CVE-2021-41524: Apache HTTP Server DoS Vulnerability

CVE-2021-41524 Overview

CVE-2021-41524 is a null pointer dereference vulnerability discovered in Apache HTTP Server version 2.4.49 during HTTP/2 request processing. The flaw was identified through fuzzing techniques and allows an external attacker to cause a Denial of Service (DoS) condition on vulnerable servers by sending specially crafted HTTP/2 requests. This vulnerability was introduced in version 2.4.49, making it a regression issue that affects organizations running this specific version.

Critical Impact

Remote attackers can crash Apache HTTP Server instances without authentication, causing service disruption for all hosted applications and websites. The network-accessible nature of this vulnerability makes it particularly dangerous for internet-facing servers.

Affected Products

• Apache HTTP Server 2.4.49
• Fedora 34 and Fedora 35
• Oracle Instantis EnterpriseTrack 17.1, 17.2, and 17.3
• NetApp Cloud Backup

Discovery Timeline

• October 5, 2021 - CVE-2021-41524 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-41524

Vulnerability Analysis

This vulnerability is classified as CWE-476 (Null Pointer Dereference), a memory corruption issue that occurs when the HTTP/2 request handler in Apache httpd attempts to access memory through a null pointer. The vulnerability was introduced as a regression in version 2.4.49 and was discovered during fuzzing activities targeting the HTTP/2 implementation.

The attack is network-based and requires no authentication or user interaction to exploit. An attacker can trigger the null pointer dereference by sending a specially crafted HTTP/2 request to a vulnerable server, causing the server process to crash. While this vulnerability affects availability rather than confidentiality or integrity, the ease of exploitation and potential for repeated attacks make it a significant concern for organizations relying on Apache HTTP Server for critical services.

Root Cause

The root cause of CVE-2021-41524 is a null pointer dereference in the HTTP/2 request processing code. During the handling of certain malformed or specially crafted HTTP/2 requests, the server fails to properly validate pointers before accessing them, leading to a crash when a null pointer is dereferenced. This programming error was introduced in the 2.4.49 release and represents a regression from previous versions that handled these conditions safely.

Attack Vector

The vulnerability is exploited remotely over the network without requiring any privileges or user interaction. An attacker targets the HTTP/2 protocol handler by crafting a malicious request that triggers the null pointer dereference condition. The attack mechanism involves:

1. Establishing an HTTP/2 connection to the target Apache server
2. Sending a specially crafted HTTP/2 request that exploits the parsing logic
3. Triggering the null pointer dereference condition in the request handler
4. Causing the server process or worker to crash

The attack can be repeated to cause sustained service disruption. For more technical details about the vulnerability mechanism, refer to the Apache HTTP Server security advisory and the Openwall OSS Security disclosure.

Detection Methods for CVE-2021-41524

Indicators of Compromise

• Unexpected Apache HTTP Server crashes or restarts, particularly in httpd worker processes
• Core dumps indicating null pointer access in HTTP/2 related modules
• Anomalous HTTP/2 traffic patterns preceding server failures
• Log entries showing abrupt connection terminations during HTTP/2 sessions

Detection Strategies

• Monitor Apache error logs for segmentation faults and null pointer dereference errors
• Implement HTTP/2 traffic inspection to identify malformed request patterns
• Deploy application performance monitoring to detect sudden availability drops
• Use web application firewalls (WAF) with HTTP/2 protocol validation capabilities

Monitoring Recommendations

• Configure crash dump collection for Apache processes to analyze exploitation attempts
• Set up automated alerting for Apache service restarts and availability degradation
• Monitor system logs for signals indicating process crashes (SIGSEGV, SIGABRT)
• Implement health checks that verify HTTP/2 protocol handling functionality

How to Mitigate CVE-2021-41524

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.50 or later immediately
• If upgrading is not immediately possible, consider disabling HTTP/2 protocol support as a temporary measure
• Review server configurations to ensure only necessary protocols are enabled
• Implement network-level rate limiting for HTTP/2 connections

Patch Information

Apache has released security patches addressing this vulnerability. Organizations should upgrade to Apache HTTP Server version 2.4.50 or later, which contains the fix for CVE-2021-41524. Detailed patching information is available in the Apache HTTP Server security advisory. Additional vendor-specific guidance is available from Oracle's January 2022 Security Alert, NetApp Security Advisory, and Fedora Package Announcements.

Workarounds

• Disable HTTP/2 protocol support by removing Protocols h2 h2c from Apache configuration until patching is complete
• Deploy a reverse proxy or load balancer in front of Apache to filter potentially malicious HTTP/2 requests
• Implement network segmentation to limit exposure of vulnerable servers
• Use web application firewalls with protocol validation to block malformed HTTP/2 traffic

# Disable HTTP/2 temporarily in Apache configuration
# Edit httpd.conf or relevant virtual host configuration
# Remove or comment out HTTP/2 protocol enablement

# Before (vulnerable):
# Protocols h2 h2c http/1.1

# After (mitigated):
Protocols http/1.1

# Restart Apache to apply changes
systemctl restart httpd