CVE-2016-9073 in Firefox
Summary
by MITRE
WebExtensions can bypass security checks to load privileged URLs and potentially escape the WebExtension sandbox. This vulnerability affects Firefox < 50.
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Analysis
by VulDB Data Team • 01/31/2018
The vulnerability identified as CVE-2016-9073 represents a critical sandbox escape mechanism within Firefox's WebExtensions architecture that undermines fundamental security boundaries designed to isolate browser extensions from privileged system resources. This flaw specifically targets Firefox versions prior to 50, where the extension framework failed to properly validate URL loading operations, allowing malicious or compromised extensions to circumvent security controls that should have prevented access to sensitive browser functionalities. The vulnerability stems from insufficient input validation and privilege escalation mechanisms within the extension loading subsystem, creating a pathway for extensions to access resources they should not be permitted to reach.
The technical implementation of this vulnerability exploits weaknesses in Firefox's WebExtensions API implementation where the security model relied on implicit trust mechanisms rather than explicit privilege checks. When extensions attempted to load privileged URLs, the browser failed to properly validate whether the extension had appropriate permissions or whether the requested access was within defined boundaries. This design flaw enabled extensions to bypass the sandbox protections by leveraging legitimate API calls that should have been restricted but were instead permitted to access sensitive browser internals. The vulnerability is classified under CWE-284, which addresses improper access control mechanisms, and specifically demonstrates the risks associated with insufficient privilege separation in browser extension architectures.
The operational impact of CVE-2016-9073 extends beyond simple privilege escalation to encompass potential full system compromise when extensions are executed with elevated privileges or when attackers can manipulate extension installation processes. Attackers could craft malicious extensions that appear legitimate but contain code designed to exploit this vulnerability, potentially gaining access to user data, cookies, local storage, and other sensitive browser resources. The sandbox escape capability means that even extensions installed from official sources could be weaponized, as the vulnerability allows for the execution of arbitrary code within the extension context that can then leverage the bypass to access privileged resources. This vulnerability directly maps to ATT&CK technique T1176, which covers the use of browser extensions for privilege escalation and persistence within compromised systems.
Mitigation strategies for this vulnerability require immediate patching of affected Firefox versions to 50 or later, where Mozilla implemented enhanced validation mechanisms and stricter privilege enforcement within the WebExtensions framework. Organizations should also implement extension whitelisting policies and regular security audits of installed extensions to reduce the risk of exploitation. The fix addresses the core issue by introducing explicit permission checks before URL loading operations and by strengthening the isolation boundaries between extension contexts and privileged browser components. Additionally, users should be educated about the risks of installing extensions from untrusted sources, as the vulnerability could be exploited through malicious extensions that appear legitimate but contain hidden malicious code designed to exploit the sandbox escape mechanism.