CVE-2026-84124 in Firefox
Summary
by MITRE • 09/01/2026
Use-after-free in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 155, Firefox ESR 140.15, 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 core and HTML parsing components of Mozilla Firefox browsers. Specifically classified as an use-after-free condition, this flaw occurs when the browser's internal data structures retain references to DOM elements or related objects that have already been deallocated from system memory. In complex web environments where dynamic content manipulation is frequent, such as single-page applications or sites utilizing heavy JavaScript frameworks, the likelihood of triggering this race condition increases significantly. The vulnerability stems from a failure in reference counting or lifecycle management within the browser engine, allowing scripts to access memory that has been freed and potentially repurposed by other processes or allocations.
From a technical perspective, use-after-free vulnerabilities are among the most dangerous classes of software defects because they can lead to arbitrary code execution with the privileges of the affected application. When an attacker crafts a malicious webpage containing specific HTML structures combined with JavaScript payloads designed to trigger rapid creation and destruction of DOM nodes, they can manipulate the timing of memory allocation. By ensuring that new data is allocated in the same memory location previously occupied by the freed object, the attacker gains control over what values are read or written through the dangling pointer. This mechanism allows for precise manipulation of internal browser state variables, which can subsequently be leveraged to bypass security controls such as Same-Origin Policy and execute arbitrary native code on the victim's machine.
The operational impact of this vulnerability is severe, potentially resulting in complete compromise of the user system if exploited successfully. An attacker could use this flaw to install malware, steal sensitive information including cookies and session tokens, or perform actions on behalf of the authenticated user without their consent. Given that Firefox is a widely used browser with significant market share, successful exploitation can lead to widespread security incidents across enterprise and personal environments. The vulnerability affects multiple versions of the software, indicating a systemic issue in how certain DOM operations are handled during page rendering and script execution phases.
Mitigation strategies primarily involve immediate application of vendor-provided patches. Users must upgrade to Firefox version 155 or later, as well as the extended support release versions 140.15 and 153.2 where these fixes have been integrated. For organizations managing large fleets of browsers, automated deployment tools should be utilized to ensure rapid rollout of these updates across all endpoints. Additionally, implementing browser hardening techniques such as disabling JavaScript for untrusted sites or using content security policies can reduce the attack surface available to potential exploiters until patches are fully deployed. Security teams should also monitor threat intelligence feeds for indicators of compromise related to this specific vulnerability class to detect any active exploitation attempts in their networks.
This incident aligns with Common Weakness Enumeration identifier CWE-416, which describes use-after-free conditions where a pointer is used after its memory has been freed leading to unpredictable behavior or security breaches. In the context of the MITRE ATT&CK framework, this vulnerability facilitates initial access and privilege escalation techniques typically associated with client-side attacks such as browser exploitation for code execution (T1203). The complexity of exploiting use-after-free bugs often requires sophisticated payload construction involving heap spraying or specific timing attacks to ensure reliable memory reuse. Therefore, while the technical fix addresses the root cause in the source code, ongoing vigilance regarding web content and strict adherence to patch management cycles remain essential components of a robust defense-in-depth strategy against such high-severity vulnerabilities.