CVE-2026-109778 in Firefox
Summary
by MITRE • 10/13/2026
Privilege escalation in the WebExtensions component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5.
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Analysis
by VulDB Data Team • 10/13/2026
The identified security issue represents a critical privilege escalation flaw located within the WebExtensions subsystem of Mozilla Firefox, specifically affecting versions prior to 158 and Firefox Extended Support Release (ESR) version 153.5. This vulnerability stems from an improper enforcement of access controls or logic errors in how extension contexts interact with browser internals, allowing maliciously crafted extensions or compromised web content leveraging WebExtensions APIs to bypass intended security boundaries. In the context of modern browsers, WebExtensions are designed to operate within a sandboxed environment that restricts their ability to modify core browser behavior or access sensitive user data outside their designated scope. When this isolation is breached due to a logic flaw in the extension host process, an attacker can elevate privileges from a low-privileged extension context to a higher-level system state, effectively gaining capabilities reserved for trusted components or even the main browser process.
From a technical perspective, privilege escalation vulnerabilities of this nature often involve improper handling of inter-process communication channels, race conditions in permission checks, or flaws in how content scripts are injected and executed within page contexts. The WebExtensions architecture relies on strict separation between extension code, background pages, and UI processes to maintain stability and security. If the validation logic governing these boundaries fails to correctly verify permissions before granting access to sensitive APIs such as those controlling browser history, cookies, or network requests, an attacker can exploit this gap to perform actions that should be prohibited. This could include reading private browsing data, modifying secure settings, or executing arbitrary code with elevated privileges within the browser process space, thereby undermining the fundamental trust model of the application.
The operational impact of such a vulnerability is severe, as it compromises both user privacy and system integrity. An attacker who successfully exploits this flaw can potentially exfiltrate sensitive information stored by extensions, manipulate browsing sessions to facilitate phishing or man-in-the-middle attacks, or install persistent malware components that survive browser restarts due to the elevated privileges obtained during exploitation. Furthermore, because WebExtensions are widely used for ad blocking, password management, and productivity enhancements, a compromise in this layer can affect millions of users simultaneously if they have installed malicious extensions from public repositories or if their legitimate extensions are compromised through supply chain attacks. The ability to escalate privileges means that the attack surface expands beyond simple data theft to include full control over browser functionality, which can be leveraged for further lateral movement within a user's environment or network infrastructure.
To mitigate this risk, users must ensure they are running Firefox version 158 or later, as well as any ESR release at version 153.5 or higher where the patch has been applied. Administrators managing enterprise deployments should prioritize updating their browser installations across all endpoints to close this window of exposure. Additionally, organizations and individual users should audit installed extensions for legitimacy and remove those that are no longer necessary or have suspicious permissions profiles. Relying on official extension stores with rigorous review processes reduces the likelihood of installing malicious code that could attempt to exploit such vulnerabilities. Security teams should also monitor for indicators of compromise related to unauthorized privilege escalation attempts, although detection may be challenging without specialized endpoint protection tools capable of monitoring browser process behavior and API calls.
This vulnerability aligns with Common Weakness Enumeration (CWE) categories such as CWE-269 Improper Privilege Management or CWE-787 Out-of-bounds Write if the flaw involves memory corruption leading to control flow hijacking, though specific classification depends on the exact technical implementation details of the fix. In terms of adversary tactics, this aligns with MITRE ATT&CK techniques related to privilege escalation and defense evasion, particularly those involving client-side exploitation or abuse of trusted software components. The resolution underscores the importance of continuous patch management in maintaining browser security posture against evolving threats that target complex subsystems like extension frameworks.