CVE-2022-25313: Libexpat DOS Vulnerability

CVE-2022-25313 Overview

CVE-2022-25313 is a stack exhaustion vulnerability affecting Expat (libexpat), a widely-used XML parsing library. In versions prior to 2.4.5, an attacker can trigger stack exhaustion in the build_model function by crafting a malicious XML document with excessive nesting depth in the DTD (Document Type Definition) element. This vulnerability can be exploited remotely to cause a denial of service condition, crashing applications that rely on libexpat for XML parsing.

Critical Impact

Attackers can cause denial of service by sending specially crafted XML documents with deeply nested DTD elements, leading to stack exhaustion and application crashes.

Affected Products

• libexpat_project libexpat (versions prior to 2.4.5)
• Debian Linux 10.0 and 11.0
• Fedora 34 and 35
• Oracle HTTP Server 12.2.1.3.0 and 12.2.1.4.0
• Oracle ZFS Storage Appliance Kit 8.8
• Siemens SINEMA Remote Connect Server

Discovery Timeline

• 2022-02-18 - CVE-2022-25313 published to NVD
• 2025-05-30 - Last updated in NVD database

Technical Details for CVE-2022-25313

Vulnerability Analysis

This vulnerability falls under CWE-674 (Uncontrolled Recursion), which occurs when a product does not properly control the amount of recursion that takes place, consuming excessive resources such as stack memory. The build_model function in libexpat processes DTD element declarations recursively. When parsing an XML document containing a DTD with deeply nested content particle definitions, each level of nesting adds a new frame to the call stack. Without proper depth limits, an attacker-controlled XML document can exhaust the available stack space, resulting in a crash.

The attack is network-exploitable and requires user interaction (such as opening a malicious XML file or processing untrusted XML input). While the vulnerability does not allow code execution or data theft, it poses a significant availability risk to any application or service that processes XML data using affected versions of libexpat.

Root Cause

The root cause lies in the recursive nature of the build_model function, which parses content model specifications in DTD element declarations. The function calls itself for each level of nesting in element content particles (sequences and choices). Prior to version 2.4.5, there was no mechanism to limit recursion depth, allowing attackers to craft XML documents with arbitrary nesting levels that exceed the system's stack capacity.

Attack Vector

The attack vector is network-based, requiring an attacker to deliver a malicious XML document to a vulnerable application. This could occur through:

• Web applications accepting XML uploads or API payloads
• Email clients or services parsing XML attachments
• Document processing systems handling untrusted XML content
• Any network service using libexpat to parse user-supplied XML data

The attacker constructs an XML document with a DTD containing deeply nested element declarations. When the vulnerable application parses this document, the recursive build_model function exhausts the stack, causing the application to crash. The attack does not require authentication and can be executed remotely, though it does require user interaction to process the malicious input.

Detection Methods for CVE-2022-25313

Indicators of Compromise

• Application crashes or segmentation faults during XML parsing operations
• Error logs indicating stack overflow or stack exhaustion conditions
• Presence of XML files with unusually deep DTD element nesting structures
• Repeated crashes when processing specific XML documents from external sources

Detection Strategies

• Monitor application logs for crash reports associated with XML parsing components
• Implement file integrity monitoring to detect suspicious XML files with abnormal structures
• Use memory monitoring tools to detect stack consumption anomalies during XML processing
• Deploy intrusion detection rules to identify XML payloads with excessive DTD nesting

Monitoring Recommendations

• Enable core dump analysis for applications using libexpat to identify crash patterns
• Set up alerting for repeated application restarts that may indicate DoS attacks
• Monitor network traffic for unusually large or complex XML documents being submitted to services
• Track libexpat version deployments across infrastructure to identify vulnerable instances

How to Mitigate CVE-2022-25313

Immediate Actions Required

• Upgrade libexpat to version 2.4.5 or later immediately across all affected systems
• Identify all applications and services that depend on libexpat for XML parsing
• Review and update vendor-specific components including Oracle HTTP Server, Siemens SINEMA Remote Connect Server, and Oracle ZFS Storage Appliance Kit
• Apply operating system patches from Debian, Fedora, and other distribution vendors

Patch Information

The vulnerability has been addressed in libexpat version 2.4.5. The fix implements depth limiting in the build_model function to prevent excessive recursion. Multiple vendors have released security updates addressing this vulnerability:

• Libexpat Project: Upgrade to version 2.4.5 or later via the GitHub Pull Request for Expat
• Oracle: Patches available in the Oracle Security Alert CPUAPR2022
• Debian: Security updates detailed in Debian DSA-5085 Security Advisory and Debian LTS Announcement
• Siemens: Patches documented in Siemens Security Patch SSA-484086
• Gentoo: Updates available via Gentoo GLSA 202209-24
• NetApp: Advisory available at NetApp Security Advisory NTAP-20220303-0008

Workarounds

• Implement input validation to reject XML documents with excessive DTD complexity before parsing
• Configure XML parsers to disable DTD processing entirely if not required for application functionality
• Deploy web application firewalls (WAF) with rules to detect and block malformed XML payloads
• Isolate XML parsing operations in sandboxed environments with limited stack resources to contain potential crashes

# Configuration example
# Check current libexpat version on Linux systems
dpkg -l | grep libexpat  # Debian/Ubuntu
rpm -qa | grep expat      # RHEL/Fedora

# Update libexpat to patched version
apt-get update && apt-get install --only-upgrade libexpat1  # Debian/Ubuntu
dnf update expat  # Fedora
yum update expat  # RHEL/CentOS