CVE-2021-23969 in Firefox
Summary
by MITRE • 02/26/2021
As specified in the W3C Content Security Policy draft, when creating a violation report, "User agents need to ensure that the source file is the URL requested by the page, pre-redirects. If that’s not possible, user agents need to strip the URL down to an origin to avoid unintentional leakage." Under certain types of redirects, Firefox incorrectly set the source file to be the destination of the redirects. This was fixed to be the redirect destination's origin. 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
The vulnerability identified as CVE-2021-23969 represents a critical information disclosure flaw in Mozilla Firefox and related products that directly impacts the implementation of Content Security Policy (CSP) violation reporting mechanisms. This security gap stems from the improper handling of redirect chains during CSP violation reporting, where the browser incorrectly includes full redirect paths in violation reports instead of adhering to W3C specifications that mandate origin-level sanitization. The flaw specifically affects the way user agents process URLs when generating CSP violation reports, creating potential exposure of sensitive information that should remain hidden from malicious actors. The vulnerability manifests when Firefox encounters certain redirect scenarios and incorrectly preserves the final redirect destination URL rather than stripping it to the origin level as required by CSP standards.
The technical implementation flaw occurs within Firefox's Content Security Policy violation reporting subsystem, where the browser fails to properly sanitize URL sources during redirect processing. According to the W3C Content Security Policy specification referenced in the vulnerability description, user agents must ensure that source file information in violation reports reflects the original URL requested by the page pre-redirects, or if that's not possible, must strip the URL down to the origin level to prevent unintentional leakage of information. However, Firefox's implementation incorrectly set the source file to the destination of redirects, effectively exposing the complete redirect path including potentially sensitive parameters, directory structures, or internal routing information that could be leveraged by attackers for reconnaissance purposes.
This vulnerability creates significant operational impact for organizations relying on Firefox for web browsing and email clients, particularly in environments where Content Security Policy enforcement is critical for application security. The exposure of full redirect paths in CSP violation reports can inadvertently leak sensitive information about internal network structures, application routing, or system configurations that attackers could use to plan more sophisticated attacks. The vulnerability affects not only standard Firefox installations but also Thunderbird email clients and Firefox Extended Support Release versions, indicating a widespread impact across Mozilla's product ecosystem. Security teams relying on CSP violation reports for monitoring and threat detection may receive misleading information that could obscure actual security incidents or provide attackers with additional reconnaissance data.
The fix implemented for CVE-2021-23969 addresses the core issue by ensuring Firefox properly handles redirect chains according to W3C specifications, specifically by setting the source file to the redirect destination's origin rather than the complete destination URL. This remediation aligns with established security practices outlined in CWE-200 (Information Exposure) and follows the principle of least privilege in information disclosure prevention. Organizations should prioritize updating affected versions to Firefox 86, Thunderbird 78.8, and Firefox ESR 78.8 to mitigate this vulnerability, as the flaw represents a direct violation of information security principles and could enable attackers to gather intelligence about web application architectures and internal routing mechanisms. The vulnerability also relates to ATT&CK technique T1068 (Exploitation for Privilege Escalation) and T1566 (Phishing) where such information disclosure could support more advanced attack vectors.