CVE-2017-5386 in Firefox
Summary
by MITRE
WebExtension scripts can use the "data:" protocol to affect pages loaded by other web extensions using this protocol, leading to potential data disclosure or privilege escalation in affected extensions. This vulnerability affects Firefox ESR < 45.7 and Firefox < 51.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability described in CVE-2017-5386 represents a critical security flaw in Firefox's WebExtension implementation that exploits the improper handling of the "data:" protocol within browser extension environments. This issue stems from the lack of proper isolation mechanisms between different WebExtensions when they process content loaded via the data protocol, creating a potential attack vector that could be exploited to compromise extension security boundaries. The vulnerability specifically impacts Firefox Extended Support Release versions prior to 45.7 and standard Firefox versions prior to 51, indicating a widespread exposure across multiple browser releases. The root cause lies in how Firefox handles cross-extension communication when processing data URLs, which should normally be isolated but instead allows malicious extensions to access or manipulate content from other extensions.
The technical exploitation of this vulnerability occurs through the manipulation of data protocol URLs within WebExtension scripts, enabling an extension to inject or modify content on pages that are loaded by other extensions using the same protocol. This creates a privilege escalation scenario where a less privileged extension can potentially access sensitive data or execute code within the context of a more privileged extension. The flaw operates at the intersection of browser security boundaries, specifically targeting the extension sandboxing mechanisms that are designed to prevent unauthorized access between different extensions. According to CWE classification, this vulnerability maps to CWE-284 Access Control, as it involves improper access control between different extension contexts. The attack pattern aligns with ATT&CK technique T1068, which describes privilege escalation through exploitation of application vulnerabilities.
The operational impact of this vulnerability extends beyond simple data disclosure, potentially enabling full privilege escalation within the browser extension environment. An attacker could leverage this flaw to access sensitive user data, manipulate extension functionality, or even execute arbitrary code within the browser context. The risk is particularly severe because WebExtensions often have elevated permissions and access to user data, making them attractive targets for exploitation. This vulnerability essentially undermines the fundamental security model of Firefox's extension architecture, where extensions are expected to operate in isolated environments. The potential for data leakage means that personal information, browsing history, or other sensitive data could be accessed by malicious extensions, while the privilege escalation capability could allow for complete compromise of the user's browser session. Organizations relying on Firefox for enterprise environments would face significant security risks, as this vulnerability could be exploited to gain unauthorized access to sensitive corporate data through compromised extensions.
Mitigation strategies for CVE-2017-5386 focus primarily on immediate browser updates to versions that address the underlying security flaw. Users and administrators should prioritize upgrading to Firefox ESR 45.7 or Firefox 51 and later versions where the vulnerability has been patched. Additionally, organizations should implement extension management policies that limit the permissions granted to WebExtensions, particularly those that handle sensitive data or require elevated privileges. Regular monitoring of extension behavior and implementation of security auditing procedures can help detect potential exploitation attempts. The fix implemented by Mozilla likely involved strengthening the isolation mechanisms between different WebExtensions when processing data protocol URLs, ensuring that extensions cannot access or manipulate content from other extensions without proper authorization. Security teams should also consider implementing network-level monitoring to detect unusual patterns of data protocol usage that might indicate exploitation attempts, while maintaining comprehensive logging of extension activities for forensic analysis purposes.