CVE-2021-23968 in Firefox
Summary
by MITRE • 02/26/2021
If Content Security Policy blocked frame navigation, the full destination of a redirect served in the frame was reported in the violation report; as opposed to the original frame URI. This could be used to leak sensitive information contained in such URIs. This vulnerability affects Firefox < 86, Thunderbird < 78.8, and Firefox ESR < 78.8.
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Analysis
by VulDB Data Team • 03/05/2021
This vulnerability represents a critical information disclosure flaw in web browsers that violates fundamental security principles of privacy protection and access control. The issue manifests when Content Security Policy (CSP) mechanisms attempt to block frame navigation while still inadvertently leaking sensitive information through violation reports. The technical implementation fails to properly sanitize the reporting mechanism, creating a scenario where attackers can extract confidential data that should remain hidden due to security policies. This flaw specifically impacts Firefox browsers and their derivatives, including Thunderbird and Firefox ESR versions prior to the mentioned security patches, representing a significant risk to users who rely on CSP protections for sensitive operations.
The underlying technical flaw stems from improper handling of redirect information within the browser's violation reporting system. When a frame navigation is blocked by CSP policies, the violation report mechanism incorrectly includes the full destination URI of the redirect rather than masking it with the original frame URI. This creates a information leakage channel where sensitive data embedded in URL parameters, fragments, or path components can be exposed to malicious actors. The vulnerability operates at the intersection of web security standards and browser implementation, where the expected behavior of CSP violation reporting conflicts with the actual implementation that fails to properly obscure sensitive information. This type of flaw aligns with CWE-200 (Information Exposure) and CWE-352 (Cross-Site Request Forgery) categories, as it enables unauthorized information disclosure through the violation reporting mechanism.
The operational impact of this vulnerability extends beyond simple information leakage, creating potential attack vectors for sophisticated reconnaissance and data exfiltration campaigns. Attackers can leverage this flaw to discover sensitive information contained in URLs that would normally be protected by CSP policies, including authentication tokens, session identifiers, personal data, or internal system references. The vulnerability is particularly dangerous in environments where users access sensitive applications through web browsers, as it undermines the security controls that organizations rely upon to protect against information disclosure attacks. This weakness affects the integrity of security policies that browsers implement to protect users from various types of malicious navigation and tracking attempts, effectively weakening the security posture of affected systems. The issue demonstrates how seemingly minor implementation details in security mechanisms can create significant vulnerabilities that compromise user privacy and data protection.
Organizations should implement immediate mitigations including updating to patched versions of Firefox, Thunderbird, and Firefox ESR as soon as possible, while also reviewing their CSP policies to ensure they do not inadvertently create additional exposure vectors. Security teams should monitor for exploitation attempts through violation report analysis and consider implementing additional network-level controls to detect unusual information disclosure patterns. The vulnerability highlights the importance of thorough testing of security policy implementations and proper validation of reporting mechanisms to prevent information leakage through indirect channels. Organizations should also consider implementing additional monitoring for CSP violation reports to identify potential exploitation attempts and maintain awareness of how browser security features interact with application security controls. This vulnerability serves as a reminder of the complexity involved in implementing robust web security and the need for comprehensive security testing across all browser components and features.