CVE-2024-45490: Libexpat XXE Vulnerability

CVE-2024-45490 Overview

An issue was discovered in libexpat before version 2.6.3 where the xmlparse.c file does not reject a negative length for XML_ParseBuffer. This input validation vulnerability allows attackers to potentially trigger unexpected behavior in applications that rely on the libexpat XML parsing library by supplying a negative length parameter.

Critical Impact

This vulnerability can be exploited remotely over the network without authentication to cause denial of service conditions in applications using vulnerable versions of libexpat.

Affected Products

• libexpat versions prior to 2.6.3
• Applications and systems that depend on the libexpat XML parsing library
• Various Linux distributions including Debian that ship with vulnerable libexpat versions

Discovery Timeline

• 2024-08-30 - CVE-2024-45490 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2024-45490

Vulnerability Analysis

This vulnerability exists in the xmlparse.c component of the libexpat XML parsing library. The core issue is an input validation flaw where the XML_ParseBuffer function fails to properly validate its length parameter, allowing negative values to be accepted. When a negative length is passed to this function, it can lead to integer overflow conditions or other undefined behavior that results in denial of service.

The vulnerability is classified under CWE-190 (Integer Overflow or Wraparound) and CWE-611 (Improper Restriction of XML External Entity Reference). The integer overflow weakness is particularly relevant here as the negative length parameter can wrap around to a very large positive value when interpreted as an unsigned integer internally, leading to memory access issues.

Root Cause

The root cause of CVE-2024-45490 is insufficient input validation in the XML_ParseBuffer function within xmlparse.c. The function does not include proper bounds checking to reject negative length values before processing. This oversight allows maliciously crafted input to bypass expected constraints and trigger integer overflow conditions.

Proper input validation should verify that the length parameter is non-negative before any further processing occurs. The absence of this check creates a pathway for attackers to manipulate buffer operations in unexpected ways.

Attack Vector

The attack vector for this vulnerability is network-based, requiring no privileges or user interaction to exploit. An attacker can remotely target applications that use libexpat for XML parsing by supplying specially crafted input that causes XML_ParseBuffer to be called with a negative length parameter.

The exploitation flow involves:

1. Identifying a target application that uses libexpat for XML parsing
2. Crafting malicious input that triggers a call to XML_ParseBuffer with a negative length
3. Sending the payload to the target over the network
4. The negative length causes integer overflow or unexpected behavior, resulting in denial of service

Technical details about the vulnerability mechanism can be found in the GitHub Issue #887 and the corresponding pull request #890 that addresses the fix.

Detection Methods for CVE-2024-45490

Indicators of Compromise

• Unexpected application crashes or restarts in services that utilize XML parsing functionality
• Abnormal memory consumption patterns in processes using libexpat
• Log entries indicating XML parsing failures or buffer-related errors
• Unusual network traffic patterns targeting XML processing endpoints

Detection Strategies

• Implement version scanning to identify systems running libexpat versions prior to 2.6.3
• Monitor application logs for XML parsing errors or unexpected buffer operation failures
• Deploy network intrusion detection signatures to identify potentially malicious XML payloads
• Use software composition analysis (SCA) tools to track libexpat dependencies across your infrastructure

Monitoring Recommendations

• Enable detailed logging for applications that process XML data from untrusted sources
• Configure alerting for abnormal crash rates in XML processing services
• Implement memory monitoring to detect potential exploitation attempts
• Review network traffic for anomalous patterns targeting XML endpoints

How to Mitigate CVE-2024-45490

Immediate Actions Required

• Upgrade libexpat to version 2.6.3 or later across all affected systems
• Inventory all applications and dependencies that rely on libexpat for XML parsing
• Prioritize patching for internet-facing services that process XML data
• Consider implementing network-level controls to restrict access to vulnerable services until patching is complete

Patch Information

The vulnerability has been addressed in libexpat version 2.6.3. The fix involves adding proper validation to reject negative length values in the XML_ParseBuffer function. The patch details are available in GitHub Pull Request #890.

For Debian-based systems, refer to the Debian LTS Announcement for distribution-specific update instructions. NetApp users should consult the NetApp Security Advisory for affected product information.

Workarounds

• Implement input validation at the application layer to sanitize XML data before passing to libexpat
• Deploy web application firewalls (WAF) with rules to inspect and filter malicious XML payloads
• Restrict network access to XML processing services to trusted sources only
• Consider using containerization or sandboxing for XML processing services to limit blast radius

# Verify installed libexpat version on Linux systems
dpkg -l | grep libexpat
# or
rpm -qa | grep expat

# Update libexpat on Debian/Ubuntu
sudo apt update && sudo apt upgrade libexpat1

# Update libexpat on RHEL/CentOS
sudo yum update expat