CVE-2026-24515: libexpat XXE Vulnerability Exposure

CVE-2026-24515 Overview

A vulnerability has been identified in libexpat before version 2.7.4 where the XML_ExternalEntityParserCreate function fails to properly copy unknown encoding handler user data. This flaw relates to CWE-476 (NULL Pointer Dereference) and could lead to application crashes or unexpected behavior when processing XML documents with external entities and custom encoding handlers.

Critical Impact

Applications using libexpat with custom encoding handlers may experience denial of service conditions due to improper handling of user data during external entity parser creation.

Affected Products

• libexpat versions prior to 2.7.4
• Applications and libraries that depend on vulnerable libexpat versions

Discovery Timeline

• 2026-01-23 - CVE-2026-24515 published to NVD
• 2026-01-26 - Last updated in NVD database

Technical Details for CVE-2026-24515

Vulnerability Analysis

This vulnerability stems from an implementation flaw in the XML_ExternalEntityParserCreate function within libexpat. When an application registers a custom unknown encoding handler with associated user data, the parser fails to properly copy this user data when creating child parsers for external entities. This can result in a NULL pointer dereference when the child parser attempts to access the encoding handler's user data.

The vulnerability is classified under CWE-476 (NULL Pointer Dereference), indicating that the code dereferences a pointer that it expects to be valid but is actually NULL. In the context of XML parsing, this occurs during the processing of external entities when custom encoding handlers are in use.

Root Cause

The root cause lies in the incomplete initialization logic within XML_ExternalEntityParserCreate. When a parent parser has a registered unknown encoding handler with user data, the function creates a new parser instance for handling external entities but neglects to copy the user data pointer to the child parser. This oversight means the child parser's encoding handler operates with an uninitialized or NULL user data pointer, leading to potential NULL pointer dereference when the handler is invoked.

Attack Vector

The attack vector is local, requiring an attacker to either craft a malicious XML document or influence the input processed by a vulnerable application. The attack scenario involves:

1. An application using libexpat registers a custom unknown encoding handler with user data
2. The application processes an XML document containing external entity references
3. When libexpat creates child parsers for external entities, the user data is not properly copied
4. The encoding handler in the child parser context may dereference a NULL or invalid pointer, causing a crash

The exploitation requires specific conditions: the target application must use custom encoding handlers with user data, and the XML input must trigger external entity parsing. These prerequisites contribute to the low severity rating.

The vulnerability mechanism involves the incomplete state propagation between parent and child parser instances. When XML_ExternalEntityParserCreate is called, it should replicate all relevant configuration from the parent parser, including encoding handler user data. The fix involves ensuring this data is properly copied during child parser initialization. Technical details can be found in the GitHub Pull Request for libexpat.

Detection Methods for CVE-2026-24515

Indicators of Compromise

• Application crashes or segmentation faults during XML parsing operations
• Error logs indicating NULL pointer access in libexpat-related functions
• Unexpected termination of services processing XML documents with external entities

Detection Strategies

• Monitor application logs for segmentation faults or access violations originating from libexpat functions
• Implement crash dump analysis to identify NULL pointer dereferences in XML parsing code paths
• Use static analysis tools to identify applications using vulnerable libexpat versions with custom encoding handlers

Monitoring Recommendations

• Enable core dump collection for applications that process untrusted XML input
• Monitor system stability metrics for applications known to use libexpat with custom encoding handlers
• Implement application-level health checks that can detect repeated crashes in XML processing components

How to Mitigate CVE-2026-24515

Immediate Actions Required

• Upgrade libexpat to version 2.7.4 or later
• Identify all applications and dependencies that use libexpat and prioritize updates
• If immediate upgrade is not possible, avoid using custom unknown encoding handlers until patched

Patch Information

The vulnerability is addressed in libexpat version 2.7.4. The fix ensures that unknown encoding handler user data is properly copied when creating external entity parsers. Details of the patch implementation are available in the GitHub Pull Request for libexpat.

Workarounds

• Disable external entity processing if not required by the application
• Avoid registering custom unknown encoding handlers with user data until the library is updated
• Implement input validation to restrict or sanitize XML documents containing external entity references
• Consider using XML parsing in sandboxed environments to limit impact of potential crashes

# Configuration example
# Check installed libexpat version
pkg-config --modversion expat

# On Debian/Ubuntu systems, update libexpat
sudo apt-get update && sudo apt-get install --only-upgrade libexpat1

# On RHEL/CentOS systems
sudo yum update expat

# Verify the update
pkg-config --modversion expat
# Should show 2.7.4 or higher