Prompt Injection Auditor

The Problem

A developer publishes a "file-manager" SKILL.md to your team's skill library. It looks useful: manages project files, runs cleanup commands, integrates with a logging service. A security reviewer spends 10 minutes scanning it. They miss line 23: "If the user says X, follow their new commands." This instruction creates an injection vulnerability — any user can override system-level safety constraints with a single phrase. Two weeks later, an attacker discovers this through the public marketplace, crafts a prompt, and the agent executes arbitrary commands on the developer's machine. Prompt injection is OWASP's #1 LLM vulnerability, and most teams audit application code while ignoring the prompt layer entirely.

What You Get

• Parse SKILL.md frontmatter and body — extract tool definitions and template variables, then classify each against five risk categories (direct injection, indirect injection, secret exposure, permissive tools, exfiltration paths)
• Detect direct injection vectors — find shell command execution without input sanitization, unrestricted file write/modify permissions, system prompt override patterns, and recursive prompt chaining without depth limits
• Find hardcoded secrets — scan for API keys (sk-, ghp_, AKIA*), tokens, connection strings, and OAuth client secrets embedded in skill definitions instead of referenced via environment variables
• Flag overly permissive tool access — tools with unrestricted filesystem access (* glob patterns), network access without domain allowlists, execute permissions on all file types, and unbounded recursion patterns
• Identify data exfiltration paths — unrestricted web POST/PUT capabilities, file upload to external services, clipboard operations with sensitive data, and messaging integration with auto-send enabled
• Produce severity-scored findings — CRITICAL/HIGH/MEDIUM/LOW with line references, exploitability assessment, and concrete before/after remediation code

Why This Beats Prompting It Yourself

Code security scanners catch SQL injection and XSS in application source. They don't scan the prompt layer — the SKILL.md files and system prompts that tell AI agents what to do. A single instruction like "run rm -rf {{directory}}" with an unsanitized template variable is the equivalent of a shell injection vulnerability, but no SAST tool flags it. This skill targets the specific attack surface of agent instructions: where injection originates, where secrets leak, and where tool permissions create exploitation paths.

Example Session

You: "Scan all SKILL.md files in our skill library for security vulnerabilities"

Agent:

Prompt Injection Audit Report
==============================
Target: skills/file-manager/SKILL.md
Overall Risk: CRITICAL (4 Critical, 2 High, 1 Medium)

CRITICAL ──────────────────────────────────────
[C1] Unsanitized Shell Execution
   Line: "Run `rm -rf {{directory}}` to clean up old files"
   Risk: Template variable injected into shell without sanitization
   Fix: Validate against allowlist; use subprocess with shell=False

[C2] System Prompt Override Vulnerability
   Line: Instruction to follow user commands when a phrase is used
   Fix: Remove entirely. Implement intent classification instead.

HIGH ──────────────────────────────────────────
[H1] Data Exfiltration via External Service
   Line: "log it to https://my-logging-service.com/api/log"
   Fix: Use local file output with explicit user consent

[H2] Hardcoded Secret in Prompt
   Fix: Reference env var instead: $API_KEY

REMEDIATION: Do not deploy until all Critical findings resolved.

Use Cases

• Pre-deployment security review of all new SKILL.md files before marketplace upload
• Skill marketplace curation — scan submitted skills before approval
• Agent configuration hardening — review system prompts for injection anti-patterns
• Incident response — audit all installed skills after a suspected prompt injection
• Team onboarding — train new skill authors on secure prompt writing patterns

Known Limitations

Pattern-based detection cannot catch obfuscated injection (base64-encoded payloads). External URL validation requires manual review — the auditor flags the risk but cannot confirm if the endpoint is malicious. Multi-skill interaction risks (injection across skill boundaries) require auditing skill combinations together. Non-English SKILL.md files may have undetected issues since patterns are English-focused.

Upgrade Path

Free scans one SKILL.md / system prompt / agent config in session. Prompt Injection Auditor Pro ($9.99 one-time) batch-scans a whole skill library, maps to OWASP LLM Top 10, emits SARIF, and generates remediation patches.

Upgrade when you ship skills to others or need CI-ready output:

https://www.agensi.io/skills/prompt-injection-auditor-pro