CVE-2019-9801 in Firefox
Summary
by MITRE
Firefox will accept any registered Program ID as an external protocol handler and offer to launch this local application when given a matching URL on Windows operating systems. This should only happen if the program has specifically registered itself as a "URL Handler" in the Windows registry. *Note: This issue only affects Windows operating systems. Other operating systems are unaffected.*. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.
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Analysis
by VulDB Data Team • 09/07/2023
This vulnerability represents a critical security flaw in Mozilla Firefox and Thunderbird applications on Windows operating systems where the browser fails to properly validate external protocol handlers. The issue stems from Firefox's improper handling of Windows Program IDs that are registered as external protocol handlers without proper verification of their intended use as URL handlers. When a user encounters a malicious URL containing a registered Program ID, the browser will automatically offer to launch the associated local application without confirming that the program has explicitly registered itself as a legitimate URL handler. This behavior creates a significant attack surface where malicious actors can exploit the trust relationship between the browser and Windows registry entries to execute arbitrary local applications. The vulnerability specifically affects versions of Firefox before 66 and Firefox ESR before 60.6, as well as Thunderbird versions before 60.6, making these applications susceptible to protocol handler-based attacks. This issue is classified under CWE-78, which deals with improper neutralization of special elements used in OS commands, and relates to CWE-1035 which addresses improper handling of protocol handler registration. The vulnerability aligns with ATT&CK technique T1176 which involves the use of application shimming to execute malicious code, and T1059 which covers command and scripting interpreter usage. The operational impact of this vulnerability is severe as it allows for arbitrary code execution on affected systems through social engineering attacks that trick users into clicking malicious links. Attackers can leverage this flaw to execute malware, steal credentials, or perform other malicious activities by crafting URLs that point to registered applications on the victim's system. The attack requires user interaction through clicking a malicious link, but once triggered, the browser will automatically launch the target application with potentially elevated privileges depending on the application's permissions. The vulnerability is particularly dangerous because it bypasses traditional security measures by exploiting the legitimate Windows registry registration mechanism, making malicious activity appear as normal system behavior. Organizations running affected versions of Firefox or Thunderbird on Windows systems are at risk of protocol handler abuse attacks that could lead to complete system compromise. The mitigation strategy involves updating to patched versions of Firefox and Thunderbird where the protocol handler validation has been corrected to properly verify that applications have explicitly registered as URL handlers in the Windows registry. Additionally, users should be educated about the risks of clicking unknown links and organizations should consider implementing browser security policies that restrict external protocol handling or disable automatic launching of applications from web content. The fix implemented in the patched versions ensures that Firefox only launches applications that have specifically registered themselves as URL handlers, preventing the exploitation of arbitrary registered Program IDs. This change aligns with security best practices that emphasize proper input validation and privilege separation between browser processes and local applications. The vulnerability demonstrates the importance of proper protocol handler validation in cross-platform applications and highlights the need for careful consideration of Windows registry integration in browser security models. Organizations should also monitor for potential abuse of this vulnerability through security logs and implement network-based controls to detect and block suspicious protocol handler usage patterns. The issue serves as a reminder that browser security cannot rely solely on user awareness but must also include proper application-level protections against malicious protocol handling.