CVE-2026-86137 in libxml2
Summary
by MITRE • 09/05/2026
In libxml2 before 2.15.4, xmlFAParsePosCharGroup has an out-of-bounds read, aka an out-of-bounds read in the NXT macro in xmlregexp.
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Analysis
by VulDB Data Team • 09/05/2026
The vulnerability identified as CVE-2024-39687 represents a critical memory safety issue within the widely used libxml2 library, specifically affecting versions prior to 2.15.4. This flaw is located in the finite automaton parser component of the regular expression engine, technically manifesting as an out-of-bounds read operation triggered by the NXT macro during the processing of character groups. The root cause lies in insufficient boundary checks when iterating through input data structures that define character ranges or sets within a regex pattern. When the parsing logic attempts to access the next element in a sequence without verifying if such an element exists, it reads memory locations beyond the allocated buffer boundaries. This type of error is classically categorized under CWE-125, which describes out-of-bounds read vulnerabilities where software accesses memory outside the intended bounds, potentially leading to information disclosure or application crashes depending on the surrounding memory layout and execution context.
From an operational perspective, this vulnerability poses significant risks to any system relying on libxml2 for parsing XML documents that contain regular expressions in their schema definitions or validation rules. An attacker could craft a maliciously constructed XML file with specific regex patterns designed to trigger the out-of-bounds read condition during parsing. The immediate impact of exploiting this flaw typically involves denial of service, as the application may crash due to accessing invalid memory addresses, resulting in segmentation faults or undefined behavior. In more sophisticated attack scenarios where the accessed memory contains sensitive data such as stack canaries, encryption keys, or internal pointers, an attacker might leverage the leaked information for further exploitation steps, including bypassing security mitigations like ASLR or achieving arbitrary code execution through subsequent heap-based attacks. This aligns with ATT&CK technique T1082, System Information Discovery, if data is exfiltrated, and potentially T1496, Resource Hijacking, in the context of denial-of-service impacts on critical infrastructure services that depend heavily on XML processing for configuration or document exchange.
The severity of this issue is amplified by the ubiquity of libxml2 across various operating systems, web servers, database management systems, and enterprise applications. Many software products embed libxml2 statically to handle XML parsing tasks ranging from SOAP message handling to HTML5 rendering engines that borrow logic from XML parsers. Consequently, a successful exploit could have widespread implications for organizations dependent on these technologies. The vulnerability highlights the importance of rigorous input validation in low-level C libraries where manual memory management is prevalent. Developers integrating libxml2 must ensure they are not using vulnerable versions and should prioritize updates to version 2.15.4 or later, which includes patches that enforce strict boundary checks within the NXT macro and related parsing routines.
Mitigation strategies primarily involve upgrading the library to a patched version where the out-of-bounds read condition is resolved by adding proper index validation before accessing array elements. For environments where immediate patching is not feasible due to dependency constraints or testing requirements, temporary mitigations include implementing strict input sanitization at the application layer to reject XML documents containing complex or deeply nested regular expression patterns that might trigger the flaw. Additionally, deploying runtime protection mechanisms such as Address Sanitizer during development and testing phases can help detect similar memory safety issues before they reach production. Security teams should also monitor for any indicators of compromise related to abnormal process termination or unexpected network activity originating from services known to utilize libxml2, ensuring rapid response in case of potential exploitation attempts targeting this specific weakness in the regular expression parsing logic.