CVE-2026-117865 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the JavaScript: GC 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 garbage collection subsystem of the Mozilla Gecko engine, specifically affecting its JavaScript execution environment. As a use-after-free flaw, this defect occurs when the application continues to reference a block of memory that has already been deallocated and potentially reused for other purposes. In the context of browser-based JavaScript engines, such as SpiderMonkey which powers Firefox, precise control over object lifecycles is paramount because these objects often contain sensitive data or executable code pointers. When the garbage collector identifies an object as unreachable and reclaims its memory, any lingering references to that object become dangling pointers. If subsequent operations attempt to access this freed memory without proper validation, it triggers undefined behavior within the runtime environment. This specific instance was addressed in Firefox version 159 and Firefox Extended Support Release (ESR) version 153.6, indicating a significant effort by Mozilla engineers to harden the engine against exploitation vectors that could lead to remote code execution or browser instability.
From a technical perspective, use-after-free vulnerabilities are particularly dangerous because they allow attackers to manipulate memory layout in ways that can bypass modern security mitigations such as Address Space Layout Randomization (ASLR) and Data Execution Prevention (DEP). By carefully crafting malicious JavaScript payloads, an attacker can trigger the premature deallocation of critical objects while maintaining references to them. Through a technique known as heap spraying or object reuse, it is possible to fill the freed memory region with controlled data structures that mimic valid objects. When the vulnerable code subsequently accesses this memory, it may interpret the attacker-controlled data as legitimate pointers or function addresses. This misinterpretation can lead to arbitrary read and write primitives, which serve as foundational building blocks for more complex exploits aimed at achieving full system compromise. The complexity of exploiting such flaws lies in the precise timing required between garbage collection cycles and subsequent object accesses, but successful exploitation results in a complete loss of browser sandbox integrity.
The operational impact of this vulnerability is severe, primarily due to its potential for remote code execution via web content. Users visiting maliciously crafted websites or those compromised by drive-by download attacks could inadvertently trigger the flaw during normal browsing activities involving complex JavaScript applications. The consequences extend beyond simple application crashes; successful exploitation allows an attacker to execute arbitrary code with the privileges of the current user, potentially leading to data theft, installation of malware, or use of the browser as a pivot point for further network intrusion. Furthermore, because this issue resides in the core engine components used across multiple Mozilla products including Firefox and Thunderbird, the blast radius is wide. The fix included in versions 159 and ESR 153.6 highlights the importance of maintaining up-to-date software to mitigate risks associated with memory corruption bugs that are increasingly targeted by advanced persistent threats and automated exploit kits found on underground markets.
To align this vulnerability with established industry standards, it is classified under Common Weakness Enumeration (CWE) ID CWE-416, which specifically defines use-after-free errors as a distinct category of software weaknesses. In the context of cyber attack tactics, techniques, and procedures, this flaw facilitates exploitation paths consistent with MITRE ATT&CK technique T1203, which covers exploitation for client execution. The vulnerability underscores the necessity for rigorous memory safety practices in high-performance scripting engines. Mitigation strategies primarily involve ensuring that all clients running affected versions are updated to the patched releases immediately upon availability. For organizations managing large fleets of endpoints, automated patch management systems should be configured to prioritize these updates due to the active exploitation potential inherent in use-after-free conditions within browser engines. Additionally, deploying content security policies and restricting JavaScript execution on untrusted domains can provide an additional layer of defense by limiting the attack surface available to malicious actors attempting to trigger such memory corruption bugs.