CVE-2026-100831 in Firefox
Summary
by MITRE • 09/29/2026
Use-after-free in the DOM: UI Events & Focus Handling 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 Document Object Model, specifically affecting the user interface events and focus handling subsystems of the Mozilla Gecko rendering engine. A use-after-free condition occurs when software continues to utilize a pointer after it has been deallocated from system memory. In this specific context, the flaw resides in how the browser manages event listeners associated with DOM elements that are subject to focus changes or other UI interactions. When an element is removed from the document tree while still having active references held by internal structures responsible for tracking focus state or pending events, the underlying memory backing those objects may be freed prematurely. However, subsequent operations triggered by user interaction or automated scripts continue to reference these now-invalid pointers, leading to access of uninitialized or recycled memory regions.
This type of vulnerability is categorized under CWE-416, which defines use-after-free as a distinct class of software weaknesses resulting from improper resource management. The technical mechanism involves the asynchronous nature of JavaScript execution and DOM manipulation. If a script removes an element that currently has focus or triggers events related to its state without properly nullifying all internal references maintained by the browser's event loop, the garbage collector may reclaim the memory associated with that element. Subsequent code paths that attempt to read properties such as offset coordinates, computed styles, or event target data will then operate on stale pointers. This can result in unpredictable behavior ranging from minor rendering glitches to severe security implications depending on how the freed memory is subsequently allocated and used by other parts of the application.
The operational impact of this vulnerability extends beyond simple stability issues. While immediate exploitation might lead to browser crashes or denial-of-service conditions, more sophisticated attacks could leverage use-after-free flaws for arbitrary code execution. By carefully controlling the allocation patterns in heap memory after the free operation, an attacker may be able to overwrite critical data structures within the browser process. This aligns with ATT&CK technique T1203, which covers exploitation for client-side execution through vulnerabilities like this one. Successful exploitation could allow a malicious website or compromised third-party content to bypass security sandboxing mechanisms, potentially leading to full compromise of the user's system if combined with other privilege escalation vectors. The risk is particularly acute in environments where users frequently interact with dynamic web applications that heavily manipulate DOM structures during focus transitions.
Mitigation strategies primarily rely on timely software updates provided by the vendor. The vulnerability was addressed and resolved in Firefox ESR 153.4 and Firefox 157, which include patches to correct the reference counting logic within the UI events module. These fixes ensure that all internal references are properly invalidated before memory is reclaimed during DOM removal operations. Organizations should prioritize patching their browser fleets to these versions or later to eliminate this attack surface. Additionally, developers integrating web technologies into desktop applications using Gecko-based engines must verify they are utilizing patched engine versions. For end-users, maintaining up-to-date browsers remains the most effective defense against exploitation of such memory corruption vulnerabilities in client-side software.