Executive Summary

CVE-2026-56132 is a security vulnerability in the libexpat library, specifically in the doProlog function within xmlparse.c. It is a heap-based buffer overflow caused by improper handling of a buffer called scaffIndex when multiple parsers share data structures. The issue arises because the variable m_groupSize, which controls the size of the scaffIndex buffer, is reused incorrectly when sub-parsers with shallower nesting levels reallocate this shared buffer. This leads to the parent parser continuing with a buffer that is too small, causing out-of-bounds writes beyond the allocated memory.

The vulnerability occurs due to mishandling of scaffold backing array reallocation when data-structure sharing happens across parsers, resulting in memory corruption. A fix has been proposed that removes the reuse of m_groupSize for scaffIndex allocation to ensure each parser independently manages its buffer size, preventing buffer overflows.

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Impact Analysis

This vulnerability can lead to a heap-based buffer overflow, which may cause memory corruption. Such memory corruption can result in application crashes, denial of service, or potentially allow an attacker to execute arbitrary code with the privileges of the affected application.

Because the vulnerability involves out-of-bounds writes in a widely used XML parsing library, any software using vulnerable versions of libexpat before 2.8.2 could be at risk if it processes untrusted XML data.

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Detection Guidance

There is no specific information provided about detection methods or commands to identify this vulnerability on your network or system.

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Mitigation Strategies

To mitigate this vulnerability, you should update libexpat to version 2.8.2 or later, where the issue has been fixed by removing the reuse of the m_groupSize variable for scaffIndex allocation, preventing the heap-based buffer overflow.

Applying the patch from the approved pull request #1272 is recommended if you cannot immediately upgrade to the fixed version.

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