CVE-2019-11701 in Firefox
Summary
by MITRE
The default webcal: protocol handler will load a web site vulnerable to cross-site scripting (XSS) attacks. This default was left in place as a legacy feature and has now been removed. *Note: this issue only affects users with an account on the vulnerable service. Other users are unaffected.*. This vulnerability affects Firefox < 67.
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Analysis
by VulDB Data Team • 06/06/2024
The vulnerability described in CVE-2019-11701 represents a significant security flaw in the Firefox web browser's handling of the webcal: protocol, which is used for calendar subscription links. This issue stems from the browser's default behavior of automatically loading webcal: protocol handlers that point to web sites susceptible to cross-site scripting attacks. The vulnerability exists because Firefox maintained a legacy feature that allowed these protocol handlers to function without proper security restrictions, creating an avenue for malicious actors to exploit user sessions through XSS attacks. The webcal: protocol is commonly used for calendar integration and subscription services, making it a potentially dangerous vector for attackers targeting Firefox users. This flaw specifically affected Firefox versions prior to 67, where the browser failed to properly validate or sanitize URLs accessed through the webcal: protocol handler, leaving users exposed to potential XSS execution.
The technical implementation of this vulnerability involves the browser's protocol handler mechanism which automatically processes webcal: URLs without sufficient security checks. When a user clicks on a webcal: link pointing to a malicious website, Firefox would execute the protocol handler and load the content without proper sandboxing or input validation. This behavior creates a path for XSS attacks where malicious scripts embedded in calendar feeds or subscription URLs could execute within the context of the user's session, potentially leading to session hijacking, credential theft, or other malicious activities. The vulnerability operates under CWE-79 which specifically addresses cross-site scripting flaws, and aligns with ATT&CK technique T1213 which covers data from information repositories. The flaw essentially allows attackers to bypass standard browser security mechanisms by leveraging the legitimate webcal: protocol handler functionality.
The operational impact of CVE-2019-11701 is significant for Firefox users who interact with calendar subscription services, particularly those who have accounts on vulnerable web services. Attackers could craft malicious calendar feeds that would automatically execute XSS payloads when users clicked on subscription links, potentially compromising user sessions and sensitive information. The vulnerability required specific conditions to be exploited, as it only affected users with accounts on vulnerable services, but this limitation did not prevent the attack from being highly effective when successful. Organizations using Firefox browsers were particularly at risk if their employees regularly subscribed to calendar services or integrated calendar functionality into their workflows. The attack vector was particularly dangerous because it could be concealed within seemingly legitimate calendar subscription links, making it difficult for users to identify the malicious nature of the content.
Mitigation strategies for this vulnerability included upgrading to Firefox version 67 or later where the problematic webcal: protocol handler was removed, as well as implementing proper input validation on web services that handle calendar subscriptions. Network administrators should have enforced browser security policies requiring regular updates and monitored for potentially malicious calendar feeds. Users needed to be educated about the risks of clicking on calendar subscription links from untrusted sources and should have been advised to verify the legitimacy of calendar feeds before subscribing. The fix implemented by Mozilla involved removing the legacy webcal: protocol handler functionality that was vulnerable to XSS attacks, aligning with security best practices for protocol handling and input validation. Organizations should have also implemented web application firewalls or content filtering solutions to prevent malicious calendar feeds from being processed by their systems. This vulnerability highlighted the importance of regularly reviewing and removing legacy features that may introduce security risks, particularly those involving protocol handlers and user interaction with external content.