Outdated Form Validation Library Can Make Your Website Unresponsive

Vulnerability Explanation

The jquery-validation library (versions before 1.19.3) contains one or more regular expressions that are vulnerable to ReDoS (Regular Expression Denial of Service). The specific regex identified by GitHub Security Lab is used for URL validation (the `url2` method). It was copied from a public gist and contains ambiguous/overlapping clauses that cause catastrophic backtracking when evaluated against a crafted input string. The vulnerable regex can be forced into O(2^n) runtime due to exponential backtracking — for example, a URL like `http://foobar.00.00.00.00.00.00...` with many repeated segments can cause the regex engine to loop near-indefinitely. If this library is used server-side (e.g., in a Node.js/Express app), an unauthenticated attacker can submit such input to any form field that triggers URL validation, blocking the event loop and making the application unresponsive for all users.

Root Cause

The URL validation regex was copied from an external, unvetted source and contains overlapping quantifiers that create ambiguity. The regex engine must explore an exponentially growing number of possible match paths when the input is crafted to exploit this ambiguity — a classic catastrophic backtracking pattern. The root cause is the use of a poorly constructed third-party regex without security review.

Technical Impact

An unauthenticated remote attacker can submit a crafted input to any form field validated by the affected URL regex. In a server-side Node.js context, this blocks the single-threaded event loop, causing a denial of service for all concurrent users. In a purely client-side context, the impact is limited to the individual user's browser tab freezing temporarily — significantly reducing real-world severity.

Severity Justification

CVSS 3.1 score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) per NVD. Network-accessible, no authentication required, high availability impact. Severity is most relevant when the library is used server-side; client-side-only deployments have materially lower real-world impact.

Affected Components

• jquery-validation < 1.19.3

Remediation Steps

1. Upgrade jquery-validation to version 1.19.3 or higher via your package manager: `npm install jquery-validation@latest` or `yarn add jquery-validation@latest`.
2. If using a CDN-hosted version, update the script tag src to reference version 1.19.3 or later (e.g., from cdnjs or the official npm CDN).
3. If using a NuGet package (ASP.NET), update via: `Update-Package jQuery.Validation`.
4. As a temporary server-side workaround (if upgrade is blocked), add input length validation middleware to reject inputs exceeding a reasonable maximum length before they reach the validation library.
5. Verify the updated version is reflected in your lock file (package-lock.json / yarn.lock) and redeploy.

Verification Steps

1. Run `npm list jquery-validation` (or `yarn list --pattern jquery-validation`) and confirm the installed version is 1.19.3 or higher.
2. Search your codebase for any vendored/copied jquery.validate.js files: `grep -r 'jquery.validate' ./public ./wwwroot ./static` — ensure no old copies remain.
3. Check your CDN script tags in HTML templates for hardcoded version numbers referencing versions below 1.19.3.

Code Examples (html)

<!-- Vulnerable: CDN reference to old version -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.14.0/jquery.validate.min.js"></script>

<!-- Fixed: Updated to 1.19.3 or later -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.19.5/jquery.validate.min.js"></script>

Best Practices

• Prefer installing front-end libraries via a package manager (npm/yarn) rather than CDN script tags — this makes version tracking and upgrades auditable and automatable.
• Enable automated dependency vulnerability scanning (e.g., `npm audit`, GitHub Dependabot, or Snyk) to catch known CVEs in dependencies before they reach production.
• Apply input length limits server-side as a defence-in-depth measure, independent of client-side validation libraries.
• Periodically audit vendored or statically copied JavaScript files — these are invisible to package managers and often go unpatched.

References

• github.com/jquery-validation/jquery-validation/security/advisories/GHSA-jxwx-85vp-gvwm
• securitylab.github.com/advisories/GHSL-2020-294-redos-jquery-validation/
• nvd.nist.gov/vuln/detail/CVE-2021-21252
• github.com/jquery-validation/jquery-validation/pull/2371