CVE-2026-43951 Overview
CVE-2026-43951 is an out-of-bounds read vulnerability [CWE-125] in the Apache HTTP Server. The flaw resides in interactions between mod_headers and mod_mime when processing multiple response languages. Affected versions span Apache HTTP Server 2.4.0 through 2.4.67. A remote attacker can trigger the condition over the network without authentication or user interaction, leading to limited information disclosure and integrity impact. Apache HTTP Server powers a significant share of public web infrastructure, making any exploitable defect in default modules relevant to operators worldwide.
Critical Impact
Remote, unauthenticated attackers can trigger an out-of-bounds read in Apache HTTP Server configurations using mod_headers and mod_mime with multiple response languages, exposing process memory and degrading response integrity.
Affected Products
- Apache HTTP Server 2.4.0 through 2.4.67
- Deployments using mod_headers together with mod_mime
- Configurations that serve multiple response languages via content negotiation
Discovery Timeline
- 2026-06-08 - CVE-2026-43951 published to the National Vulnerability Database
- 2026-06-11 - Last updated in NVD database
Technical Details for CVE-2026-43951
Vulnerability Analysis
The vulnerability is an out-of-bounds read [CWE-125] that arises when Apache HTTP Server processes responses involving both mod_headers and mod_mime while emitting multiple response languages. During response construction, the server reads beyond the bounds of an allocated buffer, returning data outside the intended structure to the caller or to subsequent processing logic.
The attack vector is network based, requires no privileges, and needs no user interaction. Successful exploitation yields limited confidentiality and integrity impact rather than full compromise. The condition can disclose adjacent process memory or alter header content in ways that influence downstream consumers such as caches and proxies.
No public exploit, proof-of-concept, or CISA Known Exploited Vulnerabilities entry exists for CVE-2026-43951 at the time of publication. Apache documents the issue in the Apache HTTP Server 2.4 vulnerabilities advisory.
Root Cause
The root cause is improper bounds checking when handling content negotiation across multiple Content-Language values combined with header manipulation performed by mod_headers. The interaction produces a state where the response handler reads beyond an allocated buffer associated with language or header metadata.
Attack Vector
An attacker sends crafted HTTP requests to a vulnerable server configured with mod_headers and mod_mime and at least one resource that resolves to multiple response languages. The server processes the request, triggers the faulty code path, and emits a response that may contain leaked bytes or malformed headers. See the OpenWall OSS-Security disclosure for additional technical context.
No verified exploit code is available. Technical mechanics are described in prose only.
Detection Methods for CVE-2026-43951
Indicators of Compromise
- Unexpected or malformed Content-Language headers returned by the origin server
- Responses containing trailing non-printable bytes or unexpected memory-like content in headers
- Repeated requests from a single source targeting resources known to support content negotiation
Detection Strategies
- Inspect Apache access and error logs for requests targeting multilingual resources accompanied by anomalous response sizes
- Compare observed response headers against expected baselines for negotiated content
- Run the running Apache version against the affected range 2.4.0 through 2.4.67 using configuration management tooling
Monitoring Recommendations
- Forward Apache access, error, and module logs to a centralized analytics platform for header anomaly detection
- Alert on bursts of requests to URIs that trigger mod_mime content negotiation
- Monitor outbound responses through a reverse proxy or WAF for malformed header structures
How to Mitigate CVE-2026-43951
Immediate Actions Required
- Identify all Apache HTTP Server instances in the affected range 2.4.0 through 2.4.67
- Upgrade to a fixed Apache HTTP Server release as listed in the official Apache HTTP Server 2.4 vulnerabilities advisory
- Restart Apache services after applying the update to ensure the patched modules are loaded
Patch Information
Apache Software Foundation publishes fixed versions and patch details on the Apache HTTP Server 2.4 vulnerabilities page. Operators should apply the vendor-supplied update for any instance running 2.4.0 through 2.4.67. Distribution packages from major Linux vendors typically follow the upstream release.
Workarounds
- Disable mod_headers where header manipulation is not required for the deployment
- Remove or restrict content negotiation that serves multiple response languages from the same resource
- Place a reverse proxy or WAF in front of Apache to normalize response headers and strip anomalous bytes
- Limit exposure of affected virtual hosts to trusted networks until patching is complete
# Configuration example: temporarily disable mod_headers until patched
a2dismod headers
systemctl restart apache2
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
