CVE-2025-49812

Executive Summary

CVE-2025-49812 is a high-severity HTTP desynchronization vulnerability affecting Apache HTTP Server versions up to and including 2.4.63. The flaw resides in specific mod_ssl configurations where SSLEngine optional is used to enable TLS upgrades. A man-in-the-middle (MITM) attacker can exploit this to hijack an HTTP session by manipulating the TLS upgrade handshake, leading to session hijacking and potential data leakage. The vulnerability is unpatched in affected versions, with the fix requiring an upgrade to version 2.4.64, which removes TLS upgrade support entirely.

Supply Chain & Package Security

This vulnerability is not directly related to NPM, PyPI, Go, or Ruby packages. However, organizations using Apache HTTP Server as a reverse proxy or load balancer for these ecosystems should prioritize patching to prevent session hijacking attacks that could compromise application-level security.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Not officially assigned)
• Attack Vector: Network (MITM)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Changed (affects HTTP session integrity)
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: None
• Threat Level: Critical (due to ease of exploitation and potential for session hijacking)

Deep Dive & Mechanics

The vulnerability arises from improper handling of TLS upgrade requests in mod_ssl when SSLEngine optional is configured. In this mode, the server allows both HTTP and HTTPS connections on the same port, with TLS upgrades initiated via the Upgrade: TLS/1.0 header. An MITM attacker can intercept the initial HTTP request, inject a malicious Upgrade header, and force the server to upgrade to TLS while maintaining the original HTTP session state. This desynchronization allows the attacker to hijack the session, intercept subsequent requests, and inject malicious responses.

The root cause is a race condition in the TLS upgrade handshake where the server does not properly validate the integrity of the upgrade request against the original HTTP session. The fix in version 2.4.64 removes TLS upgrade support entirely, eliminating the attack surface.

Vulnerable vs. Patched Code

Vulnerable Configuration (Apache HTTP Server < 2.4.64):

<VirtualHost *:80>
    SSLEngine optional
    # Other directives
</VirtualHost>

Patched Configuration (Apache HTTP Server >= 2.4.64):

<VirtualHost *:80>
    # SSLEngine optional is removed; use separate ports for HTTP and HTTPS
</VirtualHost>

The patch removes the SSLEngine optional directive and TLS upgrade support, forcing administrators to use separate ports for HTTP and HTTPS.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Apache HTTP Server 2.4.63 with SSLEngine optional enabled
• MITM position (e.g., ARP spoofing, rogue Wi-Fi)

Exploitation Steps:

1. Intercept the victim's HTTP request to the server.
2. Inject an Upgrade: TLS/1.0 header into the request.
3. Forward the modified request to the server.
4. The server upgrades to TLS, but the session state is desynchronized.
5. The attacker can now hijack the session by intercepting subsequent requests and responses.

Example MITM Attack (Python):

import socket
import ssl

def mitm_attack(target_host, target_port):
    # Intercept HTTP request
    client_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    client_sock.connect((target_host, target_port))
    request = b"GET / HTTP/1.1\r\nHost: example.com\r\nUpgrade: TLS/1.0\r\n\r\n"
    client_sock.send(request)
    response = client_sock.recv(4096)
    print("Intercepted response:", response)
    client_sock.close()

mitm_attack("192.168.1.100", 80)

Blue Team

Detection:

• Monitor for unusual Upgrade: TLS/1.0 headers in HTTP requests.
• Alert on HTTP requests that result in TLS upgrades without proper session validation.
• Use network monitoring tools to detect MITM activity (e.g., ARP spoofing).

Mitigation:

• Immediately upgrade to Apache HTTP Server 2.4.64.
• If upgrade is not possible, disable SSLEngine optional and use separate ports for HTTP and HTTPS.
• Implement strict TLS configuration with SSLEngine on for HTTPS-only virtual hosts.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests containing Upgrade: TLS/1.0 header
  • Unusual TLS handshake patterns on port 80
  • ARP spoofing or DNS poisoning indicators
• System IOCs:
  • Apache access logs showing Upgrade: TLS/1.0 in requests
  • Unexpected TLS upgrades from HTTP sessions
• File IOCs:
  • Modified Apache configuration files with SSLEngine optional

Detection Engineering

YARA Rule

rule CVE_2025_49812_HTTP_Desync {
    meta:
        description = "Detects HTTP requests with Upgrade: TLS/1.0 header indicating CVE-2025-49812 exploitation"
        author = "Valters IT"
        date = "2025-04-01"
        reference = "CVE-2025-49812"
    strings:
        $upgrade_header = "Upgrade: TLS/1.0" nocase
    condition:
        $upgrade_header
}

Sigma Rule

title: CVE-2025-49812 HTTP Desynchronization Attack
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects HTTP requests with Upgrade: TLS/1.0 header indicating exploitation of CVE-2025-49812
author: Valters IT
date: 2025-04-01
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-49812
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'GET'
        cs-uri-query|contains: 'Upgrade: TLS/1.0'
    condition: selection
falsepositives:
    - Legitimate TLS upgrade requests from known clients
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-49812 HTTP Desynchronization Attack"; flow:to_server; content:"Upgrade: TLS/1.0"; http_header; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate Action: Upgrade to Apache HTTP Server version 2.4.64 or later.
2. Workaround: If upgrade is not possible, disable SSLEngine optional and configure separate virtual hosts for HTTP and HTTPS.
3. Network Controls: Implement network segmentation and monitor for MITM attacks using ARP spoofing detection tools.
4. Configuration Hardening: Use SSLEngine on for HTTPS-only virtual hosts and avoid mixed-mode configurations.
5. Monitoring: Deploy the provided YARA, Sigma, and Suricata rules to detect exploitation attempts.

Valters IT
DevSecOps & Cyber Threat Intelligence