CVE-2021-23971 in Firefox
Summary
by MITRE • 02/26/2021
When processing a redirect with a conflicting Referrer-Policy, Firefox would have adopted the redirect's Referrer-Policy. This would have potentially resulted in more information than intended by the original origin being provided to the destination of the redirect. This vulnerability affects Firefox < 86.
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Analysis
by VulDB Data Team • 03/05/2021
The vulnerability identified as CVE-2021-23971 represents a significant security flaw in Mozilla Firefox's handling of HTTP redirect responses and referrer policy enforcement mechanisms. This issue stems from Firefox's improper processing of referrer policies when encountering redirects, specifically when the original page and the redirect destination specify conflicting referrer policies. The vulnerability exists within the browser's HTTP request processing pipeline where the referrer policy of the redirect target overrides the policy of the originating page without proper validation or consideration of the security implications. This behavior creates an unintended information disclosure scenario where sensitive referrer information may be transmitted to destinations that were not intended to receive such data, potentially compromising user privacy and data protection.
The technical flaw manifests when Firefox processes an HTTP redirect response that contains a referrer-policy header that conflicts with the referrer policy of the original request. According to the HTTP specification and security best practices, when a redirect occurs, the browser should maintain the security context established by the original origin, particularly regarding referrer information. However, Firefox version 86 and earlier incorrectly adopted the referrer policy specified in the redirect response, regardless of whether it would result in a less secure or more permissive information flow. This processing error falls under CWE-200, Information Exposure, as it leads to unintended disclosure of referrer information, and specifically relates to CWE-347, Improper Verification of Cryptographic Signature, in the context of security policy enforcement.
The operational impact of this vulnerability extends beyond simple information disclosure to potentially enable various attack vectors including tracking, user profiling, and cross-site request forgery exploitation. When users navigate through redirects, particularly those involving third-party services or advertising networks, the browser may inadvertently transmit more referrer information than originally intended. This could expose sensitive data such as search queries, personal identifiers, or session information to destinations that should not have access to such information. The vulnerability particularly affects scenarios where the original page employs a strict referrer policy to protect user privacy while the redirect target specifies a more permissive policy, creating an information leakage channel that attackers could exploit to gather intelligence about user behavior or access restricted information.
Organizations and users should prioritize immediate remediation by upgrading to Firefox version 86 or later, which contains the necessary patches to address this referrer policy handling issue. The fix implemented in Firefox 86 ensures that the browser properly validates referrer policies during redirect processing and maintains the security context of the originating page. Security teams should also consider implementing network-level monitoring to detect potential exploitation attempts involving referrer policy manipulation and review existing security policies to ensure that referrer policy configurations align with organizational security requirements. This vulnerability demonstrates the importance of proper policy enforcement in web browsers and highlights the need for comprehensive testing of HTTP response handling mechanisms to prevent unintended information flows that could compromise user privacy and security. The issue also aligns with ATT&CK technique T1531, Account Access Removal, as improper referrer policy handling can lead to unauthorized access to information through indirect means.