langfuse-security-basics

What this skill does

# Langfuse Security Basics

## Overview

Security practices for Langfuse LLM observability: credential management, PII scrubbing before tracing, self-hosted hardening, data retention, and secret scanning.

## Prerequisites

- Langfuse instance (cloud or self-hosted)
- API keys provisioned
- Understanding of data privacy requirements (GDPR, SOC2, HIPAA)

## Instructions

### Step 1: Credential Security

Langfuse uses two keys with different security profiles:

```typescript
// Startup validation -- catch misconfigurations early
function validateLangfuseCredentials() {
  const publicKey = process.env.LANGFUSE_PUBLIC_KEY;
  const secretKey = process.env.LANGFUSE_SECRET_KEY;

  if (!publicKey || !secretKey) {
    throw new Error("LANGFUSE_PUBLIC_KEY and LANGFUSE_SECRET_KEY are required");
  }

  // Catch key swap (common mistake)
  if (secretKey.startsWith("pk-lf-")) {
    throw new Error("LANGFUSE_SECRET_KEY contains a public key (pk-lf-). Keys are swapped.");
  }

  if (publicKey.startsWith("sk-lf-")) {
    throw new Error("LANGFUSE_PUBLIC_KEY contains a secret key (sk-lf-). Keys are swapped.");
  }

  return { publicKey, secretKey };
}

// Use validated credentials
const { publicKey, secretKey } = validateLangfuseCredentials();
```

**Key security rules:**

- Public key (`pk-lf-...`): Identifies the project. Safe in client-side code.
- Secret key (`sk-lf-...`): Grants write access. **Server-side only.**
- Store in environment variables or secret manager -- never in source code.
- Rotate keys immediately if exposed in logs, git, or error reports.

### Step 2: PII Scrubbing Before Tracing

Langfuse stores everything you send. Scrub PII from inputs and outputs before tracing.

```typescript
// src/lib/pii-scrubber.ts

const PII_PATTERNS: Array<{ regex: RegExp; replacement: string }> = [
  { regex: /\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z]{2,}\b/gi, replacement: "[EMAIL]" },
  { regex: /\b\d{3}[-.]?\d{3}[-.]?\d{4}\b/g, replacement: "[PHONE]" },
  { regex: /\b\d{3}-\d{2}-\d{4}\b/g, replacement: "[SSN]" },
  { regex: /\b\d{4}[\s-]?\d{4}[\s-]?\d{4}[\s-]?\d{4}\b/g, replacement: "[CARD]" },
  { regex: /\b(?:sk|pk)-[a-zA-Z0-9_-]{20,}\b/g, replacement: "[API_KEY]" },
];

export function scrubPII(text: string): string {
  let scrubbed = text;
  for (const { regex, replacement } of PII_PATTERNS) {
    scrubbed = scrubbed.replace(regex, replacement);
  }
  return scrubbed;
}

export function scrubObject(obj: any): any {
  if (typeof obj === "string") return scrubPII(obj);
  if (Array.isArray(obj)) return obj.map(scrubObject);
  if (typeof obj === "object" && obj !== null) {
    const result: Record<string, any> = {};
    for (const [key, value] of Object.entries(obj)) {
      result[key] = scrubObject(value);
    }
    return result;
  }
  return obj;
}
```

```typescript
// Usage with tracing
import { observe, updateActiveObservation } from "@langfuse/tracing";
import { scrubPII, scrubObject } from "./lib/pii-scrubber";

const tracedChat = observe(async (userMessage: string) => {
  // Scrub input before tracing
  updateActiveObservation({ input: scrubPII(userMessage) });

  // Send original to LLM (unscrubbed)
  const response = await callLLM(userMessage);

  // Scrub output before tracing
  updateActiveObservation({ output: scrubPII(response) });
  return response;
});
```

### Step 3: Self-Hosted Hardening

```yaml
# docker-compose.yml -- production-hardened
services:
  langfuse:
    image: langfuse/langfuse:latest
    environment:
      # Disable open registration
      - AUTH_DISABLE_SIGNUP=true
      # Enforce SSO for your domain
      - AUTH_DOMAINS_WITH_SSO_ENFORCEMENT=company.com
      # Least-privilege default role
      - LANGFUSE_DEFAULT_PROJECT_ROLE=VIEWER
      # Encrypt data at rest
      - ENCRYPTION_KEY=${ENCRYPTION_KEY}
      # Data retention (days)
      - LANGFUSE_RETENTION_DAYS=90
      # Database
      - DATABASE_URL=${DATABASE_URL}
      - NEXTAUTH_SECRET=${NEXTAUTH_SECRET}
      - SALT=${SALT}
```

### Step 4: Git Secret Scanning

Prevent Langfuse keys from being committed:

```bash
# .gitignore
.env
.env.local
.env.production
```

```yaml
# .github/workflows/secret-scan.yml
name: Secret Scan
on: [push, pull_request]
jobs:
  scan:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Check for Langfuse keys
        run: |
          if grep -rn "sk-lf-[a-zA-Z0-9]" --include="*.ts" --include="*.js" --include="*.py" .; then
            echo "ERROR: Secret key found in source code!"
            exit 1
          fi
```

### Step 5: Scoped API Keys and Least Privilege

```typescript
// Create separate API keys per service/environment
// In Langfuse dashboard: Settings > API Keys

// Backend API service -- full write access
// LANGFUSE_SECRET_KEY=sk-lf-backend-...

// Analytics worker -- read-only access
// Use Langfuse API with read-only key
// LANGFUSE_SECRET_KEY=sk-lf-readonly-...

// CI/CD pipeline -- scoped to test project
// LANGFUSE_SECRET_KEY=sk-lf-ci-test-...
// LANGFUSE_PUBLIC_KEY=pk-lf-ci-test-...
```

## Security Checklist

| Category | Check | Status |
|----------|-------|--------|
| Credentials | Secret key in env vars / secret manager only | |
| Credentials | Keys validated at startup (no swap) | |
| Credentials | .env files in .gitignore | |
| Data Privacy | PII scrubbed from trace inputs/outputs | |
| Data Privacy | Retention policy configured | |
| Self-Hosted | Signup disabled, SSO enforced | |
| Self-Hosted | Encryption key set for data at rest | |
| CI/CD | Secret scanning in pipeline | |
| Access | Least-privilege roles per team member | |

## Error Handling

| Issue | Cause | Solution |
|-------|-------|----------|
| PII in traces | Not scrubbing before trace | Apply `scrubPII()` to all inputs/outputs |
| Secret key leaked | Key in source code | Rotate immediately, add secret scanning |
| Unauthorized access | Default roles too permissive | Set `LANGFUSE_DEFAULT_PROJECT_ROLE=VIEWER` |
| Data accumulation | No retention policy | Set `LANGFUSE_RETENTION_DAYS` |

## Resources

- [Langfuse Data Security](https://langfuse.com/docs/data-security-privacy)
- [Self-Hosting Configuration](https://langfuse.com/self-hosting/configuration)
- [Headless Initialization](https://langfuse.com/self-hosting/administration/headless-initialization)