CVE-2026-56409 in libexpat
Summary
by MITRE • 06/21/2026
xmlwf in libexpat before 2.8.2 has an integer overflow for the output filename when -d outputDir is used.
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Analysis
by VulDB Data Team • 06/21/2026
The xmlwf utility within the libexpat library presents a critical integer overflow vulnerability that manifests when processing XML documents with the -d outputDir command line option. This flaw exists in versions prior to 2.8.2 and represents a significant security risk that can be exploited to manipulate file system operations through crafted input data. The vulnerability specifically occurs during the construction of output filenames when the utility attempts to calculate buffer sizes for directory paths, leading to potential buffer overflows and arbitrary code execution.
The technical root cause of this vulnerability lies in the improper handling of integer arithmetic when determining the required buffer size for output filenames. When xmlwf processes XML documents with the -d option, it calculates the necessary storage space for the output file path by performing arithmetic operations on integer values that represent directory and filename lengths. Under certain conditions involving large or malformed input data, these arithmetic operations can overflow the integer data type, resulting in a buffer that is insufficiently sized to accommodate the intended output filename. This overflow condition creates a predictable memory layout that can be exploited by attackers to overwrite adjacent memory regions, potentially leading to privilege escalation or remote code execution.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as it can be leveraged for more sophisticated attacks within environments where xmlwf is executed with elevated privileges or in automated processing pipelines. Systems that utilize xmlwf for XML validation or processing tasks may become vulnerable to attacks where malicious XML input triggers the overflow condition, allowing adversaries to manipulate the output directory structure or inject malicious code into the processing workflow. The vulnerability affects any environment where libexpat is used for XML processing, particularly in web applications, content management systems, and automated XML parsing tools that may invoke xmlwf as part of their processing pipeline.
This vulnerability maps to CWE-190 Integer Overflow or Wraparound, which is a well-documented weakness in software security that occurs when an integer value exceeds its maximum representable value. The attack surface aligns with ATT&CK technique T1203 Exploitation for Client Execution, as the vulnerability can be exploited to execute arbitrary code through the manipulation of file system operations. Additionally, the vulnerability falls under ATT&CK tactic TA0004 Privilege Escalation, particularly through T1068 Exploitation for Privilege Escalation, when the vulnerable system processes XML files with elevated privileges. Organizations using affected versions of libexpat should implement immediate mitigations including upgrading to version 2.8.2 or later, implementing input validation controls, and restricting xmlwf execution in privileged contexts.
The integer overflow vulnerability in xmlwf represents a serious security concern that can be exploited to compromise system integrity through memory corruption attacks. The flaw demonstrates the importance of proper integer arithmetic validation in security-critical applications, particularly those that handle user-supplied data. Organizations should prioritize patching affected systems and implementing monitoring controls to detect potential exploitation attempts. The vulnerability also highlights the need for comprehensive security testing of XML processing libraries, including boundary condition testing and memory safety validation. Given the widespread use of libexpat in various applications and systems, the impact of this vulnerability extends across multiple industries and deployment scenarios, making prompt remediation essential for maintaining overall system security posture.