CVE-2026-86139 in libxml2
Summary
by MITRE • 09/05/2026
In libxml2 before 2.15.4, xmlURIEscapeStr in uri.c has an integer overflow.
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Analysis
by VulDB Data Team • 09/05/2026
The vulnerability identified in libxml2 versions prior to 2.15.4 centers on a critical integer overflow within the xmlURIEscapeStr function located in the uri.c source file. This component is responsible for encoding Uniform Resource Identifiers by escaping special characters that are not allowed in URI strings, such as spaces or non-ASCII characters, converting them into percent-encoded sequences. The core technical flaw arises from an incorrect calculation of buffer sizes during this encoding process. Specifically, when processing input data containing multiple escape-worthy characters, the function fails to accurately account for the expanded size of the resulting encoded string relative to the original input length. This miscalculation leads to a situation where the allocated memory buffer is smaller than what is required to hold the fully escaped output, triggering an integer overflow that underestimates the necessary allocation size.
From a technical perspective, this flaw allows for a heap-based buffer over-read or potentially a write operation depending on subsequent logic in the calling functions and how the malformed data is handled by the rest of the XML parsing pipeline. When the application attempts to copy the escaped string into the undersized buffer, it writes beyond the allocated memory boundaries. This behavior aligns with CWE-190, which defines integer overflow or wraparound errors that can lead to various downstream security issues including buffer overflows and out-of-bounds accesses. The vulnerability is particularly dangerous because libxml2 is a foundational library used by numerous high-profile applications for parsing XML data, meaning the impact extends far beyond the library itself into any software stack relying on it for document processing.
The operational impact of this vulnerability can be severe, ranging from information disclosure to remote code execution depending on the context in which libxml2 operates and the nature of the input data processed by the vulnerable application. An attacker who can supply crafted XML content containing specific sequences of characters that trigger the escape routine could exploit this integer overflow to corrupt heap memory structures. In network-facing services, such as web servers or API gateways that parse incoming XML payloads, this flaw presents a vector for remote exploitation. Successful exploitation may allow an adversary to read sensitive information from adjacent memory regions, leading to data leakage of credentials or internal system details. Alternatively, if the overflow permits overwriting critical heap metadata or function pointers, it could facilitate arbitrary code execution with the privileges of the compromised process.
This vulnerability is categorized under CWE-120, Buffer Copy without Checking Size of Input Classic Buffer Overflow, as the root cause involves writing data beyond buffer limits due to flawed size calculations. In terms of adversarial tactics, this aligns with ATT&CK technique T1190 Exploit Public-Facing Application, where attackers target widely deployed software components to gain initial access or escalate privileges within a compromised environment. The lack of proper bounds checking during the URI escaping process represents a common class of errors in C-based libraries that manipulate memory manually without automatic safety checks provided by higher-level languages.
Mitigation strategies primarily involve upgrading libxml2 to version 2.15.4 or later, where this integer overflow has been patched with corrected size calculations and enhanced bounds checking during the URI escaping operation. For organizations unable to immediately update their dependencies, implementing input validation at the application layer can provide a temporary defense-in-depth measure. This includes sanitizing XML inputs before they reach the libxml2 parser, specifically restricting or validating characters that trigger complex escape sequences in URIs within those documents. Additionally, enabling compiler-based security features such as stack protectors and using AddressSanitizer during development testing can help detect similar memory safety issues early in the software lifecycle. Regular dependency audits are essential to ensure that foundational libraries like libxml2 remain current with upstream security fixes, thereby reducing the attack surface for potential exploitation by automated scanning tools or targeted attackers seeking to leverage known vulnerabilities in common infrastructure components.