CVE-2026-109739 in Firefox
Summary
by MITRE • 10/13/2026
Privilege escalation in the WebExtensions component. This vulnerability was fixed in Firefox 158.
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Analysis
by VulDB Data Team • 10/13/2026
The identified privilege escalation flaw within Mozilla Firefox's WebExtensions architecture represents a critical security deficiency that allows untrusted extension code to bypass intended permission boundaries and execute actions with elevated system privileges. In modern web browsers, the WebExtensions API is designed to sandbox third-party extensions, ensuring they operate under strict least-privilege principles where access to sensitive browser features or user data requires explicit user consent and specific manifest permissions. This vulnerability undermines that isolation model by exploiting a logic error in how permission checks are validated during runtime operations. Specifically, an attacker-controlled malicious extension could manipulate internal state variables or exploit race conditions within the extension host process to trick the browser into granting access to restricted APIs without proper authorization verification.
From a technical perspective, this flaw likely stems from insufficient validation of context boundaries between different execution environments within the Firefox engine. When extensions interact with privileged components such as the parent process or specific internal modules like nsIWebNavigation or similar low-level interfaces, the security checks may fail to account for edge cases in permission inheritance or dynamic scope manipulation. By leveraging this weakness, a malicious actor can escalate their privileges from those of a standard extension user to levels comparable to browser internals, effectively neutralizing the sandbox protections that are fundamental to preventing cross-site scripting and data exfiltration attacks. This type of vulnerability is commonly categorized under CWE-269 Improper Privilege Management or CWE-732 Incorrect Permission Assignment for Critical Resource within industry standards, as it involves granting access rights to a principal that should not possess them based on its declared capabilities.
The operational impact of this privilege escalation is severe, particularly given the widespread adoption of browser extensions in enterprise and personal computing environments. An attacker who successfully exploits this vulnerability can bypass content security policies, read sensitive user data such as cookies, session tokens, or browsing history stored by other sites, and potentially inject malicious scripts into any webpage visited by the user. In an enterprise context where managed extensions are deployed for productivity tools, this flaw could allow a compromised extension to exfiltrate corporate credentials or intercept secure communications. Furthermore, because WebExtensions often have access to browser UI elements, the attacker might also manipulate the interface to deceive users into performing unintended actions, such as authorizing financial transactions or disabling security settings.
Mitigation strategies for this vulnerability primarily rely on timely software updates and robust extension governance policies. Users must ensure that Firefox is updated to version 158 or later, where the underlying permission validation logic has been patched to correctly enforce isolation boundaries between untrusted extension code and privileged browser components. For organizations deploying extensions via enterprise policy files, it is crucial to audit installed add-ons for known malicious behavior patterns and restrict installation sources to trusted repositories only. Additionally, security teams should monitor for anomalous network traffic originating from browser processes, as privilege escalation attempts often correlate with data exfiltration activities that can be detected by endpoint detection and response systems or web proxy logs aligned with MITRE ATT&CK techniques such as T1059 Command and Scripting Interpreter if the extension executes arbitrary code after gaining elevated privileges.