CVE-2026-41080: libexpat XXE Vulnerability

CVE-2026-41080 Overview

CVE-2026-41080 is an Insecure Random Number Generation vulnerability affecting libexpat, a widely-used XML parsing library. The vulnerability exists in versions prior to 2.7.6 where insufficient entropy is used in hash table operations. This weakness allows attackers to craft malicious XML documents that trigger hash flooding conditions, potentially leading to denial of service through algorithmic complexity attacks.

Critical Impact

Attackers can exploit insufficient entropy in libexpat's hash functions to cause denial of service conditions through crafted XML documents that trigger hash collisions.

Affected Products

• libexpat versions prior to 2.7.6
• Applications and systems utilizing vulnerable libexpat libraries for XML parsing
• Software dependencies that bundle or link against affected libexpat versions

Discovery Timeline

• 2026-04-16 - CVE-2026-41080 published to NVD
• 2026-04-16 - Last updated in NVD database

Technical Details for CVE-2026-41080

Vulnerability Analysis

This vulnerability stems from insufficient entropy in libexpat's hash table implementation (CWE-331: Insufficient Entropy). Hash tables are fundamental data structures used throughout XML parsing operations to efficiently store and retrieve element names, attribute names, and namespace prefixes. When the hash function lacks sufficient randomness, attackers can predict hash values and craft inputs that deliberately cause collisions.

The local attack vector requires the attacker to have the ability to supply crafted XML documents to an application using the vulnerable library. While user interaction is not required, the attack complexity is considered high as it requires knowledge of the hash function behavior and careful construction of collision-inducing XML content. The primary impact is availability degradation, as successful exploitation causes excessive CPU consumption during XML parsing operations.

Root Cause

The root cause is CWE-331 (Insufficient Entropy) in libexpat's hash table implementation. The library uses predictable or poorly randomized seed values for its hash functions, making it possible for attackers to calculate inputs that will hash to the same bucket. Without sufficient entropy, the hash function's output becomes predictable, undermining the fundamental assumption that hash tables provide O(1) average-case lookup time.

Attack Vector

The attack vector involves submitting specially crafted XML documents to applications that parse XML using vulnerable versions of libexpat. By constructing XML content where element names, attribute names, or other parsed values all hash to the same bucket, an attacker forces the hash table into worst-case O(n) behavior for each lookup operation.

The vulnerability manifests when libexpat processes XML documents containing carefully chosen strings that produce hash collisions. This transforms normal O(n) parsing operations into O(n²) complexity, causing significant CPU resource consumption. For technical details on the vulnerability mechanism and proposed fixes, see the GitHub Issue #47 and GitHub Pull Request #1183.

Detection Methods for CVE-2026-41080

Indicators of Compromise

• Abnormal CPU utilization spikes during XML parsing operations
• Unusually long processing times for XML documents of normal size
• Application hangs or timeouts when processing XML input from untrusted sources
• Memory pressure increases in applications processing XML content

Detection Strategies

• Monitor for applications using libexpat versions prior to 2.7.6 in your software inventory
• Implement runtime performance monitoring for XML parsing operations to detect anomalous processing times
• Deploy application-level logging to track XML parsing durations and identify outliers
• Use software composition analysis (SCA) tools to identify vulnerable libexpat dependencies

Monitoring Recommendations

• Establish baseline metrics for XML parsing performance and alert on significant deviations
• Configure resource limits and timeouts for XML parsing operations to contain potential attacks
• Monitor system logs for application crashes or restarts related to XML processing components
• Track CPU utilization patterns for services that process XML from external sources

How to Mitigate CVE-2026-41080

Immediate Actions Required

• Upgrade libexpat to version 2.7.6 or later to obtain the entropy improvements
• Audit applications and dependencies to identify all instances of vulnerable libexpat versions
• Implement input validation and size limits for XML documents from untrusted sources
• Consider deploying application-level rate limiting for XML parsing endpoints

Patch Information

The vulnerability has been addressed in libexpat version 2.7.6, which implements improved entropy for hash table operations. The fix is documented in GitHub Pull Request #1183. Organizations should update to the patched version through their package manager or by rebuilding applications with the updated library.

Workarounds

• Implement strict input validation and reject unusually large or complex XML documents
• Configure XML parsing timeouts to terminate processing of documents that exceed expected durations
• Deploy resource limits (CPU time, memory) for processes that handle untrusted XML input
• Consider using XML schema validation to reject documents with unexpected structures before parsing

# Verify libexpat version on Linux systems
expat -v
# Or check shared library version
ldconfig -p | grep expat

# Update libexpat via package manager (Debian/Ubuntu)
sudo apt update && sudo apt upgrade libexpat1

# Update libexpat via package manager (RHEL/CentOS)
sudo yum update expat