CVE-2026-111809: Apache Traffic Server Smuggling Vulnerability

CVE-2026-111809 Overview

CVE-2026-111809 is an HTTP Request Smuggling vulnerability in Apache Traffic Server. The flaw lets attackers smuggle requests through the proxy by sending HEAD requests that carry Content-Length or Transfer-Encoding headers. Affected versions span 8.0.0 through 8.1.11, 9.0.0 through 9.2.15, 10.0.0 through 10.1.4, and 10.2.0. The Apache Software Foundation has released fixed versions 9.2.16, 10.1.5, and 10.2.1. The weakness is classified as [CWE-444] Inconsistent Interpretation of HTTP Requests.

Critical Impact
Attackers can desynchronize proxy and backend parsing to poison caches, bypass security controls, or hijack subsequent user requests on shared connections.

Affected Products

Apache Traffic Server 8.0.0 through 8.1.11
Apache Traffic Server 9.0.0 through 9.2.15
Apache Traffic Server 10.0.0 through 10.1.4, and 10.2.0

Discovery Timeline

2026-10-21 - CVE-2026-111809 published to NVD
2026-10-22 - Last updated in NVD database

Technical Details for CVE-2026-111809

Vulnerability Analysis

Apache Traffic Server is a widely deployed caching HTTP proxy. The vulnerability occurs when the proxy processes HEAD requests that include a message body indicator such as Content-Length or Transfer-Encoding. HTTP/1.1 defines HEAD responses as bodyless, but the specification does not forbid these headers on HEAD requests themselves. When the proxy and the upstream origin disagree on whether bytes following the HEAD request belong to a body or to the next request, request boundaries desynchronize. An attacker can exploit this desynchronization to inject a second request that the backend parses independently of the proxy.

Successful smuggling allows an attacker to influence another user's response, poison cached content, or route traffic past front-end security filters. The attack is unauthenticated and executes over the network.

Root Cause

The root cause is inconsistent interpretation of HTTP message framing between Apache Traffic Server and downstream servers when a HEAD method is combined with Content-Length or Transfer-Encoding: chunked. Per [CWE-444], one component treats the trailing bytes as part of the HEAD request, while the other treats them as a new, separate request on the same connection.

Attack Vector

The attacker sends a crafted HEAD request containing conflicting framing headers to a vulnerable Traffic Server instance. The proxy forwards or interprets the request differently than the origin server. The remaining bytes on the keep-alive connection are then consumed as a smuggled request. Review the Apache Mailing List Thread for upstream discussion.

No verified public exploit code is available at this time. The vulnerability mechanism is documented in prose rather than demonstrated here.

Detection Methods for CVE-2026-111809

Indicators of Compromise

HEAD requests in proxy access logs that contain Content-Length greater than zero or a Transfer-Encoding: chunked header.
Unexpected cache entries where the cached response does not match the originally requested URI.
Backend origin logs showing requests with no corresponding front-end proxy log entry.
Anomalous response-to-request ratios on persistent connections through Traffic Server.

Detection Strategies

Inspect HTTP traffic for HEAD methods paired with framing headers and alert on their presence.
Compare request counts between the Traffic Server front end and backend origins to surface desynchronization.
Deploy signatures tuned to CWE-444 smuggling patterns in web application firewalls fronting Traffic Server.

Monitoring Recommendations

Centralize Traffic Server access and error logs into a SIEM for correlation with origin server logs.
Monitor cache poisoning indicators such as sudden response-content changes for popular URIs.
Track upgrades to patched Traffic Server builds across the fleet to confirm coverage.

How to Mitigate CVE-2026-111809

Immediate Actions Required

Upgrade Apache Traffic Server to version 9.2.16, 10.1.5, or 10.2.1 as applicable to your deployment branch.
Inventory all Traffic Server instances, including container images and ephemeral workloads, to confirm patch status.
Rotate or invalidate cached responses after patching to remove any poisoned entries.

Patch Information

The Apache Software Foundation fixed the issue in Traffic Server 9.2.16, 10.1.5, and 10.2.1. Users on the 8.x branch should migrate to a supported fixed release, since no 8.x fix is listed. Patch details and discussion are available in the Apache Mailing List Thread.

Workarounds

Place a strict HTTP-compliant reverse proxy or WAF in front of Traffic Server to reject HEAD requests carrying Content-Length or Transfer-Encoding headers.
Disable HTTP keep-alive on vulnerable instances to limit the window for smuggled requests, accepting the performance cost until patching completes.
Restrict proxy exposure to trusted network segments where feasible until the upgrade is deployed.

bash

# Configuration example
# No vendor-published configuration workaround is available for CVE-2026-111809.
# The recommended remediation is to upgrade to a fixed Apache Traffic Server release:
#   - 9.2.16
#   - 10.1.5
#   - 10.2.1