Integer Overflow in Expat XML Parser

Executive Summary

The CVE-2026-56409 vulnerability is an integer overflow issue in the xmlwf tool of the libexpat library. It occurs when the -d outputDir option is used to specify an output directory. The code calculates the buffer size needed for the output filename by adding the lengths of the output directory and filename, but it does not properly check for integer overflow during this calculation. On systems with long directory or filenames, this can cause the size calculation to wrap around, resulting in an undersized buffer.

Because the buffer is too small, copying the directory and filename strings into it can overflow the heap buffer, potentially leading to memory corruption or other unexpected behavior. The issue was fixed by adding overflow protection checks to ensure safe memory allocation.

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Compliance Impact

The provided information does not specify any direct impact of the CVE-2026-56409 vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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Impact Analysis

This vulnerability can lead to heap-based buffer overflows when processing output filenames with long directory or file names. Such buffer overflows may cause memory corruption, crashes, or potentially allow an attacker to execute arbitrary code or cause denial of service.

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Detection Guidance

This vulnerability is related to an integer overflow in the xmlwf tool of the libexpat library when using the -d outputDir option, which can lead to heap-based buffer overflows. Detection would involve checking if your system is running a vulnerable version of libexpat (before 2.8.2) and if the xmlwf tool is used with the -d option.

Since the issue is in the construction of output file paths, direct network detection commands are not applicable. Instead, you can check the installed libexpat version with commands like:

• dpkg -l | grep libexpat (on Debian/Ubuntu systems)
• rpm -q libexpat (on RedHat/CentOS systems)

To detect if the vulnerable xmlwf tool is used with the -d option, you may review usage logs or scripts invoking xmlwf with output directory parameters.

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Mitigation Strategies

The immediate mitigation step is to upgrade libexpat to version 2.8.2 or later, where the integer overflow issue in xmlwf has been fixed.

If upgrading is not immediately possible, avoid using the xmlwf tool with the -d outputDir option, especially with long directory or file names, to prevent triggering the integer overflow.

Additionally, review and restrict usage of xmlwf in scripts or automated processes to minimize exposure.

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