CVE-2024-45491: Libexpat Buffer Overflow Vulnerability

CVE-2024-45491 Overview

CVE-2024-45491 is an integer overflow vulnerability discovered in libexpat, a widely-used XML parsing library. The vulnerability exists in the dtdCopy function within xmlparse.c, where an integer overflow can occur for the nDefaultAtts variable on 32-bit platforms where UINT_MAX equals SIZE_MAX. This flaw could allow attackers to trigger memory corruption through specially crafted XML input, potentially leading to remote code execution or denial of service conditions.

Critical Impact

This integer overflow vulnerability affects 32-bit systems running libexpat versions prior to 2.6.3, potentially allowing attackers to corrupt memory through malicious XML documents and achieve arbitrary code execution.

Affected Products

• libexpat versions prior to 2.6.3
• 32-bit platforms running vulnerable libexpat versions
• Applications and services that depend on libexpat for XML parsing

Discovery Timeline

• 2024-08-30 - CVE-2024-45491 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2024-45491

Vulnerability Analysis

This vulnerability is classified as CWE-190 (Integer Overflow or Wraparound). The flaw resides in the dtdCopy function within xmlparse.c, which handles the copying of Document Type Definition (DTD) data during XML parsing operations.

On 32-bit platforms, the nDefaultAtts counter can overflow due to the platform-specific condition where UINT_MAX equals SIZE_MAX. When processing XML documents with a large number of default attributes, the integer overflow can cause the allocated buffer size to be smaller than expected, leading to subsequent heap-based buffer overflows when data is written beyond the allocated bounds.

The network-accessible attack vector means that any application accepting XML input from untrusted sources could be vulnerable. Since no user interaction or special privileges are required, this vulnerability presents a significant risk to systems processing untrusted XML content.

Root Cause

The root cause is insufficient validation of the nDefaultAtts counter in the dtdCopy function before memory allocation operations. On 32-bit platforms, the multiplication used to calculate buffer sizes can wrap around, resulting in a smaller allocation than required. This creates a classic integer overflow leading to heap corruption scenario.

The vulnerability specifically manifests when:

1. An XML document contains a DTD with numerous default attributes
2. The parser copies the DTD structure via dtdCopy
3. The nDefaultAtts value exceeds bounds that cause integer wraparound on 32-bit systems
4. Subsequent memory operations write beyond allocated buffer boundaries

Attack Vector

The attack vector is network-based, requiring an attacker to deliver a specially crafted XML document to a vulnerable application. The malicious XML would contain a DTD structure designed to trigger the integer overflow condition during parsing.

Exploitation requires no authentication or user interaction, making this vulnerability particularly dangerous for XML-processing services exposed to the network. The attacker would craft an XML document with an excessive number of default attributes in the DTD, causing the integer overflow when the parser attempts to copy the DTD structure.

The vulnerability mechanism centers on the dtdCopy function where the nDefaultAtts counter is used in memory allocation calculations. On 32-bit systems, when this value becomes sufficiently large, the multiplication wraps around to a small value, resulting in insufficient memory allocation. For detailed technical analysis, refer to GitHub Issue #888.

Detection Methods for CVE-2024-45491

Indicators of Compromise

• Unexpected crashes or segmentation faults in applications using libexpat for XML parsing
• Unusual memory allocation patterns or heap corruption events in XML processing services
• Application logs showing malformed or excessively large XML DTD structures being processed
• Core dumps or crash reports related to dtdCopy or xmlparse.c functions

Detection Strategies

• Monitor for abnormal XML document sizes, particularly those with unusually large DTD sections
• Implement file integrity monitoring on libexpat library files to detect unauthorized modifications
• Deploy endpoint detection solutions capable of identifying heap corruption exploitation attempts
• Utilize application-level logging to track XML parsing operations and identify anomalous patterns

Monitoring Recommendations

• Configure logging for XML parsing services to capture DTD-related operations
• Implement resource monitoring for 32-bit systems processing XML content to detect memory anomalies
• Enable crash reporting and analysis for applications dependent on libexpat
• Monitor network traffic for unusually large or malformed XML documents targeting vulnerable services

How to Mitigate CVE-2024-45491

Immediate Actions Required

• Update libexpat to version 2.6.3 or later immediately across all affected systems
• Identify all applications and services that depend on libexpat and prioritize their updates
• Consider migrating vulnerable 32-bit systems to 64-bit platforms where feasible
• Implement network-level filtering to limit XML input size to trusted sources while patches are applied

Patch Information

The vulnerability has been addressed in libexpat version 2.6.3. The fix is available through GitHub Pull Request #891. System administrators should update to version 2.6.3 or later to remediate this vulnerability.

Additional security advisories have been published by downstream vendors:

• Debian LTS Announcement
• NetApp Security Advisory NTAP-20241018-0003

Workarounds

• Limit XML input size at the application or network level to reduce exposure
• Implement strict input validation for XML documents before parsing
• Consider using alternative XML parsers if immediate patching is not possible
• Restrict XML parsing services to internal network segments until patches can be applied

Systems should be configured to limit resource consumption during XML parsing operations. For applications that cannot be immediately patched, consider implementing pre-processing validation to reject XML documents with excessively large DTD structures before they reach the vulnerable parser.