CVE-2026-16396 in Firefox
Summary
by MITRE • 07/21/2026
Privilege escalation in WebExtensions. This vulnerability was fixed in Firefox 153 and Firefox ESR 140.13.
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Analysis
by VulDB Data Team • 07/21/2026
This privilege escalation vulnerability in WebExtensions represents a critical security flaw that allowed malicious extensions to bypass Firefox's security model and execute code with elevated privileges. The vulnerability stems from improper handling of extension permissions and sandbox boundaries within the browser's extension architecture, creating a pathway for attackers to escalate their privileges from regular user context to system-level access. Such flaws are particularly dangerous because they undermine the fundamental security isolation that WebExtensions are designed to provide between different parts of the browser ecosystem and the underlying operating system.
The technical implementation of this vulnerability likely involves a flaw in how Firefox validates or enforces permission boundaries when processing extension manifests, API calls, or runtime behaviors. Attackers could exploit this weakness by crafting malicious extensions that appear legitimate but contain code designed to bypass normal security restrictions. The vulnerability may involve improper validation of extension capabilities, inadequate sandboxing mechanisms, or flawed privilege delegation processes within the browser's extension management system. This type of flaw typically falls under CWE-276 which addresses improper permissions and access control, specifically targeting the weakness in how access controls are enforced for privileged operations.
The operational impact of this privilege escalation vulnerability extends far beyond simple data theft or website manipulation. Successful exploitation could enable attackers to read arbitrary files from the user's system, modify browser configuration settings, install additional malicious software, or even execute arbitrary commands with the privileges of the victim's account. The implications are particularly severe because Firefox extensions run in a trusted environment that has access to sensitive user data and browser functionality. This vulnerability creates an attack surface where malicious actors could compromise user privacy, steal credentials, or establish persistent backdoors within the browser environment.
Mitigation strategies for this specific vulnerability involve immediate deployment of updated Firefox versions, specifically Firefox 153 and Firefox ESR 140.13, which contain patches addressing the privilege escalation flaw. Organizations should also implement extension whitelisting policies, regularly audit installed extensions, and monitor for suspicious extension behavior. The fix likely involves strengthening permission validation mechanisms, improving sandbox isolation boundaries, and ensuring proper enforcement of extension capability restrictions. Additionally, users should be educated about extension installation practices and the importance of only installing extensions from trusted sources. This vulnerability highlights the critical need for continuous security assessment in browser environments and demonstrates how even seemingly minor flaws in privilege management can lead to significant security compromises.
This type of vulnerability aligns with attack patterns documented in the ATT&CK framework under privilege escalation techniques, specifically targeting the use of malicious extensions as a means to gain elevated system access. The remediation approach emphasizes the importance of maintaining up-to-date browser software and implementing layered security controls that address both the immediate vulnerability and broader extension management practices. Security teams should also consider monitoring for unusual extension behavior patterns that might indicate exploitation attempts, particularly focusing on extensions that request unusual permissions or exhibit unexpected network activity.