CVE-2026-43951

What it actually does

The vulnerability is an out-of-bounds read in Apache HTTP Server 2.4.0 through 2.4.67 when mod_headers and mod_mime are used with configurations that serve multiple response languages. A remote unauthenticated client can trigger the vulnerable response-generation path by requesting content that causes language and MIME metadata processing. The flaw may allow Apache to read memory outside the intended buffer while constructing a response, potentially exposing small amounts of adjacent process memory or causing incorrect response metadata. The CVSS vector indicates network exploitation with low complexity, no authentication, and low confidentiality and integrity impact, with no expected availability impact.

Why it matters to the business

Public-facing Apache HTTP Server instances using the affected modules and multilingual response handling may unintentionally disclose limited server-process data to remote users. In practical terms, this could expose fragments of sensitive content, headers, configuration-derived values, session-adjacent data, or other request/response material present in memory, depending on workload and memory layout. The issue is not currently listed in CISA KEV, is not reported as ransomware-used, has low EPSS, and has no known public exploit maturity in the provided data, so broad exploitation pressure appears low; however, internet-facing web servers should still be remediated because the attack requires no credentials and Apache HTTP Server is commonly deployed.

How an attack unfolds

An attacker first identifies an Apache HTTP Server version in the affected range and determines whether the server uses mod_headers, mod_mime, and multilingual content handling such as language variants or content negotiation. The attacker then sends crafted HTTP requests to resources that exercise MIME and language selection behavior, potentially using language-related request headers and paths associated with localized static content. If the server reaches the vulnerable code path, Apache may read beyond the intended memory boundary while preparing response metadata or content. The attacker compares responses across repeated requests, looking for anomalous bytes, unexpected header values, malformed output, or small memory disclosures. Because the flaw is an out-of-bounds read rather than code execution, exploitation is primarily aimed at information exposure rather than direct system takeover.

How to fix it

Upgrade Apache HTTP Server to a vendor-provided version that fixes CVE-2026-43951 as soon as an applicable fixed release is available for the deployment platform. The provided fixed-version data does not list a specific corrected version, so administrators should track official Apache HTTP Server and operating-system package advisories for releases newer than 2.4.67 or backported fixes. Until patched, reduce exposure by disabling mod_headers if it is not required, reviewing mod_mime language mappings, disabling or limiting multilingual content negotiation where feasible, removing unnecessary language variants, and restricting access to affected virtual hosts where business requirements allow. For packaged distributions, apply the distribution security update rather than relying only on the upstream version string, because vendors may backport the fix without changing the Apache base version visibly.

How to confirm the fix worked

Confirm the running Apache HTTP Server package is no longer in the vulnerable 2.4.0 through 2.4.67 range unless the operating-system vendor explicitly documents a backported fix. Use commands such as `httpd -v`, `apache2 -v`, package-manager advisory output, and distribution changelogs to verify the corrected build. Check loaded modules with `apachectl -M`, `httpd -M`, or equivalent and determine whether `headers_module` and `mime_module` are enabled. Review Apache configuration for multilingual response handling, including language mappings and content negotiation directives. After remediation or mitigation, test representative localized resources and confirm responses are stable, correctly formed, and do not contain unexpected bytes or metadata. If relying on mitigation rather than patching, verify the vulnerable module and language-negotiation combination is no longer reachable from untrusted clients.

Typical attack path

1. Scan for internet-facing Apache HTTP Server instances and fingerprint versions where possible.
2. Identify targets running Apache HTTP Server 2.4.0 through 2.4.67 with mod_headers and mod_mime enabled.
3. Probe for multilingual or language-negotiated content using requests with varied Accept-Language values and localized resource names.
4. Send crafted requests that trigger MIME and language response processing.
5. Inspect responses for unexpected data leakage, malformed headers, unusual response bodies, or inconsistent output across repeated requests.
6. Use any disclosed information to support follow-on reconnaissance, session targeting, or configuration discovery.

How to detect it

Retrospective detection is difficult because out-of-bounds read exploitation may leave no definitive server-side artifact and leaked data may only appear in the HTTP response seen by the attacker. Review Apache access logs, reverse-proxy logs, CDN logs, and WAF telemetry for repeated unauthenticated requests to localized or language-negotiated resources, especially requests with unusual, very long, malformed, or rapidly changing Accept-Language headers. Look for high request repetition against the same static resource with different language preferences, requests that produce unusual 200, 300, 406, or 500 patterns, and response-size anomalies for normally small localized files. Review Apache error logs for malformed response generation, negotiation-related warnings, module errors, or unusual header-processing messages around suspicious request times. If full packet capture, response-body logging, or web proxy response sampling is available, inspect returned headers and bodies for unexpected binary data, memory-like fragments, unrelated request content, or configuration fragments. Because no specific public exploit artifact is provided, detection should focus on behavior and exposure rather than fixed signatures.

What your security team should do

• Inventory all Apache HTTP Server deployments, prioritizing internet-facing systems and reverse proxies.
• Identify systems running Apache HTTP Server 2.4.0 through 2.4.67 and determine whether mod_headers, mod_mime, and multilingual response handling are enabled.
• Apply vendor security updates promptly once a fixed package is available; for managed Linux distributions, verify whether a backported fix has been issued.
• Temporarily disable unnecessary mod_headers usage or multilingual content negotiation on exposed virtual hosts until patched.
• Increase monitoring of Apache access logs for unusual language-negotiation probes, repeated requests with varied Accept-Language headers, and anomalous responses from localized resources.
• Preserve relevant Apache access logs, error logs, reverse-proxy logs, and web application firewall logs for retrospective review.
• Do not treat the low EPSS and absence from KEV as a reason to ignore the issue on public web infrastructure; exploitation requires no credentials and the affected software is widely deployed.