CVE-2021-26691: Apache HTTP Server Buffer Overflow Flaw

CVE-2021-26691 Overview

CVE-2021-26691 is a heap overflow vulnerability affecting Apache HTTP Server versions 2.4.0 through 2.4.46. The vulnerability occurs when the server processes a specially crafted SessionHeader sent by an origin server, potentially allowing attackers to corrupt heap memory and achieve remote code execution or cause a denial of service condition.

Critical Impact

This heap overflow vulnerability can be exploited remotely without authentication to potentially execute arbitrary code or crash affected Apache HTTP Server instances, impacting the confidentiality, integrity, and availability of web services.

Affected Products

• Apache HTTP Server versions 2.4.0 to 2.4.46
• Debian Linux 9.0 and 10.0
• Fedora 34 and 35
• Oracle Enterprise Manager Ops Center 12.4.0.0
• Oracle Instantis EnterpriseTrack 17.1, 17.2, and 17.3
• Oracle Secure Backup
• Oracle ZFS Storage Appliance Kit 8.8
• NetApp Cloud Backup

Discovery Timeline

• June 10, 2021 - CVE-2021-26691 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-26691

Vulnerability Analysis

The vulnerability resides in the session handling code of Apache HTTP Server's mod_session module. When the server acts as a reverse proxy and receives responses from a backend origin server, it processes SessionHeader values contained in those responses. A maliciously crafted SessionHeader can trigger improper memory handling, leading to a heap-based buffer overflow condition.

This flaw is classified under CWE-122 (Heap-based Buffer Overflow) and CWE-787 (Out-of-bounds Write), indicating that the vulnerability involves writing data beyond the allocated heap buffer boundaries. The network-based attack vector means exploitation can occur remotely without requiring any privileges or user interaction, making this a particularly dangerous vulnerability for internet-facing Apache deployments.

Root Cause

The root cause lies in insufficient bounds checking when parsing and processing session data from backend server responses. When the mod_session module processes a SessionHeader containing unexpected or oversized data, the server fails to properly validate the input length before copying data into a heap-allocated buffer. This allows an attacker controlling a backend server (or positioned as a man-in-the-middle) to send specially crafted session data that overflows the destination buffer.

Attack Vector

Exploitation requires the attacker to control or intercept responses from an origin server that the vulnerable Apache HTTP Server communicates with. The attack flow typically involves:

1. The Apache server is configured as a reverse proxy with session handling enabled
2. An attacker compromises or impersonates a backend origin server
3. The attacker sends an HTTP response containing a malicious SessionHeader with oversized or crafted data
4. When Apache processes this header, the heap overflow occurs
5. Depending on heap layout and exploitation technique, this can lead to code execution or denial of service

The vulnerability does not require the attacker to directly access the Apache server; instead, they exploit the trust relationship between the proxy and backend servers.

Detection Methods for CVE-2021-26691

Indicators of Compromise

• Unexpected Apache HTTP Server crashes or restarts, particularly when handling proxied requests
• Segmentation fault errors in Apache error logs related to session processing
• Anomalous or oversized SessionHeader values in HTTP traffic from backend servers
• Memory corruption indicators in core dump analysis

Detection Strategies

• Deploy web application firewalls (WAF) to inspect and limit SessionHeader sizes in proxied traffic
• Monitor Apache error logs for segmentation faults, memory allocation errors, or unexpected worker process terminations
• Implement network monitoring to detect abnormally large HTTP headers in responses from backend servers
• Use intrusion detection systems (IDS) with signatures for known Apache heap overflow attack patterns

Monitoring Recommendations

• Enable detailed logging for mod_session and mod_proxy modules to capture session-related events
• Configure alerting for Apache process crashes and automatic restart events
• Monitor backend server responses for unusual header patterns or sizes
• Implement runtime application self-protection (RASP) solutions to detect memory corruption attempts

How to Mitigate CVE-2021-26691

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.47 or later immediately
• If immediate patching is not possible, disable mod_session if session functionality is not required
• Review and restrict which backend servers can communicate with the Apache proxy
• Implement network segmentation to limit exposure of vulnerable instances

Patch Information

Apache has addressed this vulnerability in Apache HTTP Server version 2.4.47 and later releases. Organizations should apply the official patches available from the Apache HTTPD Security Advisories. Additional security updates have been released by major Linux distributions including Debian, Fedora, and Gentoo. Oracle has also released patches as part of their October 2021 and January 2022 Critical Patch Updates for affected Oracle products.

Workarounds

• Disable mod_session by commenting out or removing the LoadModule session_module directive in the Apache configuration
• Implement strict input validation on backend servers to ensure SessionHeader values conform to expected formats and sizes
• Configure reverse proxy rules to strip or sanitize SessionHeader values from backend responses
• Deploy a reverse proxy or load balancer in front of vulnerable Apache instances to filter malicious headers

# Disable mod_session in Apache configuration
# Edit /etc/httpd/conf/httpd.conf or /etc/apache2/apache2.conf

# Comment out or remove the following line:
# LoadModule session_module modules/mod_session.so

# Also disable related session modules if not needed:
# LoadModule session_cookie_module modules/mod_session_cookie.so
# LoadModule session_dbd_module modules/mod_session_dbd.so

# Restart Apache after changes
sudo systemctl restart httpd
# or
sudo systemctl restart apache2