CVE-2026-43951: Apache HTTP Server Out-of-Bounds Read in mod_headers & mod_mime

Technical summary

CVE-2026-43951 is an out-of-bounds read (CWE-125) in Apache HTTP Server's header and MIME type handling logic when processing multi-language responses. The vulnerability exists in versions 2.4.0–2.4.67 and can be exploited remotely without authentication. The flaw allows an attacker to read adjacent memory regions beyond the intended buffer boundary, potentially exposing confidential information from the server process. The condition requires no special access control or user interaction, making it readily exploitable over the network.

Business impact

Organizations running vulnerable Apache HTTP Server instances face confidentiality risk. Successful exploitation could expose sensitive data in server memory—including authentication tokens, session identifiers, private keys, or application secrets—without triggering typical attack detection signatures. While this is not a code execution or denial-of-service issue, the leakage of in-memory secrets can enable follow-on attacks such as session hijacking, credential theft, or lateral movement within the organization. Enterprises relying on Apache for critical web services should treat this as a priority remediation target.

Affected systems

All Apache HTTP Server deployments running version 2.4.0 through 2.4.67 are affected. This spans a wide range of configurations across Windows, Linux, and Unix platforms. The vulnerability is triggered when mod_headers and mod_mime modules are active (both are commonly enabled by default). Organizations should audit their Apache fleet to identify which servers are running vulnerable versions and confirm whether these modules are loaded.

Exploitability

Exploitation is straightforward and does not require authentication, special privileges, or user interaction. An attacker can craft network requests that trigger the out-of-bounds read condition from any location with network access to the server. The CVSS vector (AV:N/AC:L/PR:N/UI:N) reflects the ease of attack. However, exploitability is limited by the attacker's ability to reliably control which memory region is read and interpret the leaked data—this is not a trivial information disclosure but rather a memory leak that requires knowledge of server internals or luck to yield useful secrets.

Remediation

Upgrade Apache HTTP Server to a patched version that addresses CVE-2026-43951. Verify the exact version available from the Apache security advisory and test in a staging environment before production deployment. For organizations unable to patch immediately, consider disabling mod_headers or mod_mime if application functionality permits, or restrict network access to the server to trusted sources only. Monitor server logs for suspicious requests that might indicate exploitation attempts.

Patch guidance

Check the official Apache HTTP Server security advisory for the specific patched version number addressing this CVE. Verify compatibility with your deployed modules and extensions before upgrading. Test the patched version in a non-production environment to confirm it resolves the issue without breaking application functionality or other module interactions. Coordinate the deployment across your infrastructure to minimize service disruption. After patching, confirm that mod_headers and mod_mime are still functioning as expected.

Detection guidance

Monitor network traffic and web server logs for requests that include unusual header combinations or multi-language response parameters that might trigger the out-of-bounds condition. This may manifest as requests with multiple Accept-Language headers, Content-Language values, or edge-case MIME type specifications. Web Application Firewall (WAF) rules can help detect exploitation attempts if you understand the specific request pattern that triggers the flaw. Intrusion Detection System (IDS) signatures from vendors like Suricata or Snort may be available post-disclosure. Enable verbose error logging on Apache to capture any unusual behavior or memory access errors that might indicate exploitation.

Why prioritize this

Although the CVSS score is moderate (6.5), this vulnerability warrants priority attention due to its network accessibility, lack of authentication requirement, and confidentiality impact. The exposure of in-memory secrets can be catastrophic if attackers retrieve authentication material or cryptographic keys. The affected version range (2.4.0–2.4.67) covers many production deployments, making this a widespread concern. Organizations should patch within their standard critical-update window, particularly for internet-facing servers.

Risk score, explained

The CVSS 3.1 score of 6.5 (MEDIUM severity) reflects an exploitable network vulnerability requiring no authentication or user interaction, but with a confidentiality impact only (no integrity or availability impact). The score appropriately penalizes the ease of exploitation but does not inflate it to Critical due to the lack of code execution or denial-of-service capability. In context, however, the potential to leak sensitive in-memory data justifies treating it as a high-priority patch candidate in your patch management workflow.

Frequently asked questions