Description

Privilege escalation in the WebExtensions component. This vulnerability was fixed in Firefox 156, Firefox ESR 115.41, Firefox ESR 140.16, and Firefox ESR 153.3.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "8.8",
  "pubDate": "2026-09-15T13:16:51.100Z",
  "pubdate": "2026-09-15T13:16:51.100Z",
  "executiveSummary": "This vulnerability involves a privilege escalation flaw located within the WebExtensions component of the Firefox browser architecture.\nThe vulnerability allows a malicious actor to bypass intended security boundaries within the browser's extension framework, potentially executing code or accessing privileged APIs with elevated permissions.\nAffected products include Firefox 156, Firefox ESR 115.41, Firefox ESR 140.16, and Firefox ESR 153.3.\nThe impact of this vulnerability is significant, as successful exploitation enables an attacker to move beyond the sandbox constraints typically imposed on web extensions, potentially leading to unauthorized data access, browser compromise, or lateral movement within the user's session.\nExploitation generally requires the presence or installation of a malicious extension, which must leverage specific, undocumented behaviors or insecure API handling within the WebExtensions subsystem.\nThe vulnerability poses high risk as it undermines the browser's extension security model, which is a critical layer of defense for Firefox users.",
  "technicalDetails": "The vulnerability originates from inadequate validation or improper permission management within the WebExtensions component.\nWebExtensions in Firefox operate within a restricted environment designed to limit their ability to interact with sensitive browser internals or arbitrary host data without explicit user permission. This vulnerability indicates a flaw in the inter-process communication (IPC) or privilege-check mechanisms between the extension content process and the browser's chrome process.\nThe root cause is likely an improper trust assumption or a logic error in the handling of privileged API requests triggered by a malicious extension. Attackers can exploit this by crafting a specifically designed extension that invokes these APIs in a manner that bypasses existing access control lists (ACLs) or privilege validation logic.\nThe attack flow commences when a user installs a malicious extension. Once active, the extension triggers a sequence of API calls specifically engineered to trigger the vulnerability. These calls are processed by the browser's privileged chrome process. Due to the flaw, the browser incorrectly validates the context or origin of the request, leading it to execute functions or return data that the extension should not have authorization to access.\nSpecifically, if the WebExtensions subsystem fails to correctly distinguish between different security contexts or improperly exposes sensitive interfaces, an attacker can coerce the browser into performing actions on their behalf. This could include modifying browser settings, accessing the user's cookies, manipulating the DOM of unrelated websites, or executing arbitrary JavaScript in a privileged context.\nAffected versions include Firefox 156 and specific ESR builds: Firefox ESR 115.41, Firefox ESR 140.16, and Firefox ESR 153.3. The vulnerability does not require remote network exposure in the traditional sense, as the primary vector is the browser's extension environment itself. However, it does require a mechanism to deploy the malicious extension, such as social engineering or the exploitation of a separate vulnerability in the extension update/installation workflow.\nPost-exploitation impact includes persistent control over the browser session, potential exfiltration of sensitive browsing data, and the ability to maintain a foothold even if the malicious extension is theoretically restricted by the browser's policy engine."
}