CVE-2021-3765: Validator.js DoS Vulnerability

CVE-2021-3765 Overview

CVE-2021-3765 is an Inefficient Regular Expression Complexity vulnerability affecting the validator.js library for Node.js. This type of vulnerability, commonly known as Regular Expression Denial of Service (ReDoS), occurs when a crafted input string causes catastrophic backtracking in the regular expression engine, leading to excessive CPU consumption and application unresponsiveness.

The validator.js library is a widely-used npm package for string validation and sanitization in JavaScript applications. When exploited, this vulnerability allows remote attackers to cause a denial of service condition by sending specially crafted input that triggers exponential time complexity in vulnerable regular expression patterns.

Critical Impact

Remote attackers can cause application-level denial of service by exploiting inefficient regular expressions in the validator.js library, potentially rendering Node.js services unavailable.

Affected Products

• validator_project validator (Node.js package)
• Applications using vulnerable versions of validator.js for input validation
• Node.js services with exposed validation endpoints

Discovery Timeline

• 2021-11-02 - CVE-2021-3765 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-3765

Vulnerability Analysis

This vulnerability exists due to inefficient regular expression patterns within the validator.js library. The CWE-1333 (Inefficient Regular Expression Complexity) classification indicates that certain validation functions use regular expressions susceptible to catastrophic backtracking when processing malicious input.

In ReDoS attacks, the regular expression engine enters a state where it attempts an exponential number of matching combinations before determining that no match exists. This behavior can be triggered with relatively short input strings that are specifically constructed to exploit nested quantifiers or overlapping alternation patterns in the vulnerable regex.

The network-based attack vector means that any application exposing validator.js validation functions to user-controlled input—such as form submissions, API endpoints, or data processing pipelines—could be targeted remotely without authentication requirements.

Root Cause

The root cause of CVE-2021-3765 lies in the use of regular expressions with patterns that exhibit worst-case exponential time complexity. These patterns typically involve:

• Nested quantifiers (e.g., (a+)+)
• Overlapping alternation groups
• Greedy quantifiers combined with backtracking-heavy patterns

When user-controlled input is passed through these validation functions, an attacker can craft strings that maximize backtracking iterations, effectively blocking the event loop in Node.js applications and causing denial of service.

Attack Vector

The attack vector is network-based, requiring no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying an application endpoint that processes user input through vulnerable validator.js functions
2. Crafting a malicious string designed to trigger catastrophic backtracking
3. Submitting the payload to exhaust CPU resources on the target server

Since Node.js operates on a single-threaded event loop, a ReDoS attack can block the entire application from processing other requests, amplifying the impact beyond the single malicious request.

The vulnerability mechanism involves sending specially crafted input strings to validation functions that contain inefficient regular expression patterns. When the regex engine processes these inputs, it experiences exponential time complexity due to excessive backtracking, effectively blocking the application's event loop. For detailed technical information about the vulnerable pattern, see the Huntr Bounty Report.

Detection Methods for CVE-2021-3765

Indicators of Compromise

• Unusual CPU spikes correlating with specific API or form submission endpoints
• Prolonged request processing times for validation-heavy operations
• Application timeouts or unresponsiveness affecting all users simultaneously
• Error logs indicating Node.js event loop blockage or maximum call stack size exceeded

Detection Strategies

• Implement application performance monitoring (APM) to detect sudden CPU usage spikes
• Monitor request latency for endpoints using validator.js functions
• Deploy web application firewall (WAF) rules to detect and block known ReDoS payload patterns
• Use static analysis tools to identify vulnerable regular expression patterns in dependencies

Monitoring Recommendations

• Enable Node.js process monitoring for event loop lag detection
• Configure alerting thresholds for sustained high CPU usage on application servers
• Implement request timeout policies to terminate long-running validation operations
• Review application logs for repeated timeout errors on specific endpoints

How to Mitigate CVE-2021-3765

Immediate Actions Required

• Update validator.js to the patched version that addresses this vulnerability
• Audit application code for endpoints that pass untrusted input to validator.js functions
• Implement request timeouts and rate limiting on validation endpoints
• Consider input length restrictions as a temporary mitigation measure

Patch Information

The validator.js maintainers have released a fix for this vulnerability. The patch modifies the vulnerable regular expression patterns to prevent catastrophic backtracking. Organizations should update to the latest version of validator.js that includes commit 496fc8b2a7f5997acaaec33cc44d0b8dba5fb5e1.

To update the package, run:

npm update validator

Verify the installed version includes the security fix by checking the package version and reviewing the changelog.

Workarounds

• Implement input length validation before passing data to validator.js functions to limit potential attack payload size
• Use worker threads or process isolation for validation operations to prevent main event loop blocking
• Deploy rate limiting on endpoints that process user-provided validation input
• Consider using alternative validation libraries with ReDoS-safe regular expressions until patching is complete

# Configuration example for rate limiting (using express-rate-limit)
# Install the rate limiting middleware
npm install express-rate-limit

# Apply rate limiting to validation endpoints to mitigate DoS impact
# Configure max requests per window based on your application's needs