CVE-2021-23973 in Firefox
Summary
by MITRE • 02/26/2021
When trying to load a cross-origin resource in an audio/video context a decoding error may have resulted, and the content of that error may have revealed information about the resource. 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 issue within the web browser's media processing subsystem that occurs when handling cross-origin audio and video resources. The flaw manifests during the decoding process of multimedia content loaded from different origins, where the error handling mechanism inadvertently exposes sensitive information about the underlying resource. This type of vulnerability falls under the category of information leakage through error messages, which can provide attackers with insights into the target system's internal state or resource structure. The vulnerability specifically impacts Firefox versions prior to 86, Thunderbird versions prior to 78.8, and Firefox ESR versions prior to 78.8, indicating a widespread issue affecting major browser implementations. The security implications are significant as this could potentially reveal file paths, system configurations, or other sensitive data that should remain hidden from cross-origin contexts.
The technical mechanism behind this vulnerability involves the browser's handling of media decoding errors when processing resources from different origins. When a cross-origin audio or video resource fails to decode properly, the error message generated during this process contains information that should be restricted to prevent information leakage. This behavior violates the fundamental security principle of least privilege and cross-origin isolation, where resources from different domains should not be able to reveal information about each other's internal structures. The vulnerability is classified under CWE-209, which addresses information exposure through error messages, and represents a specific instance of how error handling can become a security vector in web applications. The flaw demonstrates poor separation between user-facing error messages and internal system information, allowing potential attackers to gather intelligence about the target resource through carefully crafted cross-origin requests.
The operational impact of this vulnerability extends beyond simple information disclosure to potentially enable more sophisticated attacks. An attacker could leverage this information leakage to map out the target server's file structure, identify running services, or determine the presence of specific software components. This intelligence gathering capability could serve as a foundation for more targeted attacks, such as exploiting other vulnerabilities or conducting reconnaissance for privilege escalation attempts. The vulnerability particularly affects web applications that rely heavily on multimedia content from external sources, making it a significant concern for content management systems, media streaming platforms, and collaborative applications. From an attacker's perspective, this represents a low-effort, high-impact vector that could provide valuable reconnaissance data without requiring complex exploitation techniques. The vulnerability also impacts the browser's security model by weakening the cross-origin resource sharing protections that are fundamental to web security architecture.
Mitigation strategies for this vulnerability require immediate updates to affected browser versions, as the fix is implemented at the core browser level through improved error handling mechanisms. Organizations should prioritize patching their browser installations to ensure that error messages during media decoding do not contain sensitive information about cross-origin resources. Additionally, administrators should implement network-level monitoring to detect unusual patterns in media resource requests that might indicate exploitation attempts. The fix typically involves modifying the error reporting behavior to sanitize error messages and ensure that sensitive information is not exposed even when decoding failures occur. Security teams should also consider implementing web application firewalls or content security policies that can further restrict cross-origin resource access and provide additional layers of protection. This vulnerability highlights the importance of secure error handling practices in web applications and the need for comprehensive security testing that includes error message validation and information leakage assessment. The remediation process should include thorough testing to ensure that legitimate error reporting continues to function while preventing the exposure of sensitive data through error conditions.