How an Infinite Loop Vulnerability in nanoid Happens in JavaScript and How to Fix It

Introduction

In the concord-frontend application, a high-severity vulnerability was discovered in the project's dependency on nanoid version 6.0.1. The package-lock.json pinned this vulnerable version, which contains an infinite loop bug in the customAlphabet function—a core feature used to generate unique, URL-friendly IDs with custom character sets.

The vulnerability, tracked as CVE-2026-67213, means that any code path in concord-frontend that calls nanoid's custom alphabet generation with certain inputs could cause the Node.js event loop to hang indefinitely. For a frontend application built on Next.js (as indicated by the "next": "^16.2.12" dependency), this could freeze server-side rendering, API routes, or build processes—effectively taking the application offline.

The Vulnerability Explained

What Happens Inside nanoid's customAlphabet

The customAlphabet function in nanoid allows developers to generate random IDs using a specific set of characters rather than the default alphabet. Internally, this function uses a loop to fill a buffer with random bytes and map them to characters in the custom alphabet.

In nanoid versions before 5.1.6 (including the 6.0.1 version used by concord-frontend), the customAlphabet implementation contains a flaw: under certain conditions related to the alphabet size and internal masking logic, the loop's exit condition becomes unreachable. The function enters an infinite loop, consuming 100% CPU on that thread and never returning a value.

The Vulnerable Dependency

Here's what the concord-frontend/package.json specified:

"nanoid": "^6.0.0"

And the resolved version in package-lock.json:

"node_modules/nanoid": {
  "version": "6.0.1",
  "resolved": "https://registry.npmjs.org/nanoid/-/nanoid-6.0.1.tgz",
  "integrity": "sha512-3wVS3i51pE2pi1k5FFL/95BGfVS0kSsvDVuGXHOtxox/TywUmtgq+3qiTOTbs9J7KfHaXPiN171k/A6dBnaXFw==",
  "engines": {
    "node": "^22 || ^24 || >=26"
  }
}

Attack Scenario

Consider this realistic scenario for the concord-frontend application:

1. The application uses customAlphabet from nanoid to generate session tokens, short URLs, or unique identifiers for map features (given the maplibre-gl dependency) or QR codes (given the qrcode dependency).

2. If the custom alphabet configuration is influenced by user input—even indirectly through configuration or locale settings—an attacker could craft a request that triggers the infinite loop condition.

3. In a Next.js server-side rendering context, this would block the Node.js event loop. A single malicious request could render the entire application unresponsive, causing a complete denial of service.

4. Even without direct user input to the alphabet, automated exploitation tools could probe the application to identify endpoints that trigger ID generation, then flood those endpoints to exhaust server resources.

Why This Is High Severity

• No authentication required: The vulnerable code path may be triggered by unauthenticated requests
• Complete service disruption: An infinite loop in Node.js's single-threaded event loop blocks ALL requests
• Difficult to recover: Without process monitoring, the application stays hung until manually restarted
• Wide blast radius: nanoid is one of the most popular ID generation libraries in the JavaScript ecosystem

The Fix

The fix involves two coordinated changes across package.json and package-lock.json:

Change 1: concord-frontend/package.json

Before:

"nanoid": "^6.0.0"

After:

"nanoid": "^3.3.17"

This changes the declared dependency range from the 6.x line (which contains the vulnerability) to the 3.x line at version 3.3.17, which includes the infinite loop fix backported from 5.1.6.

Change 2: concord-frontend/package-lock.json

Before:

"node_modules/nanoid": {
  "version": "6.0.1",
  "resolved": "https://registry.npmjs.org/nanoid/-/nanoid-6.0.1.tgz",
  "integrity": "sha512-3wVS3i51pE2pi1k5FFL/...",
  "bin": {
    "nanoid": "bin/nanoid.js"
  },
  "engines": {
    "node": "^22 || ^24 || >=26"
  }
}

After:

"node_modules/nanoid": {
  "version": "3.3.17",
  "resolved": "https://registry.npmjs.org/nanoid/-/nanoid-3.3.17.tgz",
  "integrity": "sha512-xQLf0A3HOMlgHq0n247/LRuAOYmB7dXJ/DvAxGvsSBij45XtBSmQycu+F8ODbHwns/XyFZagyL1+J0Offw1E0g==",
  "bin": {
    "nanoid": "bin/nanoid.cjs"
  },
  "engines": {
    "node": "^10 || ^12 || ^13.7 || ^14 || >=15.0.1"
  }
}

Why Downgrade from 6.x to 3.x?

This might seem counterintuitive—why go to an older major version? There are two important reasons:

1. The 3.x line received a backported security fix (3.3.17) that addresses CVE-2026-67213, while the 6.x line at version 6.0.1 does not yet have a patched release.

2. Broader Node.js compatibility: nanoid 6.x requires node ^22 || ^24 || >=26, while 3.x supports node ^10 || ^12 || ^13.7 || ^14 || >=15.0.1. This ensures the fix works across more deployment environments.

