CVE-2017-7816 in Firefox
Summary
by MITRE
WebExtensions could use popups and panels in the extension UI to load an "about:" privileged URL, violating security checks that disallow this behavior. This vulnerability affects Firefox < 56.
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Analysis
by VulDB Data Team • 11/21/2019
The vulnerability described in CVE-2017-7816 represents a critical security flaw in Firefox's WebExtensions implementation that undermined the browser's privilege separation mechanisms. This issue specifically targeted the extension user interface components, namely popups and panels, which are designed to provide interactive elements within the browser's extension ecosystem. The flaw allowed malicious extensions to bypass security controls that were specifically implemented to prevent the loading of privileged URLs, particularly those beginning with the "about:" protocol prefix that are normally restricted to browser internals. Such privileged URLs typically contain sensitive information and system-level functionality that should remain inaccessible to third-party extensions to maintain the integrity of the browser's security model.
The technical nature of this vulnerability stems from improper validation within Firefox's WebExtensions API implementation where the security checks designed to prevent loading of about: URLs in extension UI elements were circumvented. This occurred because the popup and panel mechanisms in the extension interface did not properly enforce the same privilege restrictions that apply to other browser contexts. The flaw essentially created a pathway for extensions to access privileged information that should have been protected from extension code execution. According to CWE-284, this represents an improper access control vulnerability where the extension system failed to properly enforce access restrictions for privileged resources. The vulnerability existed in Firefox versions prior to 56, indicating that the security model had been compromised for an extended period, potentially allowing attackers to exploit this weakness to gain unauthorized access to sensitive browser functionality.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it could enable attackers to craft malicious extensions that could access and potentially manipulate sensitive browser data. When an extension loads a privileged about: URL through a popup or panel, it could potentially access internal browser state, user preferences, or even system-level information that would normally be protected. This could lead to information disclosure, user tracking, or even more sophisticated attacks that leverage the extension's ability to access privileged resources. The attack surface is particularly concerning because popups and panels are commonly used interactive elements within browser extensions, making this vulnerability exploitable through normal extension installation and usage patterns. This flaw aligns with ATT&CK technique T1176 which involves the use of browser extensions to maintain persistence and access to system resources.
Mitigation of this vulnerability required updating Firefox to version 56 or later where the security checks were properly implemented to prevent loading of about: URLs through extension UI elements. Users were advised to immediately update their Firefox installations and review their installed extensions for any malicious activity that might have occurred during the vulnerability window. Security researchers recommended that administrators monitor for any extensions that might have been compromised or modified during the period when this vulnerability was present, as the ability to load privileged URLs could have enabled sophisticated attacks. The fix implemented by Mozilla involved strengthening the validation logic within the WebExtensions API to ensure that popup and panel contexts properly enforce the same privilege restrictions that apply to other browser components, preventing extensions from accessing privileged about: URLs through these user interface mechanisms. Organizations should have also reviewed their extension management policies to ensure that only trusted extensions were installed and regularly updated to maintain security posture.