CVE-2026-76641 in Expat
Summary
by MITRE • 08/20/2026
Expat through 2.8.3 contains an out-of-bounds read vulnerability that allows attackers to trigger memory corruption by processing XML with external entity parsers created via XML_ExternalEntityParserCreate. A struct size mismatch between ELEMENT_TYPE members causes storeAtts to read the attIndex member past allocated memory boundaries, resulting in failure to normalize whitespace in non-CDATA attributes or a wild pointer dereference causing a segfault. This vulnerability was introduced by the fix for CVE-2026-66046.
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Analysis
by VulDB Data Team • 08/21/2026
The Expat XML parser library through version 2.8.3 contains a critical out-of-bounds read vulnerability that poses significant risks to application stability and potentially confidentiality, depending on the specific execution context. This flaw is located within the internal handling of external entity parsers, specifically triggered when processing XML documents via functions such as XML_ExternalEntityParserCreate. The root cause stems from an incorrect struct size mismatch involving ELEMENT_TYPE members, which disrupts the expected memory layout during attribute parsing operations. When the storeAtts function executes, it attempts to access the attIndex member of a data structure that has been improperly sized or aligned due to this discrepancy. Consequently, the parser reads beyond the allocated memory boundaries associated with the current element context.
This out-of-bounds read manifests in two primary operational impacts. In less severe scenarios, the corruption prevents the proper normalization of whitespace within non-CDATA attributes, leading to data integrity issues where parsed content does not match the original source document structure. However, in more critical instances, the memory misalignment results in a wild pointer dereference. This occurs because the parser calculates an address based on incorrect offsets derived from the flawed struct definition. Dereferencing this invalid pointer typically causes a segmentation fault, resulting in application crashes and denial of service conditions for any system relying on Expat to process untrusted or semi-trusted XML input streams.
The vulnerability was inadvertently introduced by a previous patch intended to fix CVE-2026-66046. This highlights the complexity of maintaining low-level C libraries where manual memory management and struct alignment are critical. The attempt to resolve one issue created an offset miscalculation that exposed this new attack vector. From a classification perspective, this flaw aligns with CWE-125 Out-of-bounds Read, as it involves accessing memory beyond the intended buffer limits. Furthermore, given its potential to cause denial of service through application crashes, it relates to CWE-400 Uncontrolled Resource Consumption and falls under MITRE ATT&CK techniques associated with resource exhaustion or exploitation for availability impact.
Mitigation strategies must prioritize immediate software updates. Organizations utilizing Expat should upgrade to a version released after the identification of this specific regression bug, ensuring that the struct definitions are correctly aligned and sized according to the library's internal specifications. For environments where upgrading is not immediately feasible, input validation becomes crucial. Implementing strict schema validation or limiting the complexity of external entity references can reduce the likelihood of triggering the vulnerable code paths in storeAtts. Additionally, deploying runtime protection mechanisms such as Address Sanitizers during development and testing phases helps identify similar memory safety issues before they reach production environments. Security teams should also audit their XML processing pipelines to ensure that only trusted sources are allowed to trigger external entity parsing operations, thereby reducing the attack surface for potential exploitation attempts targeting this specific memory corruption flaw.