CVE-2026-117928 in Firefox
Summary
by MITRE • 10/27/2026
Same-origin policy bypass in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 159 and Firefox ESR 153.6.
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Analysis
by VulDB Data Team • 10/27/2026
The identified vulnerability represents a critical failure within the Cascading Style Sheets parsing and computation engine of Mozilla-based browsers, specifically affecting versions prior to Firefox 159 and Firefox Extended Support Release 153.6. At its core, this flaw constitutes a Same-Origin Policy bypass, which is a fundamental security mechanism designed to restrict how documents or scripts loaded from one origin can interact with resources from another origin. The CSS engine, responsible for interpreting style rules and computing final styles for DOM elements, failed to correctly enforce these isolation boundaries during the processing of specific stylesheet constructs. This technical deficiency allows an attacker-controlled webpage to read sensitive data from a different origin through side-channel attacks or direct property access that should have been blocked by the browser's security model.
The operational impact of this vulnerability is severe because it undermines one of the most important defenses against cross-site scripting and data theft on the modern web. By bypassing the same-origin policy, an attacker can potentially exfiltrate sensitive information such as session tokens, personal user details, or confidential content from a victim's active browsing sessions in other tabs or windows that are hosted on different domains. This capability effectively neutralizes the isolation between websites, allowing malicious scripts to act with the privileges of trusted sites. The vulnerability likely stems from improper validation checks within the CSS parser when handling complex selectors, computed style queries, or specific edge cases involving cross-origin stylesheets and fonts, leading to a state where restricted properties become accessible to unauthorized contexts.
From an industry standards perspective, this flaw aligns closely with CWE-200, which covers Information Exposure, as it allows for the leakage of data that should remain confined within its origin boundary. It also maps directly to MITRE ATT&CK technique T1530, Data from Local System, specifically under sub-techniques related to file and directory discovery or data exfiltration via browser-based methods. The failure is not merely a logic error but a fundamental breach of the web's trust model, categorized under CWE-942 as Permissive Cross-domain Access with Untrusted Input. This classification highlights that the vulnerability arises from insufficient restrictions on cross-origin requests initiated by untrusted sources within the rendering engine itself rather than through network-level protocols alone.
Mitigation for this issue relies primarily on immediate software updates to ensure all client systems are running Firefox 159 or later, as well as Firefox ESR 153.6 and subsequent patches. These versions contain the necessary code corrections that enforce strict origin checks during CSS computation phases. For organizations managing large fleets of browsers, deploying these updates via centralized configuration management tools is essential to close this attack vector across all endpoints. Additionally, security teams should monitor for any indicators of exploitation involving unusual cross-origin data access patterns in browser telemetry or logs. While no specific user-side workaround exists that fully mitigates the flaw without updating software, enforcing strict Content Security Policy headers on internal web applications can provide an additional layer of defense by restricting how scripts interact with resources, thereby reducing the potential blast radius if a similar vulnerability were to be discovered in other components.