How Denial of Service via Infinite Loop Happens in JavaScript Dependencies and How to Fix It

How Denial of Service via Infinite Loop Happens in JavaScript Dependencies and How to Fix It

Introduction

A recent security scan of this project using Trivy flagged a high-severity denial of service vulnerability (CVE-2026-67213) in the nanoid package listed in package-lock.json. The vulnerability exists in nanoid versions before 5.1.16 and stems from an infinite loop condition in the customAlphabet function when processing specially crafted input.

While the code repository itself doesn't directly call nanoid with malicious input, the vulnerability exists in the dependency tree and could be exploited if:

1. Any code path accepts user input that influences nanoid's alphabet parameter
2. Transitive dependencies (like vitest, next, or postcss) use the vulnerable nanoid version internally
3. An attacker sends a specially constructed request that triggers the vulnerable code path

This is a classic case where you don't need to directly use a vulnerable function for it to pose a risk—having it in your dependency tree and accessible through indirect calls creates an attack surface.

The Vulnerability Explained

What's the Problem?

Nanoid is a popular JavaScript library for generating secure, URL-friendly unique string IDs. It supports custom character alphabets through the customAlphabet function. The vulnerability occurs when this function processes input that doesn't follow expected constraints.

In versions before 5.1.6, the customAlphabet function has a critical flaw: it enters an infinite loop when processing certain malformed input patterns, particularly when the custom alphabet or ID generation parameters cause the loop exit condition to never be satisfied.

The vulnerable code pattern (conceptually, from the affected nanoid versions):

// Simplified representation of the vulnerable pattern
export function customAlphabet(alphabet, size) {
  // No proper validation of alphabet length or composition
  // No bounds checking on the loop iteration
  while (someCondition) {  // This condition may never become false
    // ID generation logic
    // If alphabet validation is missing, loop never exits
  }
}

How Could It Be Exploited?

An attacker could exploit this through several vectors:

1. Direct exploitation: If your application exposes an API endpoint that accepts a custom alphabet parameter:
   javascript // Vulnerable code path (hypothetical) app.post('/generate-id', (req, res) => { const customId = customAlphabet(req.body.alphabet, 10); res.json({ id: customId() }); });
   An attacker sends: POST /generate-id with {"alphabet": ""} or a specially crafted string, causing nanoid to enter an infinite loop.

2. Indirect exploitation: Vitest, Next.js, PostCSS, or other dependencies might call nanoid internally. If your test suite or build process uses these tools with attacker-controlled input, the vulnerability could be triggered.

3. Request flooding: Send multiple requests with malformed alphabet parameters, causing worker threads to freeze and the application to become unresponsive.

Real-World Impact

When the infinite loop is triggered:
- The affected thread/worker freezes and becomes unresponsive
- CPU usage spikes on that core/thread
- Requests waiting for ID generation timeout
- If enough threads freeze, the entire application becomes unavailable
- No exception is thrown—the process simply hangs until killed

The Fix

The fix for CVE-2026-67213 involves two critical changes:

1. Upgrade Nanoid to Version 5.1.16

The primary fix is in package-lock.json:

Before:

"node_modules/nanoid": {
  "version": "3.3.15",
  "resolved": "https://registry.npmjs.org/nanoid/-/nanoid-3.3.15.tgz",
  "integrity": "sha512-y7Wygv/7mEOvxTuEQDB8StXdMRBWf1kR/tlhAzBRUFkB2jfcLOAxO/SHmOO2zgz1pVgK29/kyupn059/bCHdjA==",
  "bin": {
    "nanoid": "bin/nanoid.cjs"
  },
  "engines": {
    "node": "^10 || ^12 || ^13.7 || ^14 || >=15.0.1"
  }
}

After:

"node_modules/nanoid": {
  "version": "5.1.16",
  "resolved": "https://registry.npmjs.org/nanoid/-/nanoid-5.1.16.tgz",
  "integrity": "sha512-kVrnsrJqMR8+oLJnGEmSWw9BivK5mt7H3FZatVRjrc5wGqFYuBxX1yG7+A7Gi5AefkX6t/oCkizcQgpu0cY1dQ==",
  "bin": {
    "nanoid": "bin/nanoid.js"
  },
  "engines": {
    "node": "^18 || >=20"
  }
}

Key improvements in v5.1.16:
- The infinite loop condition is fixed by properly validating input parameters
- Input boundaries are checked before entering the generation loop
- Early exit conditions are verified to prevent infinite iteration
- Modern JavaScript features improve robustness

2. Force Patched Version Across All Dependencies Using npm Overrides

Simply upgrading nanoid in package.json isn't enough—many transitive dependencies bundle their own version of nanoid. The fix adds npm overrides in package.json:

Before:

{
  "dependencies": { /* ... */ },
  "devDependencies": { /* ... */ }
}

After:

{
  "dependencies": { /* ... */ },
  "devDependencies": { /* ... */ },
  "overrides": {
    "@vitest/mocker": {
      "nanoid": "5.1.16"
    },
    "nanoid": {
      "nanoid": "5.1.16"
    },
    "next": {
      "nanoid": "5.1.16"
    },
    "postcss": {
      "nanoid": "5.1.16"
    },
    "vite": {
      "nanoid": "5.1.16"
    },
    "vite-node": {
      "nanoid": "5.1.16"
    },
    "vitest": {
      "nanoid": "5.1.16"
    }
  }
}

Why this matters:

• @vitest/mocker: The test mocking tool bundles nanoid and would otherwise use v3.3.15
• vitest: The test runner uses nanoid internally for test IDs and could trigger the vulnerability during test execution
• next: Next.js uses nanoid for internal ID generation; without the override, it would use the older vulnerable version
• postcss and vite: CSS processing and build tools also use nanoid; overrides ensure they all use v5.1.16
• vite-node: The Node.js runtime for Vite also needs the patched version

The npm overrides field (available in npm 8.3.0+) forces all nested dependencies to use the specified version, bypassing their own dependency constraints. This creates a unified, secure dependency tree.

