CVE-2026-69216: Http4s Ember Authentication Bypass Vulnerability

CVE-2026-69216 Overview

CVE-2026-69216 is an HTTP request smuggling vulnerability in http4s, a Scala interface for HTTP services. The flaw resides in Ember's chunked transfer-encoding decoder, which trims the chunk-size token and accepts leading + or - signs instead of strictly requiring one or more hexadecimal digits followed by CRLF. When an intermediary forwards chunked data without re-encoding and parses chunk boundaries differently than Ember, an unauthenticated attacker can craft TE.TE request smuggling payloads. The same response-path leniency enables response smuggling against an ember-client used as a gateway. Affected versions are all releases prior to 0.23.35 and 1.0.0-M47. The weakness is classified as [CWE-444] Inconsistent Interpretation of HTTP Requests.

Critical Impact
Unauthenticated attackers can bypass intermediary security controls, poison HTTP caches, or hijack queued requests behind a shared frontend proxy.

Affected Products

http4s Ember server and client, all versions prior to 0.23.35
http4s Ember server and client, all 1.0.0 milestone builds prior to 1.0.0-M47
Deployments where http4s Ember sits behind or in front of a chunk-forwarding intermediary

Discovery Timeline

2026-09-15 - CVE-2026-69216 published to NVD
2026-09-16 - Last updated in NVD database

Technical Details for CVE-2026-69216

Vulnerability Analysis

HTTP request smuggling arises when two HTTP processors in the same request path disagree on where one message ends and the next begins. Ember's chunk decoder in ember-core/shared/src/main/scala/org/http4s/ember/core/ChunkedEncoding.scala deviated from RFC 9112 by trimming whitespace around the chunk-size token and accepting numeric sign prefixes such as +1A or -0. RFC 9112 requires the chunk-size to be 1*HEXDIG followed immediately by an optional chunk extension and a CRLF.

When a strict intermediary interprets a malformed chunk-size as an error or as a differently sized chunk, and Ember accepts the same bytes as a valid but reinterpreted length, the two views of the message body diverge. An attacker exploits this divergence to smuggle a second request into the byte stream that the backend attributes to another user. The same parsing leniency on responses allows an upstream malicious server to smuggle responses through an ember-client acting as a gateway, enabling cache poisoning or cross-tenant response mixing.

Root Cause

The root cause is permissive chunk-size parsing that violates the ABNF in RFC 9112 Section 7.1. The decoder accepted tokens outside the 1*HEXDIG grammar and did not enforce a mandatory CRLF terminator after chunk-data, allowing framing ambiguity between http4s and upstream or downstream HTTP processors.

Attack Vector

Exploitation requires network reachability to a chunked HTTP path traversing at least one intermediary that forwards chunked bodies without re-encoding. No authentication or user interaction is required. Success depends on the specific chunk-parsing behavior of the paired intermediary, which raises attack complexity.

text

// Patch excerpt from ChunkedEncoding.scala
 import cats.effect.kernel.Ref
 import cats.syntax.all._
 import fs2._
+import org.http4s.internal.CharPredicate
 import org.http4s.internal.appendSanitized
 import scodec.bits.ByteVector

Source: http4s commit d78612a. The fix introduces a CharPredicate to constrain the chunk-size token to hexadecimal digits and enforces the required CRLF after chunk-data.

Detection Methods for CVE-2026-69216

Indicators of Compromise

Access logs showing HTTP requests with Transfer-Encoding: chunked and chunk-size lines containing +, -, or surrounding whitespace.
Backend responses attributed to the wrong client session, or unexpected requests appearing under an authenticated user's context.
Cache entries returning content that does not match the requested URL or Host header.

Detection Strategies

Deploy an inline HTTP validator that rejects chunk-size tokens outside 1*HEXDIG and enforces strict CRLF framing on Transfer-Encoding: chunked bodies.
Correlate frontend and backend request logs by request identifier to detect boundary mismatches indicating smuggled messages.
Fuzz staging endpoints with malformed chunk-size prefixes to identify pre-patch http4s Ember services still in production.

Monitoring Recommendations

Alert on parser errors, dropped connections, and abnormal 4xx/5xx bursts on chunked routes traversing shared proxies.
Monitor cache poisoning indicators such as sudden mismatches between response Content-Type and requested resource type.
Track http4s dependency versions across the software bill of materials and flag any host still resolving to a version below 0.23.35 or 1.0.0-M47.

How to Mitigate CVE-2026-69216

Immediate Actions Required

Upgrade http4s Ember to 0.23.35 on the 0.23.x line or to 1.0.0-M47 on the milestone line.
Inventory all services using ember-server and ember-client and prioritize any deployed behind a shared frontend proxy or CDN.
Audit intermediary configurations to disable transparent forwarding of malformed chunked bodies and enable strict RFC 9112 validation.

Patch Information

The fix is delivered in http4s v0.23.35 and http4s v1.0.0-M47. Technical details are documented in GitHub Security Advisory GHSA-jrpm-956j-96jg and the remediation commit.

Workarounds

Terminate and re-encode chunked HTTP traffic at a strict RFC 9112-compliant reverse proxy before it reaches Ember.
Disable Transfer-Encoding: chunked on request paths where clients can send only bounded, Content-Length-framed bodies.
Reject inbound requests containing both Transfer-Encoding and Content-Length headers to reduce smuggling surface until patching completes.

bash

# Example sbt dependency update
libraryDependencies ++= Seq(
  "org.http4s" %% "http4s-ember-server" % "0.23.35",
  "org.http4s" %% "http4s-ember-client" % "0.23.35"
)