CVE-2021-21252: 
JavaScript vulnerability analysis and mitigation

The jQuery Validation Plugin (jquery-validation) before version 1.19.3 contains a Regular Expression Denial of Service (ReDoS) vulnerability identified as CVE-2021-21252. The vulnerability was discovered by Erik Krogh Kristensen from the GitHub Security Lab team and was disclosed on January 13, 2021. The affected component provides drop-in validation functionality for existing forms and is published as an npm package (GitHub Advisory, NVD).

The vulnerability stems from one or more poorly constructed regular expressions that can perform extremely badly when processing specially crafted input strings. The issue specifically affects the URL validation functionality, which uses a regex pattern copied from a gist. When processing malicious input, the regex can enter an exponential backtracking state, leading to excessive CPU usage (GitHub Security Lab, GitHub Advisory). The vulnerability has been assigned a CVSS score of 7.5 (High) with vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (NetApp Advisory).

Successful exploitation of this vulnerability could lead to a Denial of Service (DoS) condition. When a malicious string is provided for validation, it can cause the regular expression engine to enter a state of excessive CPU consumption, potentially causing the application to become unresponsive (NetApp Advisory, GitHub Security Lab).

The vulnerability can be triggered by submitting a specially crafted URL for validation. A proof of concept exists where submitting a URL with repeated sequences like 'http://foobar.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00.00' to a form field can cause the regex to loop indefinitely (GitHub Security Lab).

The vulnerability has been fixed in jquery-validation version 1.19.3. Users are advised to upgrade to this version or later to address the issue. The fix involves modifying the problematic regular expression pattern to prevent exponential backtracking (GitHub Advisory).