CVE-2026-74986 in Firefox
Summary
by MITRE • 08/18/2026
Site isolation issue in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 154 and Firefox ESR 153.1.
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Analysis
by VulDB Data Team • 08/18/2026
The identified security flaw resides within the Cascading Style Sheets parsing and computation subsystem of Mozilla Firefox, specifically manifesting as a site isolation violation. Site isolation is a critical browser security architecture designed to prevent cross-site data leakage by ensuring that resources from different origins are processed in separate processes or contexts. When this mechanism fails, it creates an environment where malicious websites may potentially access sensitive information belonging to other sites loaded within the same browsing session. This type of vulnerability undermines the fundamental trust model of web browsers, which relies on strict separation between documents originating from distinct domains to protect user privacy and data integrity against cross-site scripting or side-channel attacks.
From a technical perspective, the defect likely involves an error in how style information is computed or applied across different frame contexts. In modern browser engines, CSS styles are often inherited or shared among frames under specific conditions to optimize performance. If the logic governing this sharing does not correctly verify that both the source and target of the style application belong to the same origin, it can lead to a situation where one site gains unauthorized access to computed style properties of another. These properties might include sensitive data such as layout dimensions, colors derived from user-specific themes, or other visual cues that could be leveraged for fingerprinting or information disclosure attacks. The vulnerability highlights a gap in the enforcement boundaries between different security principals within the rendering pipeline.
The operational impact of this site isolation breach is primarily centered on potential privacy violations and data exfiltration. An attacker hosting a malicious webpage could potentially exploit this flaw to read style properties from a victim's active session on a trusted website, such as an online banking portal or email service. While CSS-based attacks are often limited in scope compared to direct script execution vulnerabilities, they can still facilitate sophisticated tracking mechanisms or aid in more complex attack chains by revealing structural details of the target application. The severity is further compounded if the leaked information allows for precise user fingerprinting, making it easier for advertisers and trackers to identify users across different sites without their consent.
This vulnerability aligns with CWE-200, which classifies issues related to exposure of sensitive information to unauthorized actors, as well as CWE-1378 under improper enforcement of site isolation. In the context of the MITRE ATT&CK framework for browsers, this flaw relates to techniques involving cross-site scripting or data exfiltration through side channels, specifically falling under tactics that focus on collection and exfiltration over alternative protocols or local system access via browser vulnerabilities. The failure to maintain strict origin boundaries allows an attacker to bypass standard same-origin policies, effectively treating resources from different origins as if they were part of the same security context.
To mitigate this risk, users must ensure their browsers are updated to version 154 for Firefox and version 153.1 for Firefox ESR immediately upon availability. These releases contain the necessary patches that correct the logic errors in the CSS parsing engine, ensuring that style computations strictly adhere to origin boundaries. For organizations deploying Firefox across enterprise environments, it is imperative to automate patch management processes to minimize the window of exposure. Additionally, users should remain vigilant against phishing attempts and malicious websites, as no single vulnerability fix eliminates all potential attack vectors. Maintaining up-to-date software remains the most effective defense against such architectural flaws in browser security models.