Secure AI / LLM Features

Core assumption: prompt injection cannot be reliably prevented. Any text the model reads (user messages, web pages, emails, PDFs, RAG chunks, tool results, filenames, image text) can hijack it. Design so that a fully hijacked model still can't do damage. Security lives in deterministic code around the model, not in the prompt.

1. Keys & endpoints [LLM02, CWE-798]

• Provider keys are server-side only (env/secret manager). Never ship them in NEXT_PUBLIC_*/VITE_* or mobile apps, and never set dangerouslyAllowBrowser: true with a real key. The browser calls your backend, and the backend calls the model.
• Local model servers (Ollama :11434, vLLM, LM Studio) have no auth by default. Bind them to 127.0.0.1 or put them behind an authenticating proxy. Never expose them on 0.0.0.0 to the internet.

2. Treat model output as untrusted input [LLM05, CWE-94/79/89]

3. Tools & agents: limit agency [LLM06]

• Identity comes from the session, never from model arguments. A tool like get_order(order_id) must check order.user_id == session.user_id inside the tool.

  def get_order(order_id: str, *, ctx: RequestContext) -> dict:      # ctx injected by your code, not the model
      order = db.orders.find_one({"id": order_id, "user_id": ctx.user_id})
      if not order:
          return {"error": "not_found"}
      return {"id": order["id"], "status": order["status"]}             # minimal fields

• Least privilege: give each agent the fewest tools and the narrowest scopes (read-only DB user, per-user OAuth tokens, a specific directory). Separate read and write tools.
• Human-in-the-loop for consequential actions: payments, sending email/messages, deleting data, changing permissions, executing code, merging/deploying. Show the exact action and arguments for approval.
• The "lethal trifecta" (a term coined by Simon Willison, 2025) is private data + untrusted content + an exfiltration channel (web requests, email, image rendering, link unfurling). Never give a single agent context all three without strict egress allowlists.
• Validate tool arguments with schemas. Cap loops (max steps/tool calls), timeouts, and total tokens per task.
• Log every tool call (who, what, args, result status) for audit.

4. RAG & data access [LLM08, LLM02]

• Enforce document-level authorization at retrieval time. Filter the vector search by tenant_id/ACL metadata in the query, not by asking the model to ignore other users' data.
• Treat retrieved chunks as untrusted (indirect injection). Keep them in delimited data sections, not as instructions.
• Don't index secrets or credentials. Strip PII where not needed. Respect deletion requests in the vector store too.
• Validate and sanitize uploads before ingestion (size, type, malware scanning for office/PDF files).

5. Prompts [LLM01, LLM07]

• System prompts are not secret and not a security boundary. Don't put keys, internal URLs, or authz rules there and rely on them.
• Keep the system prompt static. Put user input in the user role. Clearly delimit untrusted content (e.g. <document>…</document>) and say it is data.
• Optional extra layers: input/output classifiers (Llama Guard, Prompt Guard, provider moderation, NeMo Guardrails), plus canary tokens to detect prompt leaks. Use them as defense in depth only.

6. Cost & abuse limits [LLM10, CWE-770]

• Authenticate AI endpoints. Use per-user rate limits and token/day budgets, plus max_tokens on every call and caps on input length (characters/tokens) and attachments.
• Concurrency caps per user, request timeouts, and retries with backoff that never loop.
• Provider-side spend limits and alerts. Cache identical requests where safe.

7. MCP servers & clients

• Servers: authenticate (OAuth 2.1 per the MCP auth spec for remote servers; local stdio servers only run tools for the local user). Validate every tool input. Use least-privilege credentials per user, never a shared admin token. No token passthrough to downstream APIs. Rate-limit. Bind local HTTP servers to 127.0.0.1 and validate the Origin header (DNS rebinding).
• Tool descriptions and results are prompt-injection vectors (tool poisoning). Keep descriptions static and honest. Don't return untrusted HTML or instructions as plain text without marking it as data.
• Clients/hosts: pin and review third-party servers, show tool calls to users, require approval for sensitive tools, and isolate servers from each other's credentials. Details: agent-tool-safety.md

8. Data & privacy [LLM02]

Tell users what is sent to third-party models. Don't send secrets or unnecessary PII. Check provider data-retention settings. Redact before logging prompts and completions, because logs often hold the most sensitive data in AI apps.

9. Supply chain for models [LLM03, LLM04]

Load weights in safetensors (never pickle/torch.load without weights_only=True). Don't use trust_remote_code=True on unvetted repos. Pin model revisions (commit hash) on Hugging Face. Verify sources.

Mapping to the OWASP LLM Top 10 (2025): llm-top10-2025.md

Sources: OWASP Top 10 for LLM Applications 2025 · MCP security best practices · Simon Willison, "The lethal trifecta for AI agents"

Before you reply

Run python3 ${CLAUDE_SKILL_DIR}/../secure-coding/scripts/secscan.py <changed files>. Confirm that keys are server-only, tools take identity from context, outputs are validated or encoded, and limits are set. Tell the user which actions need human approval.