CVE-2015-7215 in Firefox
Summary
by MITRE
The importScripts function in the Web Workers API implementation in Mozilla Firefox before 43.0 allows remote attackers to bypass the Same Origin Policy by triggering use of the no-cors mode in the fetch API to attempt resource access that throws an exception, leading to information disclosure after a rethrow.
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Analysis
by VulDB Data Team • 07/01/2022
The vulnerability CVE-2015-7215 represents a critical security flaw in Mozilla Firefox's Web Workers API implementation that fundamentally undermines the browser's Same Origin Policy enforcement mechanism. This weakness specifically affects versions prior to Firefox 43.0 and exploits a subtle interaction between the importScripts function and the fetch API's no-cors mode. The vulnerability operates through a sophisticated bypass technique that leverages the way Firefox handles cross-origin resource requests within worker contexts, creating a pathway for unauthorized information disclosure that violates core web security principles.
The technical exploitation mechanism involves the importScripts function triggering the fetch API with no-cors mode enabled, which intentionally suppresses certain error conditions that would normally prevent cross-origin access. When the fetch operation encounters a resource that would typically be blocked by CORS policies, the no-cors mode causes an exception to be thrown, but this exception is subsequently rethrown in a manner that allows attackers to extract information about the attempted resource access. This creates a side-channel information leak where sensitive data about cross-origin resources can be inferred through careful observation of the rethrown exceptions, effectively circumventing the browser's fundamental security boundaries.
From an operational impact perspective, this vulnerability enables remote attackers to perform information disclosure attacks against web applications that rely on Firefox's security model for protection. The flaw allows attackers to gather intelligence about resources hosted on different origins, potentially exposing sensitive information such as file structures, server configurations, or other data that should remain protected by the Same Origin Policy. This capability significantly weakens the security posture of web applications and can be particularly dangerous in environments where users access multiple applications or services through the same browser instance, as it creates opportunities for cross-site information leakage that can be exploited in combination with other attack vectors.
The vulnerability aligns with CWE-200 (Information Exposure) and represents a specific instance of how browser security mechanisms can be subverted through improper exception handling and API interaction patterns. From an ATT&CK framework perspective, this vulnerability maps to T1071.001 (Application Layer Protocol: Web Protocols) and T1566 (Phishing) as it enables attackers to gather intelligence about target systems that can then be used to craft more sophisticated social engineering attacks. The flaw also demonstrates characteristics of T1003 (OS Credential Dumping) and T1083 (File and Directory Discovery) through its ability to expose information about remote resources that might contain sensitive data or system information.
Mitigation strategies for CVE-2015-7215 require immediate patching of affected Firefox installations to version 43.0 or later, where the vulnerability has been resolved through proper exception handling and enforcement of CORS policies within the Web Workers API. Organizations should also implement network-level monitoring to detect unusual patterns of cross-origin resource access attempts that might indicate exploitation attempts. Additionally, web application developers should review their applications for reliance on potentially vulnerable worker contexts and consider implementing additional security controls such as Content Security Policy headers that further restrict cross-origin resource access patterns. The fix implemented by Mozilla addresses the root cause by ensuring that exceptions thrown during no-cors fetch operations within worker contexts are properly handled without leaking information about the attempted resource access, thereby restoring proper Same Origin Policy enforcement.