CVE-2026-69205: Http4s Ember Authentication Bypass Vulnerability

CVE-2026-69205 Overview

CVE-2026-69205 affects Http4s, a Scala interface for HTTP services, in versions prior to 0.23.35 and 1.0.0-M47. The flaw resides in Ember's HeaderP.parse, which applies a case-sensitive substring test to the Transfer-Encoding header value and decodes header bytes using the platform default charset. This inconsistent parsing lets attackers desynchronize Ember from RFC-compliant intermediaries, enabling HTTP request smuggling [CWE-444].

Critical Impact

Attackers can perform TE.CL or TE.0 request smuggling, bypass access controls, hijack cross-user requests, and poison caches on the server path.

Affected Products

• Http4s Ember server and client versions prior to 0.23.35
• Http4s milestone releases prior to 1.0.0-M47
• Applications relying on Ember behind RFC-compliant HTTP intermediaries

Discovery Timeline

• 2026-09-15 - CVE-2026-69205 published to NVD
• 2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-69205

Vulnerability Analysis

Ember's HeaderP.parse performs a case-sensitive substring comparison against the Transfer-Encoding header value. RFC 7230 specifies that transfer-coding tokens are case-insensitive, so front-end proxies and Ember can reach different conclusions about whether a request is chunked. Values such as Chunked are not recognized by Ember, while non-standard values such as notchunked are incorrectly accepted because the substring chunked is present.

The issue is compounded by header decoding using the Java Virtual Machine (JVM) platform default charset instead of a fixed byte-oriented parse. On systems where UTF-8 is applied, Unicode case folding can transform a Kelvin-sign byte sequence (U+212A) into an ASCII K, producing a match Ember treats as chunked. An RFC-compliant intermediary would reject or normalize the same bytes differently.

Root Cause

The root cause is improper input validation in the transfer-coding parser combined with charset-dependent header decoding. Ember does not implement RFC 7230 token rules for Transfer-Encoding, and it depends on Charset.defaultCharset() behavior rather than a deterministic ASCII decode. This creates parser disagreement between Ember and upstream proxies.

Attack Vector

An attacker sends a crafted request through an RFC-compliant intermediary to an Ember server. The intermediary interprets framing using Content-Length or zero-length semantics, while Ember interprets the same request as chunked. The remainder of the attacker's payload is then treated by Ember as the start of a subsequent request, enabling TE.CL or TE.0 smuggling, access-control bypass, cross-user request hijacking, and cache poisoning. Response smuggling through an ember-client gateway requires a malicious or compromised upstream. Technical detail is available in the Http4s Security Advisory GHSA-9998-894r-fwvr.

Detection Methods for CVE-2026-69205

Indicators of Compromise

• Requests containing Transfer-Encoding values with mixed casing such as Chunked, CHUNKED, or embedded tokens like notchunked
• Header bytes containing non-ASCII sequences (for example the Kelvin sign U+212A) inside Transfer-Encoding
• Duplicate or conflicting Transfer-Encoding and Content-Length headers on the same request
• Unexplained cross-user responses, cache entries pointing to unexpected origins, or 400-class errors from upstream proxies

Detection Strategies

• Inspect proxy and Ember access logs for divergent request boundaries and unexpected pipelined requests on the same connection
• Compare request framing decisions between the front-end proxy and the Http4s Ember backend for the same connection identifier
• Alert on any Transfer-Encoding header whose value does not exactly match the lowercase token chunked

Monitoring Recommendations

• Enable byte-level logging of Transfer-Encoding and Content-Length headers at the perimeter
• Monitor authenticated endpoints for responses delivered to sessions that did not issue the request
• Track cache-hit anomalies where a poisoned entry serves attacker-controlled content to multiple users

How to Mitigate CVE-2026-69205

Immediate Actions Required

• Upgrade Http4s Ember to 0.23.35 or 1.0.0-M47 as documented in the v0.23.35 release notes and v1.0.0-M47 release notes
• Audit build dependencies for transitive references to vulnerable Http4s Ember versions
• Review front-end proxy logs for smuggling attempts targeting the affected window

Patch Information

The fix is delivered in Http4s 0.23.35 and 1.0.0-M47. The corrected parser enforces RFC-compliant token rules for Transfer-Encoding and decodes header bytes deterministically. See the upstream commit 5e88b2e for the implementation.

Workarounds

• Terminate HTTP at a strict, RFC-compliant reverse proxy that rejects malformed Transfer-Encoding values before they reach Ember
• Configure the JVM to run with -Dfile.encoding=US-ASCII for header handling if upgrading immediately is not possible, understanding this does not fully close the parser gap
• Disable HTTP connection reuse between the proxy and Ember to limit request boundary confusion

# Example sbt dependency update to the patched Http4s version
libraryDependencies += "org.http4s" %% "http4s-ember-server" % "0.23.35"
libraryDependencies += "org.http4s" %% "http4s-ember-client" % "0.23.35"