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Khôi Nguyễn
Khôi Nguyễn

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How to Block Prompt Injection, Base64 Jailbreaks & API Key Leaks Locally in Node.js (Zero Dependencies)

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Every week, another AI wrapper or internal RAG tool accidentally leaks a production sk-proj-... key, customer SSN, or database URI to a third-party LLM provider—or gets tricked by a <|im_start|>system delimiter injection.

Most teams either ignore the problem or add a heavy 300ms external guardrail API call on every request.

To solve this locally in <1ms with zero runtime dependencies, I built PromptShield Pro—a local-first LLM Security Firewall, Reversible PII Redactor, Multi-Model Cost Simulator, and 30-Template Production System Prompt Vault.

🛡️ What It Catches Locally

  1. 16+ Credential & PII Formats (with Reversible Redaction):
    Scans for OpenAI (sk-proj-...), Anthropic (sk-ant-...), Gemini, AWS (AKIA...), GitHub, Stripe, and Slack keys, PostgreSQL/MongoDB connection URIs, JWTs, Luhn-verified credit cards, SSNs, and emails—replacing them with [REDACTED_...] tokens before calling the LLM and restoring them afterward via restoreRedacted().

  2. Multi-Layer Prompt Injection & Obfuscation Firewall:
    Detects direct instruction overrides, DAN/DevMode personas, system prompt extraction attempts, XML/ChatML delimiter spoofing, markdown image webhook exfiltration, Base64-encoded instructions, and invisible zero-width character steganography.

  3. Deterministic Prompt Compressor & Linter:
    Strips conversational filler and lints missing JSON schemas or unisolated {{variables}}, saving 15%–35% on input tokens.

  4. Multi-Model Cost & Prompt Caching Simulator:
    Compares monthly spend and prompt-caching savings across GPT-4o, o3-mini, Claude 3.7 Sonnet, Gemini 2.5 Pro/Flash, DeepSeek V3, and Llama 3.3 70B.

💻 Quick Example (Zero Dependencies in Node.js >= 18) 

const { scanAndRedact, inspectPromptSecurity, compressPrompt } = require('./src/index');

// 1. Mask API keys, DB URIs, and PII before calling any LLM API
const { redactedText, vaultMap } = scanAndRedact(userPrompt);

// 2. Block Prompt Injections, Delimiter Escapes & Base64 Jailbreaks
const security = inspectPromptSecurity(redactedText);
if (security.blocked) {
  throw new Error(`Blocked adversarial prompt (${security.riskLevel})`);
}

// 3. Strip token bloat & wrap in hardened XML data boundary
const { compressedText } = compressPrompt(redactedText);
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🚀 Try the Free CLI Starter or Grab the Complete Toolkit

I've released a Free Starter Edition ($0+) with the CLI scanner, 5 starter system prompts, and the 30-case Red-Team benchmark suite, alongside the PromptShield Pro Complete Toolkit ($29) which includes the Offline Interactive Web Studio (studio/index.html) and all 30 Schema-Enforced Production System Prompts across 6 engineering categories:

👉 Download PromptShield Lite (Free) or PromptShield Pro on Gumroad

Would love any feedback from the community on additional secret formats or adversarial injection patterns you'd like to see added!

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