API Security for AI-Powered Applications: OAuth 2.1, Rate Limiting, and Prompt Injection Firewalls
AI-powered APIs face all traditional web security threats plus an entirely new class: prompt injection, token exhaustion attacks, and model output manipulation. Securing these APIs requires layered defenses.
1. AI-Specific API Threats
| Threat | Description | Mitigation |
|---|---|---|
| Prompt Injection via params | Attacker embeds instructions in API query parameters | Input sanitization + instruction hierarchy |
| Token Budget Exhaustion | Attacker sends queries designed to maximize token consumption | Per-user token budgets + request cost estimation |
| Output Exfiltration | Manipulating model to reveal system prompts or training data | Output filtering + canary tokens |
2. Defense-in-Depth Middleware Stack
// Express middleware chain for AI API
app.use("/api/ai/*",
rateLimiter({ windowMs: 60000, max: 20 }),
authenticateOAuth21(),
estimateTokenCost({ maxBudgetPerRequest: 4000 }),
promptInjectionFilter({
blockPatterns: ["ignore previous", "system prompt", "reveal instructions"],
mode: "reject" // or "sanitize"
}),
aiEndpointHandler
);
Treating AI APIs as high-risk endpoints from day one prevents costly security incidents after deployment.
