CVE-2021-23958: 
NixOS vulnerability analysis and mitigation

CVE-2021-23958 is a security vulnerability discovered in Firefox versions prior to 85 that was disclosed on January 26, 2021. The vulnerability allowed the browser to be confused into transferring a screen sharing state into another tab, which could lead to unintended information disclosure. This security flaw affected the Firefox browser's screen sharing functionality (Mozilla Advisory).

The vulnerability was classified with a moderate severity impact and was identified as a privacy concern related to screen sharing permissions. When a user had an active screen sharing session and closed the tab, there was approximately a 20% chance that the screen capture would continue briefly into the next tab that became active, potentially exposing sensitive information from unintended tabs. The issue was timing-related and could be reproduced on both Windows 10 and macOS systems (Bugzilla).

The primary impact of this vulnerability was the potential exposure of sensitive information from unintended browser tabs during screen sharing sessions. If exploited, an attacker could capture content from tabs that the user had not intended to share, leading to privacy violations and potential information disclosure (Mozilla Advisory).

The vulnerability required specific timing conditions to be exploited, with a reported success rate of approximately 1 in 5 attempts. While proof-of-concept code was developed to demonstrate the vulnerability, there were no reported instances of this vulnerability being exploited in the wild (Bugzilla).

The vulnerability was fixed in Firefox version 85. Users were advised to update their Firefox installations to version 85 or later to receive the security patch. The fix involved modifying how the browser handles tab closing during active screen sharing sessions (Mozilla Advisory).