How Denial of Service via Infinite Loop happens in JavaScript (nanoid) and how to fix it

Introduction

In the aas-web-ui project, a high-severity Denial of Service vulnerability was lurking in the dependency tree—not in the application's own code, but in a transitive dependency pulled in by PostCSS. The pnpm-lock.yaml file pinned nanoid@3.3.16 as a dependency of postcss@8.5.25, and this version of nanoid contained a critical flaw: its customAlphabet function could enter an infinite loop during random ID generation, effectively freezing any process that triggered it.

This is a textbook example of supply chain risk. The application developers never explicitly chose nanoid 3.3.16—it was resolved automatically as a transitive dependency. Yet the vulnerability was real, exploitable, and capable of taking down the entire web UI build process or any runtime ID generation.

The Vulnerability Explained

What is nanoid?

Nanoid is one of the most popular JavaScript libraries for generating unique, URL-friendly string IDs. It's used by millions of projects, often indirectly through tools like PostCSS (which uses it to generate unique class identifiers during CSS processing).

The Infinite Loop Bug

CVE-2026-67213 affects nanoid's customAlphabet function—the API that allows developers to generate IDs from a custom set of characters. In versions before 3.3.18 (v3 line) and 5.1.6 (v5 line), specific inputs to this function could trigger a loop with an unreachable exit condition.

Here's the critical dependency chain in the vulnerable lockfile:

postcss@8.5.25:
  dependencies:
    nanoid: 3.3.16
    picocolors: 1.1.1
    source-map-js: 1.2.1

The nanoid@3.3.16 resolution was explicitly present:

nanoid@3.3.16:
  resolution: {integrity: sha512-bzlKTyNJ7+LdGIIwy8ijFpIqEQIvafahV7eYykJ8Cvh42EdJeODoJ6gUJXpQJvej1BddH8OqTXZNE/KfbWAu8Q==}
  engines: {node: ^10 || ^12 || ^13.7 || ^14 || >=15.0.1}
  hasBin: true

How Could It Be Exploited?

In the context of aas-web-ui, consider these attack scenarios:

1. Build-time DoS: If the application uses PostCSS during build (which it does—PostCSS is a direct dependency), and a crafted CSS input triggers nanoid's customAlphabet with problematic parameters, the build process hangs indefinitely. In a CI/CD pipeline, this means deployments stall.

2. Runtime DoS: If any part of the application calls nanoid's customAlphabet at runtime (e.g., for generating session tokens, unique component IDs, or request identifiers), a malicious input that influences the alphabet or size parameters could freeze the Node.js event loop entirely.

3. Supply chain amplification: Since nanoid is used by PostCSS, which is used by virtually every modern web application, this vulnerability has an enormous blast radius across the JavaScript ecosystem.

The infinite loop consumes 100% of a CPU core with no timeout or escape mechanism, making it a reliable denial of service vector.

The Fix

The fix involves three coordinated changes that force the entire dependency tree to use the patched nanoid@3.3.18:

1. Adding pnpm overrides in pnpm-workspace.yaml

# Before: No overrides
packages:
    - '.'

# After: Force nanoid resolution
packages:
    - '.'

overrides:
  "nanoid@": 3.3.18

This is the most critical change. The overrides field in pnpm tells the package manager: "No matter what version any package in the tree requests for nanoid, resolve it to 3.3.18." This catches transitive dependencies like PostCSS that would otherwise continue pulling in the vulnerable version.

2. Updating pnpm-lock.yaml

The lockfile changes accomplish three things:

Removing the vulnerable version entry:

# Removed
-  nanoid@3.3.16:
-    resolution: {integrity: sha512-bzlKTyNJ7+...}
-    engines: {node: ^10 || ^12 || ^13.7 || ^14 || >=15.0.1}
-    hasBin: true

Removing the vulnerable snapshot:

# Removed
-  nanoid@3.3.16: {}

Updating PostCSS's dependency resolution:

# Before
postcss@8.5.25:
  dependencies:
    nanoid: 3.3.16

# After
postcss@8.5.25:
  dependencies:
    nanoid: 3.3.18

3. Adding the override declaration in the lockfile header

overrides:
  nanoid@: 3.3.18

This ensures the lockfile is self-consistent and any pnpm install will respect the override even if workspace configuration is parsed differently.