Prevention & Best Practices

1. Validate Loop Exit Conditions

Always ensure loops have proper exit conditions that cannot be bypassed by user input:

// ✅ Good: Explicit iteration limit
function generateId(alphabet, size) {
  if (!alphabet || alphabet.length === 0) {
    throw new Error('Alphabet must not be empty');
  }
  if (size < 1 || size > 1000) {
    throw new Error('Size must be between 1 and 1000');
  }
  let id = '';
  for (let i = 0; i < size; i++) {
    id += alphabet[Math.floor(Math.random() * alphabet.length)];
  }
  return id;
}

2. Dependency Scanning and Regular Updates

• Use security scanners like Trivy, Snyk, or npm audit in your CI/CD pipeline
• Configure automated dependency updates (Dependabot, Renovate)
• Pin major versions but allow patch updates automatically

3. Use npm Overrides for Transitive Dependencies

When patching vulnerable nested dependencies, use overrides to enforce consistency:

{
  "overrides": {
    "vulnerable-package": "patched-version"
  }
}

4. Input Validation Before Loop Operations

Never trust user-supplied parameters that control loop behavior:

// ❌ Bad: No validation
function process(userInput) {
  while (userInput.someCondition) {
    // Process...
  }
}

// ✅ Good: Validated input
function process(userInput) {
  const sanitized = validateInput(userInput);
  const maxIterations = 10000;
  let iterations = 0;

  while (sanitized.someCondition && iterations < maxIterations) {
    iterations++;
    // Process...
  }
}

5. Monitor for Infinite Loop Indicators

Implement monitoring to detect infinite loop conditions:
- CPU usage spikes without corresponding request processing
- Threads hanging without completing operations
- Request timeouts without errors
- Memory usage changes without garbage collection

Key Takeaways

1. Transitive dependencies matter: Vulnerabilities in indirect dependencies are just as dangerous as direct ones; CVE-2026-67213 could be exploited even though your code doesn't directly call nanoid's vulnerable function.

2. npm overrides are essential for security patches: When upgrading a vulnerable nested dependency, use the overrides field to force all packages to use the patched version, not just the root dependency.

3. Infinite loops are hard to detect but devastating: Unlike exceptions that crash the app, infinite loops silently freeze threads, making them insidious DoS vectors that require explicit loop guards and input validation.

4. Upgrade from v3.3.15 to v5.1.16 includes important fixes: The version jump includes not just the infinite loop fix but also engine requirement changes (Node 18+ required), indicating a more robust implementation.

5. Dependency scanning is non-negotiable: This vulnerability was only discovered through automated scanning (Trivy); it would have remained hidden in code review without explicit security tooling.

How Orbis AppSec Detected This

Source: User-influenced input that could reach the customAlphabet function in nanoid through any of the transitive dependencies (vitest, next, postcss, vite).

Sink: The customAlphabet function in node_modules/nanoid/index.js versions <5.1.6, where the loop exit condition fails to properly validate input, causing an infinite loop.

Missing control: Input parameter validation on the alphabet and size parameters; no loop iteration bounds checking; no early exit condition validation before entering the generation loop.

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

Fix: Upgraded nanoid to version 5.1.16 (which includes proper input validation and loop exit condition fixes) and added npm overrides to force all transitive dependencies (vitest, next, postcss, vite, vite-node, @vitest/mocker) to use the patched version, ensuring consistent protection across the entire dependency tree.

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 a critical principle in application security: you are responsible for the security of your entire dependency tree, not just your own code. Even though this project didn't directly invoke nanoid's vulnerable customAlphabet function, the presence of the vulnerable version in nested dependencies created a real attack surface that needed immediate patching.

The fix—upgrading to nanoid 5.1.16 and using npm overrides to enforce the patched version across all indirect dependencies—is a best practice for handling transitive vulnerability issues. This approach ensures that build tools, test runners, and all supporting packages use the secure implementation.

By maintaining security scanning in your CI/CD pipeline, staying current with dependency updates, and using dependency management tools like npm overrides, you can prevent these invisible vulnerabilities from reaching production. The fix also highlights the importance of understanding why vulnerabilities occur: in this case, missing input validation allowed user-controlled parameters to break loop exit conditions—a pattern to watch for in your own code.

References

• CWE-835: Loop with Unreachable Exit Condition
• OWASP: Denial of Service
• npm overrides documentation
• Nanoid GitHub Repository
• Trivy Security Scanner
• Pull Request: fix: upgrade nanoid to patched version (CVE-2026-67213)