CVE-2020-35452: Apache HTTP Server Buffer Overflow Flaw

CVE-2020-35452 Overview

CVE-2020-35452 is a stack overflow vulnerability affecting Apache HTTP Server versions 2.4.0 through 2.4.46. The vulnerability exists in the mod_auth_digest module, where a specially crafted Digest nonce can trigger a single-byte stack overflow. While no confirmed exploit exists and the Apache HTTP Server team was unable to create one, certain compiler configurations or compilation options could potentially make exploitation possible, though with limited consequences due to the minimal overflow size (a single zero byte).

Critical Impact

Stack overflow in mod_auth_digest authentication module could potentially lead to denial of service or limited code execution depending on compiler and platform configurations.

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, 17.3
• Oracle ZFS Storage Appliance Kit 8.8

Discovery Timeline

• June 10, 2021 - CVE-2020-35452 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-35452

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write) and affects the Digest authentication handling in Apache HTTP Server. The mod_auth_digest module processes authentication nonce values during the HTTP Digest Authentication process. When a maliciously crafted nonce value is submitted by an attacker, the module fails to properly validate the input boundaries, resulting in a stack buffer overflow.

The overflow itself is constrained to a single byte with a null value (0x00), which significantly limits the practical impact of exploitation. The Apache Security Team conducted internal testing and could not produce a working exploit. However, the theoretical risk remains because certain compiler optimizations, stack layouts, or platform-specific memory arrangements could potentially make this single-byte overflow exploitable under specific conditions.

Root Cause

The root cause lies in improper boundary checking within the nonce parsing logic of mod_auth_digest. When processing Digest authentication headers, the module allocates a fixed-size stack buffer for the nonce value. The vulnerability occurs because the code does not adequately validate the length of the incoming nonce before writing it to the buffer, allowing an off-by-one write condition that results in a single null byte being written past the allocated buffer boundary.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by sending specially crafted HTTP requests containing a malformed Digest authentication nonce to a vulnerable Apache HTTP Server with mod_auth_digest enabled.

The attack requires:

1. Target server running Apache HTTP Server 2.4.0 through 2.4.46
2. The mod_auth_digest module must be enabled and configured
3. Attacker sends HTTP requests with crafted Digest authentication headers containing malicious nonce values

While the vulnerability can be triggered remotely, the practical exploitability is severely limited by the single-byte overflow constraint. The exploitation would depend heavily on memory layout determined by compiler options, optimization levels, and target platform architecture.

Detection Methods for CVE-2020-35452

Indicators of Compromise

• Unusual or malformed Digest authentication headers in HTTP access logs
• Repeated authentication failures with abnormally long nonce values in request headers
• Apache httpd process crashes or unexpected restarts associated with authentication requests
• Memory corruption signatures in core dump files from Apache processes

Detection Strategies

• Monitor Apache error logs for segmentation faults or memory-related errors during authentication processing
• Implement web application firewall (WAF) rules to detect oversized or malformed Digest authentication nonce values
• Deploy intrusion detection system (IDS) signatures to identify HTTP requests with suspicious Digest authentication headers
• Use static code analysis tools to verify Apache HTTP Server version and module configuration

Monitoring Recommendations

• Enable verbose logging for mod_auth_digest module to capture authentication attempt details
• Implement real-time log analysis for Apache access and error logs to detect anomalous authentication patterns
• Configure application performance monitoring (APM) to alert on unexpected Apache process behavior or crashes
• Establish baseline metrics for authentication traffic and alert on significant deviations

How to Mitigate CVE-2020-35452

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.47 or later immediately
• If upgrade is not immediately possible, disable mod_auth_digest and switch to alternative authentication methods such as mod_auth_basic with HTTPS
• Apply vendor-specific patches from Debian, Fedora, or Oracle as applicable to your environment
• Review server configurations to identify all instances where Digest authentication is enabled

Patch Information

Apache has addressed this vulnerability in Apache HTTP Server version 2.4.47 and later releases. Organizations should upgrade to the latest stable version as soon as possible. Security advisories and patch information are available from the Apache HTTP Server Vulnerabilities page.

Additional vendor patches are available:

• Debian users should refer to Debian Security Advisory DSA-4937 and the Debian LTS Announcement
• Fedora users can find updates in the Fedora Package Announcements
• Oracle customers should apply the Oracle Critical Patch Update from October 2021

Workarounds

• Disable mod_auth_digest module by commenting out the LoadModule directive in Apache configuration: # LoadModule auth_digest_module modules/mod_auth_digest.so
• Migrate to mod_auth_basic with TLS/HTTPS encryption as an alternative authentication mechanism
• Implement network-level access controls to restrict access to authenticated endpoints from trusted IP ranges only
• Deploy a reverse proxy or WAF in front of Apache to filter and validate authentication headers before they reach the server

# Configuration example
# Disable mod_auth_digest in Apache configuration
# Edit /etc/httpd/conf/httpd.conf or /etc/apache2/apache2.conf

# Comment out or remove the following line:
# LoadModule auth_digest_module modules/mod_auth_digest.so

# Alternative: Switch to Basic Auth with HTTPS
<Directory "/var/www/protected">
    AuthType Basic
    AuthName "Restricted Area"
    AuthUserFile /etc/apache2/.htpasswd
    Require valid-user
</Directory>

# Restart Apache to apply changes
# systemctl restart httpd
# or
# systemctl restart apache2