CVE-2024-53868: Apache Traffic Server DOS Vulnerability

CVE-2024-53868 Overview

CVE-2024-53868 is an HTTP request smuggling vulnerability in Apache Traffic Server (ATS), the open-source forward and reverse proxy widely deployed as a content delivery and caching layer. The flaw stems from improper parsing of malformed chunked transfer-encoded messages, allowing attackers to desynchronize how the proxy and backend interpret request boundaries. Affected versions include Apache Traffic Server 9.2.0 through 9.2.9 and 10.0.0 through 10.0.4. The Apache Software Foundation released fixed versions 9.2.10 and 10.0.5. The weakness is classified under [CWE-444] (Inconsistent Interpretation of HTTP Requests).

Critical Impact

Remote unauthenticated attackers can smuggle HTTP requests through Apache Traffic Server, bypassing security controls and poisoning caches or hijacking sessions on backend servers.

Affected Products

• Apache Traffic Server 9.2.0 through 9.2.9
• Apache Traffic Server 10.0.0 through 10.0.4
• Deployments using ATS as a reverse proxy or CDN edge

Discovery Timeline

• 2025-04-03 - CVE-2024-53868 published to NVD
• 2025-04-29 - Last updated in NVD database

Technical Details for CVE-2024-53868

Vulnerability Analysis

The vulnerability is an HTTP Request Smuggling flaw triggered by malformed chunked transfer-encoded messages. Apache Traffic Server processes chunked HTTP message bodies in a way that can diverge from how an upstream origin server interprets the same request. When a chunked message contains malformed framing, such as invalid chunk size lines, unexpected whitespace, or trailing characters, ATS and the backend can disagree on where one request ends and the next begins.

An attacker exploits this disagreement by sending a single crafted HTTP request that ATS treats as one message but the backend parses as two. The smuggled second request executes in the context of a subsequent legitimate client connection. This enables cache poisoning, security control bypass, credential theft, and unauthorized access to internal endpoints.

The issue carries an EPSS score of 0.292% with a percentile of 52.78, reflecting moderate exploit prediction. No public proof-of-concept is currently catalogued, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Root Cause

The root cause is inconsistent parsing of chunked transfer encoding between ATS and downstream origin servers. ATS accepts chunked message constructs that deviate from RFC 9112 strict framing, producing a different request boundary than a stricter or differently lenient backend parser. This parser disagreement is the foundation of all HTTP request smuggling attacks.

Attack Vector

The attack vector is network-based, requires no authentication, and no user interaction. An attacker submits a crafted HTTP request containing a malformed chunked body to any ATS listener. The exploitation mechanism relies on sending hop-by-hop framing primitives that ATS and the backend disagree about, causing the trailing bytes of the attacker's request to be prepended to the next victim request on the reused upstream connection.

No verified public exploit code is available. Refer to the Apache Mailing List Thread and the OpenWall OSS-Security Discussion for vendor-provided technical context.

Detection Methods for CVE-2024-53868

Indicators of Compromise

• Unexpected HTTP responses returned to clients that do not correspond to their original requests, indicating cross-request contamination.
• ATS access logs containing chunked requests with unusual chunk size syntax, embedded CRLF sequences, or duplicate Transfer-Encoding headers.
• Cache entries serving content for URLs that legitimate clients never requested, suggesting cache poisoning.
• Backend application logs showing requests with mismatched Host headers or methods compared to the ATS upstream record.

Detection Strategies

• Deploy network-layer inspection rules that flag HTTP requests containing both Transfer-Encoding: chunked and Content-Length headers traversing ATS.
• Compare ATS access logs against backend origin logs for request count and boundary mismatches over the same keep-alive connection.
• Run version inventory queries against deployed ATS instances to identify hosts running 9.2.0 through 9.2.9 or 10.0.0 through 10.0.4.

Monitoring Recommendations

• Alert on anomalous spikes in HTTP 400 responses from ATS, which often accompany smuggling probes during reconnaissance.
• Monitor for non-standard chunk size lines and malformed Transfer-Encoding headers at the ingress WAF or reverse proxy tier.
• Track cache hit ratios and cache key collisions for unexpected variance that may indicate poisoning activity.

How to Mitigate CVE-2024-53868

Immediate Actions Required

• Upgrade Apache Traffic Server to version 9.2.10 if running the 9.2.x branch, or 10.0.5 if running the 10.0.x branch.
• Inventory all ATS deployments, including container images and CDN edge nodes, to confirm patched versions are in use.
• Review ATS and origin access logs for the past 90 days for indicators of chunked-encoding abuse and cache poisoning.

Patch Information

The Apache Software Foundation has released fixed versions Apache Traffic Server 9.2.10 and 10.0.5. Patch details are documented in the Apache Mailing List Thread. Administrators should apply the upgrade through standard package management or rebuild from source using the patched release tags.

Workarounds

• If immediate patching is not feasible, place a strict HTTP parser such as a WAF in front of ATS to reject requests containing both Transfer-Encoding and Content-Length headers.
• Disable HTTP keep-alive between ATS and origin servers to reduce the impact of smuggled requests on subsequent connections.
• Configure backend origins to enforce strict RFC 9112 chunked encoding parsing and reject malformed framing.

# Verify Apache Traffic Server version after upgrade
traffic_server --version

# Expected output should show 9.2.10 or 10.0.5 or later
# Restart ATS to apply the patched binary
sudo systemctl restart trafficserver