CVE-2020-12407: 
NixOS vulnerability analysis and mitigation

CVE-2020-12407 is a vulnerability discovered in Mozilla Firefox's WebRender component that affects versions prior to Firefox 77. The vulnerability was discovered by Mozilla Developer Nicolas Silva and disclosed on June 2, 2020. When using WebRender, Firefox would under certain conditions leak arbitrary GPU memory to the visible screen when using the border-image CSS directive (Mozilla Advisory, NVD).

The vulnerability occurs when processing border-image CSS directives in WebRender, causing uninitialized parts of GPU texture memory to be displayed on the screen. The issue was 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:H/I:N/A:N. The vulnerability is specifically related to WebRender's handling of zero slice size in border-image segments (Mozilla Bug).

The vulnerability results in leaked GPU memory content being visible to the user on the screen. In some cases, this could include content from other processes, including live video streams from other browser windows. However, the leaked memory content was not observable or accessible by web content, limiting the security impact (Mozilla Advisory, Mozilla Bug).

While the vulnerability allows GPU memory content to be displayed on screen, exploitation is limited as web pages cannot read the leaked pixels back from WebRender. Access to the leaked content would require additional privileged APIs or explicit user permissions, such as screen sharing features. The vulnerability is considered moderately difficult to exploit in a meaningful way (Mozilla Bug).

The vulnerability was fixed in Firefox 77. Users should upgrade to Firefox version 77 or later to receive the fix. The patch prevents drawing border-image segments with zero slice size, which was the root cause of the memory leak (Mozilla Advisory).