CVE-2022-24070: Apache Subversion Use-After-Free Flaw

CVE-2022-24070 Overview

Subversion's mod_dav_svn is vulnerable to memory corruption due to a use-after-free condition. While looking up path-based authorization rules, mod_dav_svn servers may attempt to use memory which has already been freed. This vulnerability affects Subversion mod_dav_svn servers versions 1.10.0 through 1.14.1 (inclusive). Servers that do not use mod_dav_svn are not affected by this vulnerability.

Critical Impact

This use-after-free vulnerability in Apache Subversion's mod_dav_svn module can be exploited remotely over the network without authentication, potentially causing denial of service conditions on affected Subversion servers.

Affected Products

• Apache Subversion 1.10.0 through 1.14.1
• Debian Linux 10.0 and 11.0
• Fedora 35 and 36
• Apple macOS (bundled Subversion)

Discovery Timeline

• 2022-04-12 - CVE-2022-24070 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-24070

Vulnerability Analysis

The vulnerability exists within the mod_dav_svn module, which provides WebDAV/DeltaV protocol support for Apache Subversion repositories served over HTTP/HTTPS. The core issue is a use-after-free (CWE-416) memory corruption vulnerability that occurs during the path-based authorization (authz) rule lookup process.

When mod_dav_svn processes requests requiring path-based access control evaluation, it allocates memory structures to handle authorization decisions. Under certain conditions, the module incorrectly references memory that has already been deallocated, leading to undefined behavior. This can result in server crashes or unpredictable behavior, effectively denying service to legitimate users attempting to access Subversion repositories.

The vulnerability is particularly concerning because it can be triggered remotely over the network without requiring authentication, making it accessible to unauthenticated attackers who can reach the affected Subversion server.

Root Cause

The root cause is a use-after-free memory management error in the mod_dav_svn module's path-based authorization code. During authz rule processing, memory containing authorization context or rule data is freed prematurely while still being referenced by other parts of the code. When subsequent operations attempt to access this freed memory, the server encounters corrupted or invalid data, leading to crashes or unpredictable behavior.

Attack Vector

The attack vector is network-based, allowing remote exploitation without user interaction or authentication. An attacker can send specially crafted HTTP/HTTPS requests to a vulnerable mod_dav_svn server that trigger the path-based authorization lookup code path. By manipulating request parameters or repository paths, the attacker can cause the server to enter a state where the use-after-free condition is triggered.

The vulnerability specifically requires:

• Target server running Apache HTTP Server with mod_dav_svn module enabled
• Subversion versions 1.10.0 through 1.14.1
• Path-based authorization (authz) configured for the repository

Servers not using mod_dav_svn (such as those using svnserve directly) are not affected.

Detection Methods for CVE-2022-24070

Indicators of Compromise

• Unexpected Apache HTTP Server crashes or segmentation faults in mod_dav_svn module
• Error log entries indicating memory access violations during Subversion repository operations
• Repeated service restarts of Apache processes handling Subversion requests
• Core dump files generated by httpd processes with mod_dav_svn in the stack trace

Detection Strategies

• Monitor Apache error logs for segmentation fault messages related to mod_dav_svn processing
• Implement application-level monitoring to detect abnormal crash frequencies on Subversion servers
• Deploy intrusion detection rules to identify unusual patterns of HTTP requests targeting SVN repositories
• Use memory debugging tools (such as Valgrind or AddressSanitizer) in development environments to detect use-after-free conditions

Monitoring Recommendations

• Configure automated alerting for Apache child process crashes on servers running mod_dav_svn
• Implement log aggregation to correlate crash events with incoming request patterns
• Monitor server availability metrics for Subversion repositories to detect denial of service conditions
• Enable core dump collection and analysis for post-incident forensic investigation

How to Mitigate CVE-2022-24070

Immediate Actions Required

• Upgrade Apache Subversion to version 1.14.2 or later to address this vulnerability
• If immediate patching is not possible, consider temporarily disabling mod_dav_svn and using alternative access methods
• Review server logs for signs of exploitation attempts targeting Subversion repositories
• Implement network-level access controls to limit exposure of Subversion servers to trusted networks

Patch Information

Apache has released patched versions of Subversion that address this vulnerability. Users should upgrade to Subversion 1.14.2 or later. Multiple Linux distributions have also released security updates:

• Debian Security Advisory DSA-5119 provides patched packages for Debian 10 and 11
• Fedora Package Announcements include updates for Fedora 35 and 36
• Apple Support Security Update HT213345 addresses the vulnerability in macOS

Additional technical details are available in the Apache Bugzilla Report and Apache JIRA Issue SVN-4880.

Workarounds

• Restrict network access to Subversion servers using firewall rules or VPN requirements
• Disable mod_dav_svn and use svnserve with SSH tunneling as an alternative access method
• Implement rate limiting on the web server to reduce the impact of potential exploitation attempts
• Configure Apache to run in a sandboxed environment to limit the impact of crashes

# Configuration example: Disable mod_dav_svn temporarily
# In Apache configuration (httpd.conf or apache2.conf)
# Comment out or remove the LoadModule directive:
# LoadModule dav_svn_module modules/mod_dav_svn.so

# Alternative: Restrict access to trusted networks only
<Location /svn>
    Require ip 10.0.0.0/8 192.168.0.0/16
</Location>