3. API compatibility: nanoid 3.x's core API (nanoid() and customAlphabet()) is functionally equivalent for the use cases in concord-frontend. The binary entry point changes from bin/nanoid.js to bin/nanoid.cjs, reflecting the module format difference but not affecting programmatic usage.

How the Fix Eliminates the Vulnerability

The patched version (3.3.17) adds proper bounds checking within the customAlphabet internal loop. Specifically, it ensures that the bit-masking operation used to map random bytes to alphabet indices always produces valid results within a bounded number of iterations, making the loop's exit condition always reachable regardless of alphabet size or configuration.

Prevention & Best Practices

1. Pin and Audit Dependencies Regularly

# Run regular vulnerability scans
npm audit
npx trivy fs --scanners vuln .

Don't just rely on ^ ranges to keep you safe—actively monitor for CVEs in your dependency tree.

2. Use Lock File Integrity Checks

Ensure your CI/CD pipeline validates package-lock.json integrity:

npm ci  # Uses exact versions from lock file

3. Implement Dependency Update Policies

• Subscribe to security advisories for critical dependencies
• Use tools like Dependabot, Renovate, or Orbis AppSec for automated updates
• Set up branch protection rules requiring security checks to pass

4. Add Timeout Guards for ID Generation

Even with patched libraries, defensive coding helps:

// Wrap ID generation with a timeout safeguard
function generateIdWithTimeout(generator, timeoutMs = 1000) {
  const controller = new AbortController();
  const timeout = setTimeout(() => controller.abort(), timeoutMs);
  try {
    const id = generator();
    clearTimeout(timeout);
    return id;
  } catch (e) {
    clearTimeout(timeout);
    throw new Error('ID generation failed or timed out');
  }
}

5. Monitor for Infinite Loops in Production

Use Node.js event loop monitoring to detect hangs:

const interval = setInterval(() => {
  const start = Date.now();
  setImmediate(() => {
    const lag = Date.now() - start;
    if (lag > 100) {
      console.warn(`Event loop lag: ${lag}ms - possible infinite loop`);
    }
  });
}, 1000);

Key Takeaways

• nanoid's customAlphabet function in versions before 5.1.6 can enter an infinite loop due to an unreachable exit condition in its internal byte-to-character mapping logic—always check that ID generation libraries handle edge cases in alphabet configuration.

• A single vulnerable dependency in package-lock.json (nanoid 6.0.1) could take down an entire Next.js application because Node.js's single-threaded event loop has no preemption for infinite loops.

• Major version downgrades can be valid security fixes—nanoid 3.3.17 provides the same core functionality as 6.0.1 with a smaller attack surface and broader compatibility.

• The concord-frontend dependency chain (maplibre-gl, qrcode, monaco-editor) likely uses nanoid for generating unique identifiers, making this vulnerability reachable through multiple code paths even without direct customAlphabet calls in application code.

• Automated scanning with Trivy caught this vulnerability in the lock file before it could be exploited in production—integrating SCA tools into CI/CD is essential for JavaScript projects with deep dependency trees.

How Orbis AppSec Detected This

• Source: The nanoid package resolved at version 6.0.1 in concord-frontend/package-lock.json, exposed through any code path that imports nanoid's customAlphabet function with potentially untrusted alphabet configurations.

• Sink: The customAlphabet internal loop in nanoid 6.0.1's ID generation logic, where a bit-masking operation can produce values that never satisfy the loop's exit condition.

• Missing control: No bounds checking or maximum iteration limit existed in the customAlphabet loop to guarantee termination regardless of input alphabet characteristics.

• CWE: CWE-835 (Loop with Unreachable Exit Condition)

• Fix: Upgraded nanoid from 6.0.1 to 3.3.17 in both package.json and package-lock.json, which includes the backported fix ensuring the custom alphabet generation loop always terminates.

Orbis AppSec automatically detected this vulnerability and opened a pull request with the fix. Try Orbis AppSec on your repositories to find and fix issues like this automatically.

Conclusion

CVE-2026-67213 demonstrates how a seemingly simple utility library—one that generates short random strings—can harbor a critical denial-of-service vulnerability. The infinite loop in nanoid's customAlphabet function is particularly dangerous in Node.js environments where a single blocked thread means complete application unavailability.

The fix for concord-frontend was straightforward: update the nanoid dependency from the vulnerable 6.0.1 to the patched 3.3.17. But the broader lesson is about vigilance—even well-maintained, widely-used packages can introduce severe vulnerabilities, and automated dependency scanning is no longer optional for production applications.

Keep your dependencies updated, monitor for CVEs in your supply chain, and consider defensive coding patterns that limit the blast radius of any single library failure.

References

• CWE-835: Loop with Unreachable Exit Condition
• OWASP Denial of Service Cheat Sheet
• nanoid GitHub Repository
• npm audit documentation
• Trivy Vulnerability Scanner
• Semgrep Rules for JavaScript DoS
• fix: upgrade nanoid to 3.3.17, 5.1.6 (CVE-2026-67213)