CVE-2024-42516: Apache HTTP Server Response Split Flaw

CVE-2024-42516 Overview

CVE-2024-42516 is an HTTP response splitting vulnerability in the core of Apache HTTP Server. An attacker who can manipulate the Content-Type response headers of applications hosted or proxied by the server can split the HTTP response, potentially leading to cache poisoning, cross-site scripting (XSS), or other injection attacks.

This vulnerability was originally described as CVE-2023-38709, but the patch included in Apache HTTP Server 2.4.59 did not adequately address the issue. Users are recommended to upgrade to version 2.4.64, which properly fixes this vulnerability.

Critical Impact

Attackers can exploit this HTTP response splitting vulnerability to inject malicious content into HTTP responses, potentially compromising web application security and enabling downstream attacks such as cache poisoning or session hijacking.

Affected Products

• Apache HTTP Server versions prior to 2.4.64
• Applications hosted or proxied by vulnerable Apache HTTP Server instances
• Systems running Apache HTTP Server 2.4.59 with the incomplete CVE-2023-38709 fix

Discovery Timeline

• 2025-07-10 - CVE-2024-42516 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2024-42516

Vulnerability Analysis

This vulnerability stems from improper input validation (CWE-20) in the Apache HTTP Server core when processing Content-Type response headers. HTTP response splitting occurs when an attacker can inject carriage return (CR) and line feed (LF) characters into HTTP response headers, effectively terminating the current response and beginning a new one under the attacker's control.

The vulnerability is particularly concerning because it affects the server core, meaning any application hosted or proxied by a vulnerable Apache HTTP Server instance could potentially be exploited. The network-accessible nature of web servers combined with the ability to compromise response integrity without requiring authentication makes this a significant security concern.

The incomplete patch in version 2.4.59 (intended to address CVE-2023-38709) failed to properly sanitize all injection vectors in Content-Type headers, leaving the vulnerability exploitable in environments that had applied what was believed to be a complete fix.

Root Cause

The root cause is improper input validation in the Apache HTTP Server core when handling Content-Type response headers. The server fails to adequately sanitize header values, allowing specially crafted input containing CRLF sequences to be interpreted as header terminators. This enables attackers to inject arbitrary HTTP headers and body content into the server's response stream.

Attack Vector

The attack is network-based and can be executed without authentication or user interaction. An attacker needs the ability to influence the Content-Type response headers of applications running on or proxied through the vulnerable Apache HTTP Server. This could be achieved through:

• Manipulating application parameters that are reflected in Content-Type headers
• Exploiting vulnerable backend applications that pass unsanitized data to header generation
• Leveraging proxy configurations where user-controlled data influences response headers

When successful, the attacker can inject additional HTTP headers or even complete HTTP responses, potentially leading to cache poisoning attacks affecting multiple users, cross-site scripting through injected content, or session hijacking scenarios.

Detection Methods for CVE-2024-42516

Indicators of Compromise

• Unusual CRLF sequences (%0d%0a or \r\n) appearing in web server access logs, particularly in Content-Type related parameters
• Unexpected additional HTTP headers in responses that were not configured by administrators
• Cache entries containing suspicious or unexpected content that differs from the legitimate application responses
• Reports of users receiving unexpected content or being redirected to malicious sites when accessing trusted applications

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block CRLF injection attempts in HTTP headers
• Configure intrusion detection systems to alert on patterns consistent with HTTP response splitting attacks
• Enable detailed logging of HTTP response headers and monitor for anomalies
• Deploy SentinelOne Singularity Platform for real-time behavioral analysis of web server processes and network traffic

Monitoring Recommendations

• Monitor Apache HTTP Server access and error logs for evidence of header injection attempts
• Track unusual patterns in Content-Type headers across application responses
• Implement application-layer monitoring to detect response splitting attempts before they reach clients
• Regularly audit proxy configurations for potential injection vectors

How to Mitigate CVE-2024-42516

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.64 or later immediately
• Review and validate all application code that generates or modifies Content-Type response headers
• Implement input validation at the application layer to sanitize any user-controllable data used in HTTP headers
• Deploy web application firewall rules to block CRLF injection patterns

Patch Information

Apache has released version 2.4.64 which properly addresses this vulnerability. This update supersedes the incomplete fix in version 2.4.59 that was intended to resolve CVE-2023-38709. Organizations should update all Apache HTTP Server installations to 2.4.64 or later. Detailed patch information is available from the Apache HTTPD Vulnerability List.

Additional advisories and discussions are available from the Openwall OSS Security mailing list and Debian LTS Security Announcement.

Workarounds

• Implement strict input validation in all applications to prevent CRLF sequences from reaching header generation code
• Configure reverse proxy or WAF rules to strip or reject requests containing potential header injection payloads
• Consider implementing Content Security Policy (CSP) headers to limit the impact of any successful injection
• Restrict access to applications that allow user-controlled input in response headers until patching is complete

# Verify Apache HTTP Server version
httpd -v

# Check for available updates (RHEL/CentOS)
yum check-update httpd

# Check for available updates (Debian/Ubuntu)
apt list --upgradable | grep apache2

# Update Apache HTTP Server (RHEL/CentOS)
yum update httpd

# Update Apache HTTP Server (Debian/Ubuntu)
apt update && apt upgrade apache2