CVE-2021-3803: 
JavaScript vulnerability analysis and mitigation

CVE-2021-3803 is a vulnerability discovered in the nth-check NodeJS module, which is used to parse and compile nth-checks found in CSS 3's nth-child() and nth-last-of-type(). The vulnerability was identified on September 15, 2021, affecting the nth-check library's parsing functionality (CVE Mitre).

The vulnerability stems from inefficient regular expression complexity in the nth-check library's parsing mechanism. The original implementation used a regex-based parser that could lead to excessive resource consumption. The issue was addressed by replacing the regex-based parser with a hand-rolled parser implementation (GitHub Commit).

When exploited, this vulnerability could lead to consuming a significant amount of system resources during the execution of nth-check parsing operations, potentially resulting in a denial of service condition (Red Hat).

The vulnerability has been fixed in various versions across different distributions. For Debian 10 (buster), the fix was implemented in version 1.0.1-1+deb10u1. The core fix involved replacing the vulnerable regex-based parser with a more efficient hand-rolled parser implementation (Debian LTS, GitHub Commit).