CVE-2021-33502: Normalize-url ReDoS Vulnerability

CVE-2021-33502 Overview

CVE-2021-33502 is a Regular Expression Denial of Service (ReDoS) vulnerability affecting the normalize-url package for Node.js. The vulnerability exists because the package exhibits exponential performance degradation when processing specially crafted data: URLs. An attacker can exploit this flaw by supplying malicious input that causes the vulnerable regular expression to perform excessive backtracking, leading to CPU exhaustion and denial of service conditions.

Critical Impact

This vulnerability allows remote attackers to cause denial of service by sending crafted data URLs, potentially rendering Node.js applications unresponsive and impacting service availability.

Affected Products

• normalize-url versions before 4.5.1
• normalize-url versions 5.x before 5.3.1
• normalize-url versions 6.x before 6.0.1

Discovery Timeline

• 2021-05-24 - CVE-2021-33502 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-33502

Vulnerability Analysis

The normalize-url package is a widely-used Node.js utility that normalizes URLs to a canonical form, making them suitable for comparison and deduplication. The vulnerability stems from inefficient regular expression patterns used to parse and process data: URLs. When a carefully crafted input string is provided, the regex engine enters a state of catastrophic backtracking, where the number of possible matching paths grows exponentially with input length.

This type of algorithmic complexity vulnerability is classified under CWE-1333 (Inefficient Regular Expression Complexity). The attack can be executed remotely over a network without requiring any authentication or user interaction, making it particularly dangerous for internet-facing applications that process user-supplied URLs.

Root Cause

The root cause is an inefficient regular expression pattern within the normalize-url package that lacks proper bounds checking for data: URL processing. When the regex engine encounters certain input patterns, it attempts to match the input in exponentially many ways before ultimately failing. This results in CPU time growing exponentially with input size, rather than linearly as expected for well-designed regular expressions.

Attack Vector

The vulnerability is exploitable over the network. An attacker can send a malicious HTTP request containing a specially crafted data: URL to any application that uses the vulnerable normalize-url package to process user input. The attack requires no privileges and no user interaction. While the vulnerability does not compromise confidentiality or integrity, it causes complete denial of the availability aspect by exhausting server CPU resources.

Applications that normalize user-provided URLs for caching, logging, or deduplication purposes are particularly susceptible. A single malicious request could potentially block the Node.js event loop, affecting all concurrent users of the application.

Detection Methods for CVE-2021-33502

Indicators of Compromise

• Abnormally high CPU utilization on Node.js application servers
• Application timeouts or unresponsiveness correlated with URL processing requests
• Requests containing unusually long or malformed data: URLs in server logs
• Repeated requests with similar data: URL patterns from the same source

Detection Strategies

• Implement dependency scanning using tools like npm audit to identify vulnerable normalize-url versions
• Deploy application performance monitoring (APM) to detect regex-related CPU spikes
• Use Software Composition Analysis (SCA) tools to continuously monitor for known vulnerable packages
• Review application logs for requests containing suspicious data: URL patterns

Monitoring Recommendations

• Set up alerts for CPU usage anomalies on Node.js application instances
• Monitor request latency percentiles to detect processing delays indicative of ReDoS attacks
• Implement rate limiting on endpoints that process user-supplied URLs
• Use SentinelOne Singularity to detect and respond to denial of service patterns targeting applications

How to Mitigate CVE-2021-33502

Immediate Actions Required

• Update normalize-url to version 4.5.1, 5.3.1, or 6.0.1 or later depending on your major version
• Run npm audit or yarn audit to identify all instances of the vulnerable package in your dependency tree
• Review applications for any code paths that process untrusted data: URLs
• Implement input validation to limit URL length and reject suspicious patterns before processing

Patch Information

The normalize-url maintainer has released patched versions that address this ReDoS vulnerability:

• For version 4.x: upgrade to 4.5.1 or later
• For version 5.x: upgrade to 5.3.1 or later
• For version 6.x: upgrade to 6.0.1 or later

The fix involves optimizing the regular expression patterns to prevent catastrophic backtracking. See the GitHub Release v6.0.1 for release notes and the NetApp Security Advisory NTAP-20210706-0001 for additional vendor guidance.

Workarounds

• Implement URL length limits on user input before passing to normalize-url
• Use URL validation middleware to reject malformed or suspicious data: URLs
• Consider implementing request timeouts at the application or reverse proxy level to limit impact
• Deploy Web Application Firewall (WAF) rules to filter suspicious URL patterns

# Update normalize-url to a patched version
npm update normalize-url

# Check installed version
npm list normalize-url

# Force update to specific safe version (for v6.x)
npm install normalize-url@6.0.1