CVE-2026-86143 in libxml2
Summary
by MITRE • 09/05/2026
In xmlIO in libxml2 before 2.15.4, an inconsistency in xmlOutputWriteCallback and xmlBufUse causes negative lengths to reach write callbacks, aka a lack of a check for integer overflow before calling writecallback. This has security relevance for many types of uses of that length value within a callback.
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Analysis
by VulDB Data Team • 09/05/2026
The vulnerability identified in libxml2 prior to version 2.15.4 centers on an integer handling flaw within the xmlIO module, specifically affecting the interaction between the xmlOutputWriteCallback and the internal buffer management function xmlBufUse. Libxml2 is a widely used C library for parsing XML documents, serving as a foundational component in numerous enterprise applications, web servers, and data processing pipelines that rely on structured text input. The core technical flaw arises from an inconsistency where the length value derived from xmlBufUse is not validated against integer overflow conditions before being passed to write callbacks. In many programming contexts, particularly those involving C-style memory management, failing to check for arithmetic overflows can lead to unexpected data types or values when large numbers wrap around negative boundaries due to signed integer limitations.
When an attacker provides a crafted XML input that triggers this specific code path, the calculation of the buffer length may result in a value that exceeds the maximum limit for a signed integer. Instead of raising an error or truncating the request safely, the system proceeds with this miscalculated negative length. This negative length is then passed directly to the xmlOutputWriteCallback function. Since most write callbacks expect a non-negative size parameter indicating how many bytes to process, receiving a negative value creates a critical inconsistency in the operational logic of the library. The callback mechanism assumes valid input and attempts to execute operations based on this erroneous metric, leading to undefined behavior within the application layer that consumes libxml2.
The security implications of this flaw are significant because they enable potential memory corruption attacks. Depending on how the specific write callback is implemented by the developer using libxml2, a negative length value might be interpreted as an extremely large unsigned integer due to type coercion or implicit conversion rules in C. This can lead to buffer over-reads if the system attempts to read data beyond allocated boundaries, or it may cause crashes that result in denial of service conditions. In more sophisticated scenarios where memory is manipulated based on these lengths, attackers could potentially achieve arbitrary code execution by exploiting the resulting memory state inconsistencies. The lack of a pre-call validation check means that any downstream logic relying on this length parameter inherits this vulnerability, making it a systemic risk across all applications utilizing affected versions of libxml2 for XML processing tasks involving custom output handlers.
This issue aligns with CWE-190, which describes Integer Overflow or Wraparound vulnerabilities where arithmetic operations produce results too large to be represented by the destination type. Furthermore, from an offensive security perspective such as the MITRE ATT&CK framework, this flaw facilitates techniques related to memory corruption and potentially remote code execution if combined with other exploitation vectors like buffer overflow exploits. The vulnerability highlights the importance of rigorous input validation at every stage of data processing, particularly when dealing with low-level system calls or callbacks that interact directly with memory buffers.
To mitigate this risk, organizations must immediately upgrade libxml2 to version 2.15.4 or later, where the inconsistency has been resolved by implementing proper checks for integer overflow before invoking write callbacks. Developers integrating libxml2 should also review their custom xmlOutputWriteCallback implementations to ensure they validate length parameters and handle edge cases gracefully. Additionally, employing static analysis tools that detect potential integer overflows during development can help prevent similar issues in future codebases. Regular patching of third-party libraries is essential to maintain the integrity and security posture of systems dependent on robust XML parsing capabilities.