Executive Summary

CVE-2026-6653 is a use-after-free vulnerability in the libxml2 library, specifically in the xmlParseInternalSubset function. It affects versions from 2.9.11 up to 2.11.0. This flaw allows a remote attacker to cause memory corruption by processing maliciously crafted XML input that improperly handles entity resolution.

The vulnerability involves accessing memory that has already been freed (heap-use-after-free), which can lead to unexpected behavior or crashes. It was discovered during research on distributed fuzzing techniques and was silently fixed in version 2.11.0.

Useful 0 Not Useful 0

Impact Analysis

This vulnerability can be exploited remotely by an attacker who sends specially crafted XML data to an application using the affected libxml2 versions. The impact is primarily a denial-of-service condition caused by memory corruption, which can crash the application or system processing the XML.

Because the vulnerability involves unauthorized memory access, it could potentially be leveraged for more severe attacks, but the primary confirmed impact is denial-of-service.

Useful 0 Not Useful 0

Detection Guidance

This vulnerability involves a heap-use-after-free in the xmlParseInternalSubset function of libxml2 when processing malformed XML input. Detection can involve analyzing XML inputs that trigger memory corruption or crashes in libxml2 versions 2.9.11 to 2.11.0.

A proof-of-concept (PoC) exists that demonstrates the issue, which can be used to test if your system is vulnerable by feeding crafted XML inputs to applications using libxml2.

Since the vulnerability is related to malformed XML processing, you can attempt to detect it by running fuzzing tools or test scripts that send malicious XML payloads to services or applications using libxml2.

No specific commands are provided in the available resources, but you can check your libxml2 version with commands like:

• dpkg -l | grep libxml2
• xml2-config --version

If your version is between 2.9.11 and 2.11.0 (exclusive), your system is potentially vulnerable.

Useful 0 Not Useful 0

Mitigation Strategies

The vulnerability was silently fixed in libxml2 version 2.11.0. Therefore, the immediate mitigation step is to upgrade libxml2 to version 2.11.0 or later.

If upgrading is not immediately possible, consider restricting or sanitizing XML inputs to applications using libxml2 to prevent processing of maliciously crafted XML that could trigger the vulnerability.

Additionally, monitor for updates or patches from your operating system vendor, such as Ubuntu, which has addressed this issue in libxml2 version 2.9.14+dfsg-1.3ubuntu3.7 for Ubuntu 24.04.3 LTS.

Useful 0 Not Useful 0