CVE-2026-100825 in Firefox
Summary
by MITRE • 09/29/2026
Use-after-free in the JavaScript Engine: JIT component. This vulnerability was fixed in Firefox ESR 153.4 and Firefox 157.
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Analysis
by VulDB Data Team • 09/29/2026
The identified vulnerability represents a critical memory management error within the Just-In-Time compilation subsystem of the Mozilla Gecko engine, specifically affecting the handling of JavaScript objects during execution optimization phases. As a use-after-free condition, this flaw occurs when the application continues to reference a memory block after it has been deallocated or freed by the garbage collector. In the context of JIT compilers like IonMonkey, which dynamically translate high-level JavaScript code into optimized machine code for performance gains, such errors are particularly dangerous because they can be triggered through complex script execution paths that manipulate object lifecycles and type inference mechanisms. The vulnerability allows an attacker to craft a malicious web page containing specially designed JavaScript code that forces the engine to free memory associated with a JIT-compiled function or data structure while retaining a pointer to that same memory region for subsequent operations.
From a technical perspective, this flaw typically arises when there is a mismatch between the garbage collection markers and the actual usage of objects within the compiled bytecode. When the JavaScript runtime determines an object is no longer reachable, it reclaims its memory space. However, if the JIT compiler has cached references to this object or assumes its validity for optimization purposes without proper synchronization with the garbage collector's state, subsequent access attempts will read from or write to invalid memory addresses. This can lead to arbitrary code execution in the context of the browser process, as an attacker may leverage heap spraying techniques combined with precise control over the freed memory contents to overwrite function pointers or vtables. The severity is amplified by the fact that JIT engines operate at a low level close to the hardware, making exploitation vectors more direct and potentially bypassing higher-level security mitigations implemented in the browser's sandbox architecture.
The operational impact of this vulnerability is severe, posing a significant risk to user confidentiality, integrity, and availability. Successful exploitation could allow remote attackers to execute arbitrary code on the victim's system with the privileges of the current user. This can lead to full system compromise, including data theft, installation of malware, or use as part of a botnet. Given that Firefox is widely used across enterprise environments and personal devices, this vulnerability affects a broad attack surface. The issue was addressed in Firefox ESR 153.4 and Firefox 157 through rigorous code reviews and the implementation of stricter memory safety checks within the JIT compilation pipeline. These updates likely involve enhanced validation of object lifecycles before accessing them during optimization passes and improved synchronization between the garbage collector and the compiler to prevent stale references from being utilized.
In terms of industry classification, this vulnerability aligns with CWE-416 Use After Free, which describes situations where a program uses memory after it has been freed, leading to undefined behavior that can be exploited for malicious purposes. Furthermore, in the context of the MITRE ATT&CK framework, such vulnerabilities are often associated with Tactic 7: Discovery or Tactic 3: Persistence if leveraged for initial access and subsequent foothold establishment on a target system. Security professionals should prioritize patching affected systems immediately to mitigate the risk of exploitation via malicious websites or drive-by download attacks. Organizations relying on Firefox ESR versions must ensure their update policies are configured to automatically deploy these security patches, as delayed updates leave users exposed to zero-day exploits that may be actively weaponized in the wild before public disclosure is fully propagated across all distribution channels.