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

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.6%, percentile 47: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 47% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes org.http4s:http4s-ember-core_2.12 (Maven) <= 0.23.34, fixed in 0.23.35; org.http4s:http4s-ember-core_2.13 (Maven) <= 0.23.34, fixed in 0.23.35; org.http4s:http4s-ember-core_3 (Maven) <= 0.23.34, fixed in 0.23.35; org.http4s:http4s-ember-core_2.13 (Maven) >= 1.0.0-M1, <= 1.0.0-M46, fixed in 1.0.0-M47. Check your lockfile, including transitive dependencies.

What it is

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.

From the GitHub advisory

Summary

Ember's HTTP/1.1 request parser does not reject a message that carries both a Transfer-Encoding and a Content-Length header. RFC 9112 §6.1 requires a server to treat such a message as a framing error and close the connection. An intermediary that follows the RFC's CL-strip-and-forward path (or that prioritises Content-Length) will frame the body differently from Ember, enabling HTTP request smuggling (CL.TE).

Impact

Server

Request smuggling when ember-server is an origin behind an intermediary that forwards both headers over a keep-alive backend connection and frames by Content-Length while Ember frames by chunked:

Weakness type: CWE-444.

Product: org.http4s:http4s-ember-core_2.12 (Maven) · Assigned by: GitHub_M · Published: 2026-09-15

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: present (depends on deployment conditions)
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.57% probability of exploitation activity in the next 30 days; percentile 47.

Timeline

• 2026-09-15 — CVE record last updated
• 2026-09-15 — CVE record published by GitHub_M
• 2026-09-15 — GitHub-reviewed advisory GHSA-8h4c-x2wg-6xp8 published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-8h4c-x2wg-6xp8 github.com
2. CVE.org record CVE-2026-69204 cve.org
3. NVD entry CVE-2026-69204 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com

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