CVE-2026-84125 in Firefox
Summary
by MITRE • 09/01/2026
Use-after-free in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 155 and Firefox ESR 153.2.
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Analysis
by VulDB Data Team • 09/01/2026
The identified vulnerability represents a critical memory management error within the Document Object Model, specifically affecting the core and HTML components of the browser engine. A use-after-free condition occurs when software continues to reference a pointer after the memory it points to has been reallocated or freed by the system. In this specific context, the flaw likely stems from an internal object lifecycle mismatch where references to DOM nodes are retained in JavaScript contexts even after those underlying C++ objects have been destroyed during garbage collection or explicit removal operations. This discrepancy creates a window of opportunity for malicious actors to exploit dangling pointers that point to freed memory regions rather than valid data structures.
From a technical perspective, this flaw allows an attacker who can trick a user into visiting a specially crafted webpage to execute arbitrary code with the privileges of the browser process. The exploitation vector typically involves constructing a complex HTML and JavaScript payload designed to trigger rapid creation and destruction of DOM elements while simultaneously maintaining references through event handlers or closures. When the garbage collector reclaims the memory associated with these destroyed nodes, any subsequent access via the lingering reference results in undefined behavior. Depending on how the browser handles this invalid read or write operation, an attacker can potentially achieve arbitrary code execution, leading to a complete compromise of the user's system security posture.
The operational impact of such vulnerabilities is severe as they bypass many modern browser sandboxing mechanisms if not properly mitigated by exploit mitigation techniques like ASLR and DEP. Although contemporary browsers employ rigorous memory safety checks and fuzzing during development cycles, sophisticated attackers can often find edge cases where these protections are circumvented or delayed long enough to execute malicious payloads. The vulnerability affects both Firefox 155 and the Extended Support Release version 153.2, indicating that the flaw existed in stable releases used by a significant portion of the user base who may not immediately update their browsers upon notification. This delay between discovery and patch deployment increases the risk window for widespread exploitation across enterprise and personal environments relying on these specific versions.
Industry standards classify this type of memory corruption under CWE-416, which defines use-after-free errors as a distinct category of software weaknesses resulting from improper management of dynamic memory allocation. Furthermore, in terms of tactical classification within the MITRE ATT&CK framework, this vulnerability facilitates initial access and execution phases by allowing remote code execution through social engineering tactics such as phishing or drive-by downloads. The ability to execute arbitrary code effectively neutralizes the browser's role as a secure container for web content, transforming it into an attack vector against the underlying operating system.
Mitigation strategies primarily involve immediate software updates to apply the vendor-provided patches that correct the memory management logic within the DOM handling routines. Organizations should enforce automated patching policies to ensure all endpoints are running versions of Firefox later than 153.2 and 155 respectively, depending on their release channel preferences. Additionally, deploying browser hardening configurations such as disabling JavaScript execution for untrusted sites or utilizing content security policies can reduce the attack surface available to exploit this specific flaw. Security teams should also monitor network traffic for indicators of compromise associated with known exploitation kits that target similar memory corruption vulnerabilities in web browsers.