CVE-2026-84119 in Firefox
Summary
by MITRE • 09/01/2026
Sandbox escape due to use-after-free in the DOM: Navigation component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, and Firefox ESR 153.2.
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Analysis
by VulDB Data Team • 09/01/2026
The identified security flaw represents a critical memory corruption issue within the Document Object Model navigation subsystem of Mozilla Firefox browsers. Specifically, this is an use-after-free vulnerability that occurs during DOM navigation operations. In software engineering terms, an use-after-free condition arises when a program continues to reference a pointer after the memory it points to has been deallocated or freed. This typically happens due to improper lifecycle management where references are not invalidated immediately upon resource release, allowing stale pointers to remain accessible in memory. When such a dangling pointer is dereferenced, either by reading from or writing to the previously allocated memory region, the application enters an undefined state. In the context of browser engines like Gecko, which handle complex rendering and scripting environments, this corruption can lead to arbitrary code execution if exploited successfully, as attackers may manipulate heap contents to control program flow.
From a technical perspective, the vulnerability resides in how the navigation component manages object lifecycles during page transitions or history manipulations. When a user navigates between pages or interacts with dynamic content that triggers DOM updates, the browser allocates memory structures to track state and rendering data. If these structures are freed prematurely while still being referenced by active scripts or internal engine processes, subsequent access attempts trigger the flaw. This specific instance highlights a failure in reference counting or smart pointer management within the navigation stack. The exploitation potential is significant because DOM manipulation is a common vector for web-based attacks. An attacker could craft a malicious webpage that triggers rapid navigation events or history changes designed to race against memory deallocation routines, thereby gaining access to freed heap chunks and potentially overwriting critical function pointers or virtual table entries.
The operational impact of this vulnerability extends beyond simple application crashes. While unintended behavior such as browser instability is the most common outcome for unexploited flaws, successful exploitation allows a remote attacker to execute arbitrary code within the security sandbox of the browser process. This effectively breaks the isolation model that protects users from malicious websites. By achieving code execution in the context of the browser, an adversary can potentially access sensitive user data such as cookies, session tokens, and stored credentials. Furthermore, depending on operating system protections like Address Space Layout Randomization or Data Execution Prevention, exploitation might be difficult but not impossible for sophisticated threat actors who employ advanced heap grooming techniques to stabilize memory layouts and ensure reliable control flow hijacking.
This vulnerability aligns with CWE-416, which defines use-after-free errors as a class of software weaknesses where pointers are used after their associated memory has been freed. It also maps closely to MITRE ATT&CK technique T1203, specifically the exploitation phase for client-side attacks such as browser exploits or drive-by downloads. The navigation component serves as an attack surface because it is frequently invoked by standard web browsing activities and JavaScript APIs like history.pushState or window.location changes. These features are essential for modern single-page applications but introduce complexity in memory management that can be abused if not rigorously audited.
Mitigation strategies primarily involve the immediate application of vendor-provided patches, as indicated by the availability of fixes in Firefox version 155 and specific Extended Support Release versions including ESR 115.40, ESR 140.15, and ESR 153.2. Organizations relying on these browser distributions must ensure their update policies are configured to deploy these patches promptly across all endpoints. For developers maintaining custom builds or forks of the Gecko engine, code reviews focusing on memory management in navigation-related modules are essential. Implementing static analysis tools that detect dangling pointer usage and dynamic testing with sanitizers like AddressSanitizer can help identify similar issues before deployment. Additionally, enforcing strict Content Security Policies helps mitigate the impact by restricting script execution sources, thereby reducing the likelihood of successful exploitation even if a vulnerability exists.