%7D](https://api.github.com/users/ERobertGII/gists%7B/gist_id%7D)
- [https://api.github.com/users/ERobertGII/repos](https://api.github.com/users/ERobertGII/repos)
- [https://avatars.githubusercontent.com/u/93225934?v=4](https://avatars.githubusercontent.com/u/93225934?v=4)
- [https://api.github.com/users/ERobertGII/events%7B/privacy%7D](https://api.github.com/users/ERobertGII/events%7B/privacy%7D)

## Related threats

- [Http4s Ember HTTP/2 frame buffering denial of service](https://junglewise.ai/threats/cve-2026-88975-http4s-ember-http-2-frame-buffering-denial-of-service)
- [Http4s Ember HTTP/2 unbounded continuation frame accumulation](https://junglewise.ai/threats/cve-2026-69218-http4s-ember-http-2-unbounded-continuation-frame-accumulation)
- [Http4s DigestAuth replay attack in authorization header](https://junglewise.ai/threats/cve-2026-69206-http4s-digestauth-replay-attack-in-authorization-header)
- [Http4s Ember HTTP request smuggling via Transfer-Encoding parsing](https://junglewise.ai/threats/cve-2026-69205-http4s-ember-http-request-smuggling-via-transfer-encoding-parsing)
- [Http4s Ember HTTP/2 stream limit enforcement bypass](https://junglewise.ai/threats/cve-2026-69203-http4s-ember-http-2-stream-limit-enforcement-bypass)