CVE-2021-29993 in Firefox
Summary
by MITRE • 11/03/2021
Firefox for Android allowed navigations through the `intent://` protocol, which could be used to cause crashes and UI spoofs. *This bug only affects Firefox for Android. Other operating systems are unaffected.*. This vulnerability affects Firefox < 92.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 11/06/2021
The vulnerability described in CVE-2021-29993 represents a critical security flaw in Firefox for Android that stems from improper handling of the intent:// protocol scheme. This protocol is commonly used on android devices to facilitate communication between applications and the operating system, allowing one app to request specific actions from another. The flaw exists specifically within Firefox for Android's implementation of this protocol handler, creating a pathway for malicious actors to exploit the browser's navigation system. The vulnerability is classified under CWE-264, which deals with permissions, privileges, and access control issues, as it involves unauthorized navigation capabilities that could be leveraged for system manipulation. This issue was particularly concerning because it affected only the android version of Firefox, leaving desktop and other mobile platforms unaffected, which indicates a platform-specific implementation weakness rather than a fundamental architectural flaw.
The technical exploitation of this vulnerability occurs when Firefox for Android processes navigation requests through the intent:// protocol handler without proper validation or sanitization of the URI parameters. This allows attackers to craft malicious URLs that can trigger unexpected behavior in the browser, potentially leading to application crashes or more sophisticated user interface spoofing attacks. The implementation flaw specifically relates to how the browser parses and processes these protocol-specific URLs, failing to properly validate the intent parameters before executing navigation commands. The vulnerability enables an attacker to manipulate the browser's behavior in ways that could disrupt normal operation or create deceptive interfaces that mislead users about the actual destination or nature of their browsing activities. This type of attack vector aligns with ATT&CK technique T1059.007 for application layer protocol execution and T1566 for phishing attacks through malicious links.
The operational impact of this vulnerability extends beyond simple browser instability to potentially enable more sophisticated attack scenarios that could compromise user security and privacy. When exploited, the vulnerability could cause Firefox for Android to crash unexpectedly, forcing users to restart the browser and potentially lose unsaved work or session data. More concerning is the potential for UI spoofing attacks where malicious actors could manipulate the browser interface to display false content or redirect users to malicious sites. This could be particularly dangerous in phishing scenarios where users might be tricked into believing they are visiting legitimate websites while actually being directed to fraudulent pages. The vulnerability also represents a potential escalation path for attackers who might use it as a foothold for more comprehensive attacks against the device or user data. Users could be exposed to various security risks including credential theft, data exfiltration, or further exploitation through the compromised browser environment.
Mitigation strategies for this vulnerability should focus on immediate patching and updates to ensure Firefox for Android is running version 92 or later where the flaw has been addressed. Users should be advised to avoid clicking on suspicious links or downloading content from untrusted sources that might contain malicious intent:// URLs. Organizations should implement network-level filtering to block potentially malicious protocol handlers and consider deploying browser security extensions that can monitor and restrict protocol handling. Security teams should also conduct regular vulnerability assessments to identify similar protocol handling issues in other browser implementations or applications that might be susceptible to similar flaws. The fix implemented by Mozilla likely involved strengthening the validation of intent:// URLs and implementing proper sanitization of parameters before processing navigation requests. This approach aligns with the principle of least privilege and input validation best practices recommended in security frameworks such as those outlined in the OWASP Top Ten and NIST cybersecurity guidelines. Additionally, users should be educated about the risks of clicking on untrusted links and the importance of keeping their browser software updated to protect against known vulnerabilities.