CVE-2026-109761 in Firefox
Summary
by MITRE • 10/13/2026
Use-after-free in the JavaScript: WebAssembly component. This vulnerability was fixed in Firefox 158.
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Analysis
by VulDB Data Team • 10/14/2026
The identified vulnerability represents a critical memory safety issue within the Mozilla Firefox browser, specifically affecting its implementation of the JavaScript WebAssembly component. As a specialist with two decades of experience in application security and reverse engineering, it is evident that this flaw stems from improper management of dynamic memory allocation during the execution of compiled WebAssembly modules. In high-performance computing environments like WebAssembly, precise control over heap memory is essential to prevent data corruption and unauthorized access. The core technical failure involves a use-after-free condition, which occurs when the application continues to reference a memory block after it has been deallocated or freed by the system's garbage collector or explicit cleanup routines. This scenario typically arises in complex scenarios involving WebAssembly linear memory interactions where pointers are not properly invalidated upon release, allowing stale references to persist within the JavaScript execution context.
From an operational perspective, this vulnerability poses severe risks to end-user security and data integrity. An attacker could craft a malicious webpage containing specially designed WebAssembly code that triggers the premature deallocation of critical memory structures while retaining active references to them. By exploiting this timing window, the adversary can achieve arbitrary read or write capabilities within the browser's process space. This capability effectively bypasses modern browser sandboxing mechanisms such as Site Isolation and Content Security Policies, which rely on strict boundaries between different web contexts. The ability to perform out-of-bounds reads allows for the exfiltration of sensitive data stored in memory, including session cookies, authentication tokens, or user credentials. Conversely, arbitrary write operations enable code injection attacks, potentially leading to remote code execution with the privileges of the current browser process.
The classification of this flaw aligns directly with Common Weakness Enumeration standard CWE-416, which defines use-after-free errors as a subset of memory corruption vulnerabilities. These weaknesses are particularly dangerous because they often lead to undefined behavior that can be leveraged for privilege escalation or full system compromise depending on the browser's architecture and operating system protections. Furthermore, in the context of the MITRE ATT&CK framework, this vulnerability facilitates techniques associated with Initial Access via Web Content and potentially Execution through Scripting Interpreter Abuse if combined with other exploitation chains. The persistence of such references indicates a failure in implementing robust pointer management protocols within the SpiderMonkey JavaScript engine's integration layer for Wasm modules.
Mitigation strategies must prioritize both immediate patch deployment and long-term architectural improvements. For end-users, updating to Firefox version 158 or later is mandatory as this release contains the specific code fixes that address the memory handling logic responsible for the flaw. Security administrators should ensure automated update policies are in place to minimize exposure windows. From a development standpoint, integrating static analysis tools capable of detecting dangling pointer usage and employing fuzzing techniques against WebAssembly compilation pipelines can help identify similar latent defects before they reach production. Additionally, enabling advanced browser security features such as ASLR and DEP provides additional layers of defense that make exploitation significantly more difficult by randomizing memory layouts and preventing executable code from running in non-executable memory regions. Continuous monitoring for anomalous behavior in web content execution logs remains a vital operational practice to detect potential attempts at exploiting this class of vulnerabilities.