CVE-2026-109729 in Firefox
Summary
by MITRE • 10/13/2026
Use-after-free in the JavaScript Engine: JIT component. This vulnerability was fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5.
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Analysis
by VulDB Data Team • 10/13/2026
The identified vulnerability represents a critical memory management error within the Just-In-Time compilation subsystem of the JavaScript engine, specifically classified as an out-of-bounds write or use-after-free condition. This type of flaw typically arises when the JIT compiler generates machine code that incorrectly handles object lifecycles during optimization phases such as inline caching or type inference. In this scenario, a reference to a JavaScript object is retained by compiled native code even after the garbage collector has reclaimed the underlying memory allocation in the heap. When the malicious script triggers execution of the optimized path, the JIT-compiled function attempts to dereference the stale pointer, leading to access of freed memory regions. This constitutes a classic use-after-free vulnerability where the program continues to operate on data that is no longer valid or may have been reallocated for different purposes by subsequent allocations.
From an offensive security perspective, this flaw aligns with CWE-416 Use After Free and often overlaps with CWE-787 Out-of-Bounds Write if the freed memory was reused in a way that allows arbitrary data to be written into controlled locations. The operational impact is severe because modern browsers employ various exploit mitigation techniques such as Address Space Layout Randomization, Data Execution Prevention, and heap hardening measures like zone isolation or free-list randomization. However, sophisticated attackers can leverage this vulnerability to achieve arbitrary code execution by carefully crafting a sequence of allocations that place attacker-controlled data into the freed memory slot before it is accessed again. This technique allows for control over instruction pointers or function return addresses, potentially bypassing browser security boundaries and leading to full system compromise if combined with other vulnerabilities like sandbox escapes.
The vulnerability has been addressed in Firefox version 158 as well as extended support release versions 115.43 and 153.5 through rigorous code audits and the implementation of stricter memory safety checks within the JIT compiler infrastructure. The fix likely involves adding explicit null-checks, updating reference counting mechanisms to ensure objects are not collected while still in use by compiled code paths, or employing safe pointer wrappers that detect dangling references at runtime. These mitigations prevent the execution flow from proceeding into invalid memory spaces and instead trigger a controlled crash or error handling routine that preserves system stability without exposing sensitive data.
For organizations relying on these browser versions, immediate patching is essential to mitigate the risk of remote code execution attacks delivered via malicious web content. Security teams should verify deployment status across all endpoints using automated vulnerability management tools configured to detect specific version numbers associated with this fix. Additionally, deploying advanced endpoint detection and response solutions that monitor for anomalous memory access patterns or JIT compiler behavior can provide an additional layer of defense against exploitation attempts targeting unpatched systems. Continuous monitoring for indicators of compromise related to browser-based exploits remains critical given the high value placed on web browsers as initial attack vectors in many cyber threat landscapes.