CVE-2024-40896: Xmlsoft Libxml2 XXE Vulnerability

CVE-2024-40896 Overview

CVE-2024-40896 is a critical XML External Entity (XXE) vulnerability discovered in libxml2, a widely-used XML parsing library. The vulnerability exists in the SAX (Simple API for XML) parser component, which can produce events for external entities even when custom SAX handlers attempt to override entity content by setting the "checked" flag. This behavior allows attackers to execute classic XXE attacks, potentially leading to unauthorized data access, server-side request forgery, and denial of service conditions.

The vulnerability affects multiple versions of libxml2 across the 2.11.x, 2.12.x, and 2.13.x branches, impacting a wide range of applications and systems that depend on this fundamental XML processing library.

Critical Impact

This XXE vulnerability enables attackers to read arbitrary files from the server, conduct internal network scanning via SSRF, and potentially cause denial of service conditions on affected systems running vulnerable versions of libxml2.

Affected Products

• xmlsoft libxml2 versions 2.11 before 2.11.9
• xmlsoft libxml2 versions 2.12 before 2.12.9
• xmlsoft libxml2 versions 2.13 before 2.13.3
• NetApp HCI Compute Node
• NetApp SolidFire & HCI Management Node
• NetApp SolidFire & HCI Storage Node
• NetApp H300s, H410s, H500s, H700s, H410c (firmware and hardware)

Discovery Timeline

• 2024-12-23 - CVE-2024-40896 published to NVD
• 2025-11-25 - Last updated in NVD database

Technical Details for CVE-2024-40896

Vulnerability Analysis

The vulnerability resides in libxml2's SAX parser implementation, which is designed to provide event-driven XML parsing. In properly secured configurations, application developers can implement custom SAX handlers to control how external entities are processed, including the ability to override entity content by setting the "checked" flag. However, due to a flaw in the parser logic, the SAX parser continues to produce events for external entities regardless of whether custom handlers have attempted to neutralize them.

This means that even when developers believe they have implemented proper XXE protections through custom SAX handlers, malicious XML documents containing external entity declarations can still trigger entity resolution. The parser's failure to respect the "checked" flag essentially bypasses application-level security controls, making traditional XXE attack patterns viable against applications that would otherwise be protected.

The impact of successful exploitation includes the ability to exfiltrate sensitive files from the server's filesystem, perform server-side request forgery (SSRF) attacks against internal services, and potentially cause denial of service through recursive entity expansion or by referencing external resources that cause processing delays.

Root Cause

The root cause of CVE-2024-40896 lies in improper handling of the entity resolution mechanism within the SAX parser. Specifically, when custom SAX handlers set the "checked" flag to indicate that entity content has been overridden, the parser fails to honor this setting and continues to process external entity declarations. This represents a CWE-611 (Improper Restriction of XML External Entity Reference) vulnerability where the parser does not adequately restrict the processing of external entities despite application-level attempts to do so.

Attack Vector

The attack vector for this vulnerability is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by sending a crafted XML document containing external entity declarations to an application that uses a vulnerable version of libxml2 for XML parsing. The malicious XML document would define external entities pointing to local files (for data exfiltration), internal network services (for SSRF), or recursive entity definitions (for denial of service).

The vulnerability is particularly dangerous because applications that have implemented XXE protections using custom SAX handlers may incorrectly assume they are secure. When the application processes the malicious XML using the vulnerable libxml2 library, the SAX parser generates events for the external entities despite the handler's attempts to override them, allowing the attack to succeed.

For detailed technical information about this vulnerability, refer to the GNOME libxml2 Issue tracker and the GNOME libxml2 Commit that addresses this flaw.

Detection Methods for CVE-2024-40896

Indicators of Compromise

• Unusual file access attempts to sensitive system files such as /etc/passwd, /etc/shadow, or application configuration files
• Outbound network connections from XML processing components to unexpected internal or external destinations
• XML parsing errors or exceptions indicating failed entity resolution attempts
• Log entries showing requests containing XML with <!DOCTYPE> declarations or entity references (&entity;)

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block XML documents containing <!ENTITY declarations or DOCTYPE definitions
• Monitor application logs for XML parsing exceptions related to external entity resolution or file access
• Deploy network monitoring to identify unexpected outbound connections originating from XML processing services
• Use SentinelOne's behavioral AI to detect anomalous file access patterns from processes that typically handle XML data

Monitoring Recommendations

• Enable detailed logging for all XML processing components and monitor for entity-related events
• Configure alerts for any attempts to access sensitive system files from application contexts
• Monitor network traffic for DNS queries or HTTP requests that may indicate SSRF attempts through XXE exploitation
• Review application dependencies regularly to identify instances of vulnerable libxml2 versions

How to Mitigate CVE-2024-40896

Immediate Actions Required

• Inventory all systems and applications using libxml2 and identify vulnerable versions (2.11.x < 2.11.9, 2.12.x < 2.12.9, 2.13.x < 2.13.3)
• Prioritize patching internet-facing applications and services that process XML from untrusted sources
• Implement network segmentation to limit the potential impact of SSRF attacks if exploitation occurs
• Review and restrict file system permissions for processes that handle XML parsing

Patch Information

The libxml2 maintainers have released patched versions that properly respect custom SAX handler overrides for external entity content. Organizations should upgrade to the following versions or later:

• libxml2 2.11.9 or later for the 2.11.x branch
• libxml2 2.12.9 or later for the 2.12.x branch
• libxml2 2.13.3 or later for the 2.13.x branch

The fix is documented in the GNOME libxml2 Commit. NetApp customers should refer to the NetApp Security Advisory for guidance on affected products and available updates.

Workarounds

• Disable external entity processing entirely at the libxml2 configuration level using XML_PARSE_NOENT and XML_PARSE_DTDLOAD flags
• Implement input validation to reject XML documents containing DOCTYPE declarations before they reach the parser
• Deploy a security proxy or WAF to filter malicious XML payloads before they reach vulnerable applications
• Consider using alternative XML parsing libraries with stronger default security configurations until patches can be applied

# Example: Checking libxml2 version on Linux systems
xml2-config --version

# Example: Finding applications linked against libxml2
ldd /path/to/application | grep libxml2

# Example: Package manager updates for Debian/Ubuntu
sudo apt update && sudo apt install --only-upgrade libxml2

# Example: Package manager updates for RHEL/CentOS
sudo yum update libxml2