CVE-2021-21252: jQuery Validation Plugin DoS Vulnerability

CVE-2021-21252 Overview

CVE-2021-21252 is a Regular Expression Denial of Service (ReDoS) vulnerability affecting the jQuery Validation Plugin, a widely-used npm package (jquery-validation) that provides drop-in form validation capabilities for web applications. The vulnerability exists in versions prior to 1.19.3 and stems from one or more regular expressions that are susceptible to catastrophic backtracking when processing maliciously crafted input strings.

Critical Impact

Attackers can exploit vulnerable regular expressions in the URL validation method to cause excessive CPU consumption, potentially leading to application unresponsiveness or denial of service conditions affecting all users of the web application.

Affected Products

• jQuery Validation Plugin versions prior to 1.19.3 (npm package jquery-validation)
• NetApp SnapCenter (affected downstream)
• Applications using jqueryvalidation:jquery_validation for Node.js

Discovery Timeline

• January 13, 2021 - CVE-2021-21252 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-21252

Vulnerability Analysis

The jQuery Validation Plugin contains a ReDoS vulnerability in its URL validation functionality. Regular Expression Denial of Service occurs when a regex pattern exhibits exponential time complexity due to catastrophic backtracking. When certain input strings are processed against vulnerable patterns, the regex engine repeatedly backtracks through numerous possible matches, consuming excessive CPU resources.

In this case, the vulnerable regular expression is used in the URL validation method within src/core.js. The original regex pattern contained constructs that, when given specially crafted input, would cause the validation function to hang or consume disproportionate processing time relative to the input size.

Root Cause

The root cause is an inefficiently designed regular expression in the URL validation logic. The original pattern (?:[a-z\\\u00a1-\\\uffff0-9]-*)*[a-z\\\u00a1-\\\uffff0-9]+ contains nested quantifiers and alternations that create ambiguous matching paths. When the regex engine encounters input that nearly matches but ultimately fails, it must explore exponentially many backtracking paths before determining a non-match. This algorithmic complexity vulnerability (CWE-400: Uncontrolled Resource Consumption) allows attackers to trigger denial of service with relatively small payloads.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can submit malicious input to any form field validated using the vulnerable URL validation method. The crafted string exploits the regex backtracking behavior, causing the client-side JavaScript to hang or the server (if validation runs server-side) to become unresponsive.

The following shows the security patch that addresses the ReDoS vulnerability in src/core.js:

// https://gist.github.com/dperini/729294
// see also https://mathiasbynens.be/demo/url-regex
// modified to allow protocol-relative URLs
// BEFORE (vulnerable):
// return this.optional( element ) || /^(?:(?:(?:https?|ftp):)?\/\/)(?:\S+(?::\S*)?@)?(?:(?!(?:10|127)(?:\.\d{1,3}){3})(?!(?:169\.254|192\.168)(?:\.\d{1,3}){2})(?!172\.(?:1[6-9]|2\d|3[0-1])(?:\.\d{1,3}){2})(?:[1-9]\d?|1\d\d|2[01]\d|22[0-3])(?:\.(?:1?\d{1,2}|2[0-4]\d|25[0-5])){2}(?:\.(?:[1-9]\d?|1\d\d|2[0-4]\d|25[0-4]))|(?:(?:[a-z\\u00a1-\\uffff0-9]-*)*[a-z\\u00a1-\\uffff0-9]+)(?:\.(?:[a-z\\u00a1-\\uffff0-9]-*)*[a-z\\u00a1-\\uffff0-9]+)*(?:\.(?:[a-z\\u00a1-\\uffff]{2,})).?)(?::\d{2,5})?(?:[/?#]\S*)?$/i.test( value );

// AFTER (fixed):
return this.optional( element ) || /^(?:(?:(?:https?|ftp):)?\/\/)(?:\S+(?::\S*)?@)?(?:(?!(?:10|127)(?:\.\d{1,3}){3})(?!(?:169\.254|192\.168)(?:\.\d{1,3}){2})(?!172\.(?:1[6-9]|2\d|3[0-1])(?:\.\d{1,3}){2})(?:[1-9]\d?|1\d\d|2[01]\d|22[0-3])(?:\.(?:1?\d{1,2}|2[0-4]\d|25[0-5])){2}(?:\.(?:[1-9]\d?|1\d\d|2[0-4]\d|25[0-4]))|(?:(?:[a-z0-9\\u00a1-\\uffff][a-z0-9\\u00a1-\\uffff_-]{0,62})?[a-z0-9\\u00a1-\\uffff]\.)+(?:[a-z\\u00a1-\\uffff]{2,}\.?))(?::\d{2,5})?(?:[/?#]\S*)?$/i.test( value );

Source: GitHub Commit - Security Patch

The fix replaces the vulnerable nested quantifier pattern with a more constrained version that bounds the repetition length ({0,62}) and restructures the character class matching to prevent catastrophic backtracking.

Detection Methods for CVE-2021-21252

Indicators of Compromise

• Unusual CPU spikes in client browsers or server-side Node.js processes during form validation operations
• Web application forms becoming unresponsive when processing URL input fields
• Increased page load times or timeouts specifically related to form submission endpoints
• Error logs showing JavaScript execution timeouts or worker thread exhaustion

Detection Strategies

• Audit package.json and package-lock.json files for jquery-validation versions below 1.19.3
• Use software composition analysis (SCA) tools to identify vulnerable dependencies in your application stack
• Implement client-side performance monitoring to detect anomalous JavaScript execution times during form validation
• Deploy web application firewalls (WAF) with rules to detect abnormally long or malformed URL strings in form submissions

Monitoring Recommendations

• Configure alerting on JavaScript error rates and execution timeouts in application performance monitoring (APM) tools
• Monitor Node.js process CPU utilization for server-side validation scenarios
• Track form validation response times and establish baselines to identify anomalous patterns
• Enable logging for rejected form submissions to identify potential exploitation attempts

How to Mitigate CVE-2021-21252

Immediate Actions Required

• Upgrade jquery-validation to version 1.19.3 or later immediately
• Audit all applications and dependencies that may include the jQuery Validation Plugin
• Review application code for any custom URL validation logic that may have copied the vulnerable regex pattern
• Consider implementing input length limits on URL fields as a defense-in-depth measure

Patch Information

The vulnerability is fixed in jQuery Validation Plugin version 1.19.3. The patch modifies the URL validation regular expression to eliminate the catastrophic backtracking condition. Organizations should update their dependencies using their package manager:

For npm: npm update jquery-validation
For yarn: yarn upgrade jquery-validation

Additional resources:

• GitHub Security Advisory (GHSA-jxwx-85vp-gvwm)
• GitHub Pull Request #2371
• NetApp Security Advisory

Workarounds

• If immediate upgrade is not possible, implement server-side input length restrictions on URL fields to limit regex execution time
• Apply custom timeout wrappers around validation functions to prevent prolonged execution
• Consider temporarily disabling URL validation or replacing it with a safer alternative regex until the patch can be applied
• Use Content Security Policy (CSP) headers to limit the impact of client-side DoS conditions

# Configuration example - Update package to patched version
npm install jquery-validation@1.19.3 --save

# Verify the installed version
npm list jquery-validation

# Audit for remaining vulnerabilities
npm audit