CVE-2026-84137 in Firefox
Summary
by MITRE • 09/01/2026
Spoofing issue 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 security flaw resides within the Document Object Model core and HTML components of Mozilla Firefox, specifically manifesting as a spoofing issue that undermines the integrity of user interface elements and navigation indicators. This vulnerability allows an attacker to manipulate how web content is rendered or presented to the end-user in a manner that deceives them regarding the true origin or nature of the data being displayed. By exploiting weaknesses in how the browser processes DOM structures and HTML parsing rules, malicious actors can craft specially designed web pages that trick users into interacting with fake interfaces, such as counterfeit login prompts, deceptive download buttons, or misleading address bar displays. This type of attack is particularly dangerous because it leverages the trust users place in their browser's interface to facilitate social engineering attacks, credential harvesting, and malware distribution without requiring any additional software installation on the victim's machine beyond a standard web browsing session.
From a technical perspective, this flaw likely stems from improper validation or sanitization of DOM elements during rendering, allowing malicious scripts or HTML structures to override legitimate UI components. The attacker can inject content that mimics native browser dialogs or navigation bars, effectively creating a man-in-the-middle scenario within the client-side environment. This aligns with CWE-20, Improper Input Validation, as the browser fails to adequately distinguish between trusted interface elements and untrusted web content. Furthermore, it relates closely to CWE-798, Use of Hard-coded Credentials, if the spoofed interfaces are designed to capture authentication details, or more broadly to CWE-1346, Insecure Implementation of Interactive Authentication Mechanisms. The core issue is a failure in the browser's security model to maintain strict boundaries between privileged UI code and untrusted web content, leading to potential confusion for the user about which entity they are communicating with.
The operational impact of this vulnerability is significant, as it directly compromises the confidentiality and integrity aspects of information security by facilitating phishing attacks at scale. Users who encounter spoofed interfaces may inadvertently disclose sensitive personal information, financial credentials, or corporate secrets under the false impression that they are interacting with a legitimate service provider. This erodes user trust in the browser ecosystem and increases the success rate of targeted social engineering campaigns. Attackers can deploy these exploits via drive-by downloads, malicious advertisements, or compromised websites to harvest session cookies, passwords, and two-factor authentication codes. The widespread adoption of Firefox makes this vulnerability particularly impactful, affecting a large segment of internet users who rely on the browser for secure online transactions and communications.
To mitigate the risks associated with this spoofing issue, it is imperative that all users update their Mozilla Firefox installations to version 155 or later, including the Extended Support Release (ESR) version 153.2 where applicable. These updates contain specific patches designed to harden the DOM and HTML parsing engines against such manipulation techniques. Organizations should enforce automated patch management policies to ensure that all endpoint devices running Firefox are kept up-to-date with the latest security fixes. Additionally, users can employ browser extensions or built-in features like phishing protection warnings to add an extra layer of defense against deceptive interfaces. Security awareness training remains critical, as technical controls alone cannot fully prevent sophisticated social engineering; educating users on how to verify URL authenticity and recognize signs of interface spoofing complements the software-level mitigations provided by the vendor updates.