CVE-2026-92021 in Firefox
Summary
by MITRE • 09/15/2026
Use-after-free in the JavaScript Engine: JIT component. This vulnerability was fixed in Firefox ESR 140.16.
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Analysis
by VulDB Data Team • 09/15/2026
The identified vulnerability represents a critical memory safety issue within the Just-In-Time compilation subsystem of the Mozilla SpiderMonkey JavaScript engine, specifically categorized under Use-After-Free conditions. In software architecture, particularly within high-performance scripting engines like those used in Firefox ESR 140.16, JIT compilers optimize code execution by translating dynamic bytecode into native machine instructions at runtime. This process involves complex memory management where objects are allocated on the heap for temporary storage during compilation and optimization phases. A Use-After-Free flaw occurs when the engine fails to properly invalidate a pointer or reference after the underlying memory has been deallocated, allowing subsequent operations to access that freed memory region.
From a technical perspective, this vulnerability likely stems from a race condition or logic error in how JIT-generated code manages object lifecycles during garbage collection cycles or stack unwinding events. When an object is marked for deletion by the garbage collector but remains referenced by active JIT-compiled frames or speculative execution paths, the memory address becomes dangling. If the application subsequently writes to or reads from this invalid pointer before it is reallocated for a different purpose, undefined behavior ensues. This can manifest as data corruption, crashes due to segmentation faults, or more critically, arbitrary code execution if an attacker can control the contents of the freed memory block and trick the engine into executing that content as instructions.
The operational impact of this vulnerability is severe because it resides in a core component responsible for performance optimization. Attackers could potentially craft malicious JavaScript payloads designed to trigger specific JIT compilation paths that expose the timing window between object deallocation and pointer invalidation. Successful exploitation would allow an attacker to bypass modern browser security mitigations such as Data Execution Prevention (DEP) and Address Space Layout Randomization (ASLR), leading to remote code execution on the victim's system. This poses a significant risk in enterprise environments where Firefox ESR is deployed, potentially compromising sensitive corporate data or serving as an entry point for further lateral movement within internal networks.
This flaw aligns with Common Weakness Enumeration identifier CWE-416, which defines Use After Free vulnerabilities resulting from improper memory management practices. In the context of offensive security frameworks, this vulnerability maps to MITRE ATT&CK techniques related to Execution via Web Shells or Scripting Interpreters, specifically leveraging browser-based attack vectors such as CVE-driven exploits for Remote Code Execution (T1059). The exploitation chain typically involves social engineering tactics like phishing emails containing malicious links that host the exploit kit, which then leverages this JIT engine flaw to gain initial access.
Mitigation strategies primarily involve immediate patching of all affected systems to version Firefox ESR 140.16 or later, where the memory management logic in the JIT compiler has been corrected to ensure pointers are nullified immediately upon deallocation and that speculative execution paths do not retain stale references. Additionally, organizations should enforce strict content security policies (CSP) to restrict inline script execution and disable unnecessary JavaScript features if feasible within their operational constraints. Regular vulnerability scanning and monitoring for indicators of compromise related to browser exploit kits are also recommended to detect potential attempts to leverage this flaw before successful exploitation occurs.