CVE-2020-26967: 
NixOS vulnerability analysis and mitigation

CVE-2020-26967 is a security vulnerability discovered in Mozilla Firefox's Screenshots feature that affects Firefox versions prior to 83.0. The vulnerability was disclosed on November 17, 2020, and involves a potential security issue where a malicious web page could manipulate Firefox Screenshots through Mutation Observers (Mozilla Advisory, NVD).

The vulnerability occurs when listening for page changes with a Mutation Observer, where a malicious web page could confuse Firefox Screenshots into interacting with elements other than those it injected into the page. The issue stems from the preview iframe lacking proper URL validation checks that were present in other iframes used by the Screenshots feature. The vulnerability has been assigned a CVSS v3.1 Base Score of 6.5 (Medium) with vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N (NVD).

When exploited, this vulnerability could lead to internal errors and unexpected behavior in the Screenshots code. While rated as low impact by Mozilla, it could potentially allow attackers to manipulate the Screenshots feature's functionality and cause unintended interactions with page elements (Mozilla Advisory).

The vulnerability requires user interaction and can be triggered when a malicious web page uses Mutation Observers to interfere with the Screenshots feature's normal operation. The exploit involves manipulating the preview iframe's behavior through specific timing and element manipulation (Bugzilla).

The vulnerability was fixed in Firefox 83.0. The fix involved implementing proper validation checks for the preview iframe to ensure it matches the expected blank.html URL, similar to existing checks in other Screenshots feature iframes. Users are advised to upgrade to Firefox 83.0 or later to receive the security fix (Mozilla Advisory).