CVE-2026-86140 in libxml2
Summary
by MITRE • 09/05/2026
In libxml2 before 2.15.4, xmlSnprintfElements in valid.c has a strcat stack-based buffer overflow.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 09/05/2026
The vulnerability identified in libxml2 prior to version 2.15.4 involves a critical memory safety flaw within the xmlSnprintfElements function located in the valid.c source file. This component is responsible for formatting XML element names and attributes, often during the process of validating or serializing document structures. The core technical issue stems from an improper handling of buffer boundaries when concatenating strings using strcat operations without adequate prior length verification. Specifically, the function fails to ensure that the destination buffer has sufficient allocated space to accommodate the combined length of the source string and the existing content. This oversight leads directly to a stack-based buffer overflow condition where data is written beyond the allocated limits of the local variable array on the call stack.
From a technical perspective, this flaw aligns with CWE-121, which classifies stack-based buffer overflows resulting from insufficient bounds checking during memory operations. The strcat function used in this context does not perform any internal checks to prevent writing past the end of the destination buffer. When an attacker can influence the input data passed to xmlSnprintfElements, they may supply a string that causes the cumulative length to exceed the fixed-size stack allocation. This results in adjacent memory locations being overwritten with arbitrary data. In many execution environments, this corruption affects critical control flow information such as return addresses or frame pointers stored on the stack, thereby providing a potential pathway for remote code execution if the overflow is precisely controlled and exploitable within the specific runtime context of the application utilizing libxml2.
The operational impact of this vulnerability depends heavily on how libxml2 is integrated into host applications. Since libxml2 is widely used as an underlying library for parsing XML documents in various software ecosystems, including web browsers, document processing tools, and network services, exploitation could lead to severe consequences such as application crashes leading to denial of service or arbitrary code execution with the privileges of the affected process. Attackers might exploit this by crafting maliciously formatted XML content that triggers the overflow during validation or serialization phases. The risk is particularly acute in scenarios where untrusted input is processed without sufficient sanitization before reaching vulnerable parsing routines, allowing adversaries to potentially compromise system integrity and confidentiality through successful exploitation of the memory corruption.
Mitigation strategies primarily involve upgrading libxml2 to version 2.15.4 or later, which includes patches addressing this specific buffer overflow by implementing proper bounds checking within xmlSnprintfElements. Developers integrating this library should ensure that their build pipelines enforce these updated dependencies and verify that no legacy versions remain in use across production environments. Additionally, employing compiler-level protections such as stack canaries, Address Sanitizers during testing phases, and non-executable memory pages for the stack can help mitigate the exploitability of similar vulnerabilities even if they exist undetected. Security teams should also review input validation logic to ensure that all XML inputs are strictly validated against expected schemas before being processed by vulnerable parsing functions, thereby reducing the attack surface available to potential adversaries leveraging this weakness as part of a broader exploitation chain consistent with ATT&CK techniques related to initial access and execution via malformed files or network-based payloads.