CVE-2026-118370 in Libexpat
Summary
by MITRE • 10/28/2026
libexpat through 2.9.0 contains a signed integer overflow vulnerability in storeAtts() in lib/xmlparse.c that allows attackers to trigger out-of-bounds writes via oversized attribute counts. Attackers can supply a multi-gigabyte document with a start tag carrying over 2^30 attributes to overflow attIndex and nPrefixes, crashing applications or corrupting heap memory.
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Analysis
by VulDB Data Team • 10/28/2026
The vulnerability identified in libexpat versions through 2.9.0 represents a critical security flaw rooted in improper handling of signed integer arithmetic within the XML parsing engine. Specifically, the defect resides in the storeAtts function located in the xmlparse.c source file. This component is responsible for managing attribute data structures during the processing of XML documents. The core technical issue arises when an attacker supplies a maliciously crafted XML document containing an exceptionally large number of attributes on a single start tag, specifically exceeding 2 to the power of 30 attributes. Under normal circumstances, libexpat expects reasonable limits on attribute counts per element. However, due to the use of signed integer types for tracking these counts and indices, the accumulation of such a massive quantity triggers a signed integer overflow. This arithmetic error causes internal variables like attIndex and nPrefixes to wrap around or assume incorrect negative values rather than continuing to increment correctly.
The operational consequence of this integer overflow is severe memory corruption within the application utilizing libexpat. When the internal indices are corrupted due to the overflow, subsequent array access operations proceed with invalid offsets. This leads directly to out-of-bounds writes into heap memory structures that were not allocated for those specific attribute data slots. The immediate impact includes the potential crash of the host application through segmentation faults or unhandled exceptions caused by accessing unmapped or protected memory regions. More critically, if the execution flow can be manipulated further, this heap corruption allows for arbitrary code execution. An attacker could potentially overwrite function pointers or other control structures within the heap to redirect program execution, thereby achieving remote code execution on systems that process untrusted XML input using vulnerable versions of libexpat.
From a classification perspective, this vulnerability aligns with CWE-190, which defines integer overflow or wraparound errors. The specific exploitation vector involves supplying oversized attribute counts to trigger the arithmetic error, leading to CWE-787, an out-of-bounds write condition. In terms of tactical mapping within the MITRE ATT&CK framework, this vulnerability facilitates initial access and privilege escalation techniques that rely on memory corruption exploits. It is particularly dangerous in environments where XML parsing is performed on untrusted data sources without strict input validation or size limits enforced prior to passing the document to the parser. The ability to crash applications makes it a viable denial-of-service vector, while the heap corruption potential elevates it to a high-severity remote code execution risk depending on the specific memory layout and exploitation capabilities of the attacker.
Mitigation strategies must focus primarily on upgrading libexpat to version 2.9.1 or later, where this integer overflow has been addressed by implementing proper bounds checking and using appropriate data types that prevent wraparound behavior during attribute count calculations. For applications unable to immediately upgrade, defensive programming practices should be adopted at the integration layer. This includes enforcing strict limits on the number of attributes allowed per XML element before passing the document to libexpat for parsing. Input validation mechanisms can inspect the structure of incoming XML documents and reject those with excessive complexity or attribute counts that exceed predefined safe thresholds. Additionally, enabling compiler-level protections such as stack canaries and heap hardening features like ASLR and DEP can help mitigate the impact of memory corruption attempts by making exploitation more difficult, although these are compensatory controls rather than definitive fixes for the underlying logic error.