Why Each File Change Was Necessary

Prevention & Best Practices

1. Use Dependency Override Mechanisms

Every major package manager supports forcing transitive dependency versions:

# pnpm (pnpm-workspace.yaml or package.json)
overrides:
  "nanoid@": ">=3.3.18"

# npm (package.json)
"overrides": {
  "nanoid": ">=3.3.18"
}

# yarn (package.json)
"resolutions": {
  "nanoid": ">=3.3.18"
}

2. Automate Dependency Scanning

Run SCA tools in CI/CD pipelines:
- Trivy (used to detect this issue)
- npm audit / pnpm audit
- Snyk or Dependabot

3. Pin and Audit Transitive Dependencies

Don't assume transitive dependencies are safe just because you didn't choose them. The pnpm-lock.yaml file is your audit surface—review it during security assessments.

4. Implement Build Timeouts

Even with patched dependencies, add timeouts to build processes so that any future infinite loop vulnerability causes a fast failure rather than an indefinite hang:

# GitHub Actions example
- name: Build
  run: pnpm build
  timeout-minutes: 10

5. Monitor CWE-835 Patterns

When writing custom loop logic (especially in ID generation, parsing, or retry mechanisms), always ensure:
- Loops have a maximum iteration bound
- Exit conditions are mathematically provable
- Timeout mechanisms exist for non-deterministic loops

Key Takeaways

• Transitive dependencies are attack surface: nanoid was never directly imported by aas-web-ui, yet it exposed the application to a high-severity DoS through PostCSS's dependency on nanoid@3.3.16.
• pnpm overrides are essential for supply chain security: Without the "nanoid@": 3.3.18 override in pnpm-workspace.yaml, PostCSS would have continued resolving to the vulnerable version regardless of any direct dependency updates.
• Infinite loops in ID generation libraries are particularly dangerous: nanoid's customAlphabet is called frequently and often in hot paths—a single infinite loop call freezes the entire Node.js event loop.
• Lockfile integrity matters: The fix required removing both the resolution entry AND the snapshot for nanoid@3.3.16 to ensure no install path could accidentally resolve the old version.
• SCA tools catch what code review misses: No amount of application code review would have revealed this vulnerability—it required scanning the dependency tree against CVE databases.

How Orbis AppSec Detected This

• Source: The pnpm-lock.yaml dependency resolution for postcss@8.5.25, which transitively pulled in nanoid@3.3.16 as a required dependency for CSS class ID generation.
• Sink: nanoid's customAlphabet() function internal loop, which under specific input conditions enters an infinite loop with no exit condition (CWE-835).
• Missing control: No pnpm override existed to force the patched nanoid version across transitive dependencies; the lockfile allowed resolution of the known-vulnerable 3.3.16 version.
• CWE: CWE-835 (Loop with Unreachable Exit Condition)
• Fix: Added "nanoid@": 3.3.18 override in pnpm-workspace.yaml and updated the lockfile to eliminate all references to the vulnerable nanoid@3.3.16 version.

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 is a stark reminder that modern JavaScript applications are only as secure as their deepest transitive dependency. A single vulnerable version of nanoid—pulled in automatically by PostCSS—could have allowed an attacker to freeze the application's build pipeline or runtime with a carefully crafted input that triggers an infinite loop.

The fix was surgical: a pnpm override forcing nanoid@3.3.18 across the entire dependency tree, combined with lockfile cleanup to ensure no installation path could resolve the vulnerable version. This pattern—using package manager overrides to patch transitive vulnerabilities—is a critical tool in every JavaScript developer's security toolkit.

Keep your dependencies scanned, your overrides intentional, and your build pipelines protected with timeouts. Supply chain security isn't optional—it's the foundation your application stands on.

References

• CWE-835: Loop with Unreachable Exit Condition
• OWASP Dependency Check
• pnpm overrides documentation
• Trivy Vulnerability Scanner
• Semgrep rules for JavaScript DoS patterns
• fix: upgrade nanoid to 3.3.18, 5.1.6 (CVE-2026-67213)