CVE-2025-6021: Xmlsoft Libxml2 Buffer Overflow Vulnerability

CVE-2025-6021 Overview

A flaw was found in libxml2's xmlBuildQName function, where integer overflows in buffer size calculations can lead to a stack-based buffer overflow. This vulnerability affects the widely-used XML parsing library that is a core dependency in countless applications across Linux distributions, containerized environments, and enterprise platforms. When processing crafted XML input, the vulnerability can result in memory corruption or denial of service conditions.

Critical Impact

This stack-based buffer overflow vulnerability enables remote attackers to cause denial of service through memory corruption when vulnerable systems parse maliciously crafted XML documents, with potential for broader exploitation depending on application context.

Affected Products

• xmlsoft libxml2 (all versions prior to patch)
• Red Hat Enterprise Linux 7, 8, 9, and 10 (including EUS, AUS, and TUS variants)
• Red Hat OpenShift Container Platform 4.12 through 4.18 (including ARM64, IBM Z, LinuxONE, and Power variants)
• Red Hat JBoss Core Services
• Red Hat In-Vehicle Operating System 1.0

Discovery Timeline

• June 12, 2025 - CVE-2025-6021 published to NVD
• November 29, 2025 - Last updated in NVD database

Technical Details for CVE-2025-6021

Vulnerability Analysis

The vulnerability resides in libxml2's xmlBuildQName function, which is responsible for constructing qualified names during XML parsing operations. The function performs buffer size calculations that are susceptible to integer overflow conditions. When processing specially crafted XML input with excessively long namespace prefixes or local names, the arithmetic operations used to determine the required buffer size can wrap around, resulting in an undersized buffer allocation on the stack.

Once the undersized buffer is allocated, subsequent write operations overflow the stack boundary, corrupting adjacent memory regions. This memory corruption can manifest as application crashes (denial of service) or potentially enable more sophisticated attacks depending on how the calling application handles the corrupted data structures.

The network-accessible nature of this vulnerability is particularly concerning given libxml2's ubiquitous presence in web servers, API services, document processors, and container orchestration platforms that routinely parse XML from untrusted sources.

Root Cause

The root cause is an integer overflow vulnerability (CWE-190) in the buffer size calculation logic within xmlBuildQName. When computing the total buffer size needed to store a qualified name (combining prefix and local name), the function fails to properly validate that the arithmetic result does not exceed integer bounds before allocation. This leads to a stack-based buffer overflow (CWE-121) when the calculated size wraps to a small value, and the function subsequently writes data beyond the allocated buffer.

Attack Vector

An attacker can exploit this vulnerability by supplying a maliciously crafted XML document to any application that uses libxml2 for parsing. The attack vector is network-based, requiring no authentication or user interaction. The attacker constructs an XML document with specially chosen namespace prefix and local name lengths that trigger the integer overflow during qualified name construction.

The exploitation flow involves:

1. Attacker sends a crafted XML document to a vulnerable application endpoint
2. The application invokes libxml2 to parse the XML content
3. During parsing, xmlBuildQName calculates buffer size with values that cause integer overflow
4. A stack buffer smaller than required is allocated
5. Data is written past the buffer boundary, corrupting stack memory
6. The application crashes or exhibits undefined behavior due to memory corruption

Detection Methods for CVE-2025-6021

Indicators of Compromise

• Application crashes or segmentation faults in processes using libxml2 during XML parsing operations
• Core dumps showing stack corruption in libxml2-related function call chains
• Abnormal termination of services that process XML input (web servers, API gateways, document converters)
• System log entries indicating buffer overflows or memory access violations in XML processing contexts

Detection Strategies

• Monitor for unexpected process terminations in applications known to use libxml2 for XML parsing
• Implement XML input validation to reject documents with abnormally long namespace prefixes or element names before passing to libxml2
• Deploy intrusion detection rules to identify XML payloads with suspiciously long qualified names targeting the overflow condition
• Use memory-safe tooling (AddressSanitizer, Valgrind) in testing environments to detect buffer overflows during XML parsing

Monitoring Recommendations

• Enable crash reporting and monitoring for all services that process XML from untrusted sources
• Configure application-level logging to capture XML parsing errors and exceptions
• Implement rate limiting on XML processing endpoints to reduce potential denial of service impact
• Monitor for unusual patterns in XML document characteristics (excessively long element or attribute names)

How to Mitigate CVE-2025-6021

Immediate Actions Required

• Inventory all systems and applications using libxml2 and prioritize patching based on exposure to untrusted XML input
• Apply vendor-provided security patches from Red Hat, Debian, or the upstream libxml2 project immediately
• For containerized environments, rebuild container images with patched libxml2 packages
• Review and restrict XML processing endpoints to authenticated users where possible

Patch Information

Multiple vendors have released security patches addressing this vulnerability. For Red Hat systems, security updates are available through the following advisories:

• RHSA-2025:10630 and related advisories for Enterprise Linux
• OpenShift Container Platform updates through RHSA-2025:12098 and subsequent advisories
• Debian LTS security announcement for Debian-based distributions

For upstream information, refer to the GNOME GitLab Issue #926 tracking this vulnerability.

Workarounds

• Implement input validation to reject XML documents with namespace prefixes or local names exceeding reasonable length thresholds
• Deploy a web application firewall (WAF) with rules to filter XML requests containing abnormally long element names
• Consider using alternative XML parsing libraries with stronger integer overflow protections for critical applications if immediate patching is not possible
• Isolate XML processing services in sandboxed environments to limit the impact of potential exploitation

# Check installed libxml2 version on RHEL/CentOS
rpm -qa | grep libxml2

# Check installed libxml2 version on Debian/Ubuntu
dpkg -l | grep libxml2

# Update libxml2 on RHEL/CentOS
sudo yum update libxml2

# Update libxml2 on Debian/Ubuntu
sudo apt-get update && sudo apt-get upgrade libxml2