CVE-2026-117916 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the XSLT component. This vulnerability was fixed in Firefox 159 and Firefox ESR 153.6.
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Analysis
by VulDB Data Team • 10/27/2026
The identified vulnerability represents a critical memory management error within the Extensible Stylesheet Language Transformations (XSLT) processing engine, specifically classified as an out-of-bounds write resulting from use-after-free conditions. This type of flaw occurs when software continues to utilize a pointer after it has been deallocated or freed by the system's memory manager. In the context of XSLT processors, which are responsible for transforming XML documents into other formats such as HTML or plain text using stylesheet definitions, complex parsing operations can lead to scenarios where internal data structures are prematurely released while still being referenced by active execution contexts. When an attacker crafts a maliciously constructed XML document paired with specific XSL stylesheets, they can trigger this race condition or logical error during the transformation process. The immediate technical consequence is that the application attempts to write data into memory regions that have already been returned to the operating system for reuse. This unauthorized access allows the attacker to corrupt adjacent heap structures, overwrite function pointers, or manipulate object headers within the browser's memory space.
From a security impact perspective, this vulnerability poses a severe risk to user confidentiality and integrity. Because XSLT processing often involves complex DOM manipulation and script execution contexts in modern browsers, exploiting an out-of-bounds write can lead to arbitrary code execution with the privileges of the victim process. An attacker could potentially bypass browser sandboxing mechanisms if they successfully control memory layout through heap spraying techniques or other exploitation primitives associated with the specific version of the engine. This aligns closely with CWE-416, which defines use-after-free as a design flaw where an application uses a pointer to deallocated memory, and CWE-787, which covers out-of-bounds writes that allow for buffer overflow attacks. The potential for remote code execution makes this particularly dangerous in scenarios involving phishing campaigns or compromised websites that serve malicious content designed specifically to trigger the XSLT engine's parsing routines without user interaction beyond visiting a page.
The operational impact extends beyond immediate exploitation, affecting the overall trustworthiness of the browser as a platform for viewing structured data. Users relying on Firefox and its Extended Support Release (ESR) variants for enterprise environments or sensitive browsing activities are exposed to significant risk until patched. The vulnerability highlights the inherent complexity of maintaining robust memory safety in C++ based browsers where manual memory management is still prevalent despite modern best practices encouraging safer alternatives like Rust. For organizations, this necessitates immediate attention to patching cycles and endpoint security configurations. Mitigation strategies primarily involve applying the vendor-provided updates released for Firefox version 159 and Firefox ESR 153.6. These patches address the underlying logic errors in the XSLT processor that fail to properly validate object lifecycles before dereferencing them. Additionally, users should ensure their browsers are configured to automatically update or be managed by enterprise policy tools that enforce timely patch deployment. Security teams should also monitor for indicators of compromise related to unusual memory allocation patterns or unexpected network connections originating from browser processes, which may indicate active exploitation attempts targeting this specific flaw before patches were applied.