CVE-2019-9809 in Firefox
Summary
by MITRE
If the source for resources on a page is through an FTP connection, it is possible to trigger a series of modal alert messages for these resources through invalid credentials or locations. These messages cannot be immediately dismissed, allowing for a denial of service (DOS) attack. This vulnerability affects Firefox < 66.
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Analysis
by VulDB Data Team • 09/07/2023
This vulnerability exists in Firefox versions prior to 66 and represents a denial of service condition that can be triggered through malicious FTP connections. The flaw occurs when web pages load resources through FTP protocols, creating a scenario where invalid credentials or non-existent FTP locations can generate cascading modal alert dialogs. These alerts are designed to be persistent and cannot be immediately dismissed by users, effectively blocking normal browser functionality and creating a denial of service condition that prevents users from interacting with the affected web page or browser interface. The vulnerability specifically impacts the browser's handling of FTP resource loading mechanisms and demonstrates a critical flaw in how Firefox manages error conditions for external protocol connections.
The technical implementation of this vulnerability exploits the browser's resource loading behavior when encountering FTP connections that fail to authenticate or resolve properly. When Firefox attempts to load resources from an FTP server using invalid credentials or connecting to a non-existent location, the browser generates modal dialog boxes that require user interaction to dismiss. However, in this case, these dialogs remain active and cannot be dismissed through normal user interface interactions, creating a persistent blocking condition that prevents normal browser operations. This behavior violates standard user interface design principles and represents a failure in the browser's error handling mechanisms for external protocol connections.
The operational impact of CVE-2019-9809 extends beyond simple user inconvenience to represent a significant security concern for web browsing environments. Attackers can craft malicious web pages that establish FTP connections to non-responsive or malicious FTP servers, triggering the persistent modal dialog behavior that effectively freezes the browser interface. This condition can be exploited across multiple user sessions and may require complete browser restarts to resolve, making it particularly disruptive to normal web browsing activities. The vulnerability affects the browser's ability to maintain responsive user interfaces and can be leveraged in social engineering attacks where users are forced to abandon browsing sessions or experience complete browser lockups.
Organizations and users should immediately upgrade to Firefox version 66 or later to address this vulnerability, as no effective workarounds exist for the underlying issue. The fix implemented in Firefox 66 addresses the modal dialog persistence problem by ensuring that FTP connection errors generate dismissible alerts or are handled through alternative error presentation mechanisms. Security teams should monitor for exploitation attempts targeting this vulnerability in environments where older Firefox versions remain in use, particularly in corporate settings where browser updates may be delayed. This vulnerability aligns with CWE-254, which addresses security weaknesses in error handling and user interface design, and represents a specific implementation gap in the ATT&CK framework's defense evasion techniques category where browser-based denial of service attacks can be executed through user interface manipulation.