CVE-2021-34798: Apache HTTP Server Use-After-Free Flaw

CVE-2021-34798 Overview

CVE-2021-34798 is a NULL pointer dereference vulnerability in Apache HTTP Server versions 2.4.48 and earlier. This vulnerability allows remote attackers to cause a denial of service condition by sending specially crafted malformed HTTP requests to the server. When the server attempts to process these malicious requests, it dereferences a NULL pointer, resulting in a crash and service disruption.

Critical Impact

Unauthenticated remote attackers can crash Apache HTTP Server instances with malformed requests, causing service outages for web applications and services hosted on affected servers.

Affected Products

• Apache HTTP Server 2.4.48 and earlier versions
• Fedora 34 and 35
• Debian Linux 9.0, 10.0, and 11.0
• NetApp Cloud Backup, Clustered Data ONTAP, and StorageGRID
• Tenable.sc
• Oracle HTTP Server 12.2.1.3.0 and 12.2.1.4.0
• Oracle Enterprise Manager Base Platform 13.4.0.0 and 13.5.0.0
• Oracle PeopleSoft Enterprise PeopleTools 8.58
• Oracle Instantis EnterpriseTrack 17.1, 17.2, and 17.3
• Oracle ZFS Storage Appliance Kit 8.8
• Broadcom Brocade Fabric Operating System Firmware
• Siemens RUGGEDCOM NMS, SINEC NMS, SINEMA Remote Connect Server, and SINEMA Server 14.0

Discovery Timeline

• September 16, 2021 - CVE-2021-34798 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-34798

Vulnerability Analysis

This vulnerability stems from inadequate input validation when Apache HTTP Server processes incoming HTTP requests. When the server receives a malformed request with specific characteristics, the request parsing logic fails to properly handle certain edge cases, leading to a NULL pointer being dereferenced during processing.

The attack can be executed remotely over the network without requiring any authentication or user interaction. While the vulnerability does not allow attackers to access or modify data on the server, it can reliably crash the httpd process, causing a denial of service condition. Organizations running Apache HTTP Server in production environments face potential service disruptions that could affect all hosted websites and applications.

Root Cause

The root cause is classified as CWE-476 (NULL Pointer Dereference). The vulnerability exists because the Apache HTTP Server request handling code does not adequately validate certain request parameters before attempting to access memory through pointers. When a malformed request triggers a code path where an expected pointer value is NULL, the server attempts to dereference this NULL pointer, causing the process to crash.

Attack Vector

The attack vector is network-based, requiring no privileges or user interaction. An attacker can exploit this vulnerability by sending specially crafted HTTP requests to a vulnerable Apache HTTP Server. The malformed requests are designed to trigger the NULL pointer dereference condition during request processing.

The attack can be performed remotely from any network location that can reach the target server on its HTTP/HTTPS ports. Since the vulnerability affects the core request handling functionality, any publicly accessible Apache server running a vulnerable version is potentially at risk. The low complexity of exploitation combined with no authentication requirements makes this vulnerability particularly concerning for internet-facing servers.

Detection Methods for CVE-2021-34798

Indicators of Compromise

• Unexpected Apache httpd process crashes or restarts in system logs
• Segmentation fault entries in Apache error logs referencing NULL pointer access
• Increased frequency of server restarts without corresponding configuration changes
• Unusual patterns of malformed HTTP requests in access logs

Detection Strategies

• Monitor Apache error logs for segmentation fault messages and NULL pointer dereference errors
• Implement Web Application Firewall (WAF) rules to detect and block malformed HTTP requests
• Use intrusion detection systems (IDS) to identify anomalous request patterns targeting Apache servers
• Deploy SentinelOne Singularity to detect exploitation attempts and process crashes

Monitoring Recommendations

• Configure alerting on Apache service availability and automatic restart events
• Enable verbose logging on Apache HTTP Server to capture detailed request information
• Monitor system-level crash reports and core dump generation for httpd processes
• Track Apache process stability metrics including uptime and restart frequency

How to Mitigate CVE-2021-34798

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.49 or later to address this vulnerability
• Apply vendor-specific patches for affected Oracle, NetApp, Siemens, and other bundled products
• Implement rate limiting and request validation at the network edge to reduce attack surface
• Enable Web Application Firewall (WAF) protection with rules targeting malformed HTTP requests

Patch Information

Apache Software Foundation has released Apache HTTP Server version 2.4.49, which addresses this vulnerability along with several other security issues. Organizations should consult the Apache HTTP Server Vulnerabilities page for official patch information. Additionally, multiple vendors have released advisories and patches for their products that bundle Apache HTTP Server:

• Oracle Security Alerts January 2022
• Oracle Security Alerts April 2022
• Siemens Product Security Advisory
• Debian Security Advisory DSA-4982
• NetApp Security Advisory NTAP-20211008-0004

Workarounds

• Deploy a reverse proxy or load balancer in front of Apache servers to filter malformed requests
• Implement network-level access controls to restrict access to Apache servers from untrusted sources
• Configure mod_reqtimeout to limit request processing time and reduce impact of malicious requests
• Use connection limiting directives to throttle incoming requests from individual IP addresses

# Configuration example - Add to Apache configuration for additional protection
# Enable mod_reqtimeout to limit request handling time
<IfModule mod_reqtimeout.c>
    RequestReadTimeout header=20-40,MinRate=500 body=20,MinRate=500
</IfModule>

# Limit connections per IP using mod_limitipconn (if available)
<IfModule mod_limitipconn.c>
    MaxConnPerIP 10
</IfModule>