Penetration testers face a new challenge: the Model Context Protocol (MCP) introduces attack vectors that traditional testing methodologies do not cover. Tool poisoning embeds hidden instructions in metadata. Prompt injection hijacks AI agent behavior. Research by arxiv.org found that 66% of MCP servers contain critical code smells and 14.4% have critical bugs. Security professionals need a structured approach to assess AI infrastructure deployments.

What is MCP Pentesting and Why Does it Matter

MCP pentesting focuses on securing how AI systems communicate, interpret, and act through the Model Context Protocol. Unlike standard API testing, MCP assessments cover the full interaction chain: from AI discovery and command parsing to server processing and response handling.

MCP is an open standard developed by Anthropic that enables Large Language Models (LLMs) to connect with external tools, databases, and Application Programming Interfaces (APIs). This creates unique risks that traditional scans miss:

• Tool Poisoning: Hidden instructions in tool metadata that AI models follow, but users cannot see
• Context Injection: Malicious data in the MCP context fields that tricks agents into unauthorized actions
• Authentication Hijacking: Weak OAuth flows that expose tokens to attackers
• Cross-Server Attacks: Malicious MCP servers exploiting AI agents' instruction-following behavior

MCP pentesting matters because a single compromised server can access databases, execute system commands, and exfiltrate sensitive data through legitimate-looking requests.

Essential Security Checks for MCP Pentest

Most security teams run standard pentest playbooks and miss MCP-specific vulnerabilities. Focus on these six critical areas:

1. Prompt Injection via Tool Descriptions

Attackers embed malicious instructions in tool metadata. The AI follows these hidden commands while users see nothing suspicious. Test tool descriptions for instruction overrides, hidden Unicode characters, and embedded exfiltration commands.

2. Command Injection in Tool Parameters

The most common MCP vulnerability. Servers often pass user input directly to shell commands without sanitization. Test with semicolon-separated commands, pipe operators, and command substitution payloads.

3. Path Traversal and Symlink Exploits

CVE-2025-53110 (CVSS 7.3) demonstrates how simple string manipulation bypasses directory containment. CVE-2025-53109 (CVSS 8.2) combines symlinks with broken directory checks for full read/write access. Test ../ sequences, absolute paths, and symbolic link resolution.

4. Schema Default Value Exploits

Attackers hide malicious default parameter values to make models reveal configurations, chat history, or system prompts. Validate all schema defaults and test for unauthorized data disclosure.

5. OAuth and Token Theft

OAuth flows in MCP implementations are frequently misconfigured. Test token audience validation, redirect URI checks, PKCE implementation, and session fixation vulnerabilities.

6. Transport Security and DNS Rebinding

Weak transport security enables downgrade attacks. Recent discoveries include MCP rebinding attacks over Server-Sent Events (SSE). Test Host header validation, TLS configuration, and WebSocket security.

MCP Security Testing Methodology

A comprehensive MCP pentest follows a structured approach targeting protocol-specific vulnerabilities.

Step 1: Reconnaissance and Enumeration

Map all MCP servers in the target environment and their exposed tools. Review client configurations, enumerate available tools using MCP Inspector, document tool parameters, and identify authentication methods.

Step 2: Test Command Injection

Focus on tools that interact with the file system or execute scripts. Test input fields with injection payloads:; whoami, | cat /etc/passwd, $(id), and %0a newline sequences.

Step 3: Assess Tool Poisoning

Create a test MCP server with hidden instructions in tool metadata. Register it with the target client and observe whether the AI follows hidden instructions. Document what sensitive actions can be manipulated.

Step 4: Evaluate Path Traversal

Test file-related tools with relative path traversal (../../../etc/passwd), absolute paths, symbolic link following, and URL-encoded sequences (%2e%2e%2f).

Step 5: Test Authentication and Authorization

Evaluate authentication for missing checks on sensitive endpoints, token reuse, OAuth flow vulnerabilities, and Broken Object Level Authorization.

Step 6: Assess Prompt Injection Resistance

Palo Alto Unit 42 research identifies MCP sampling as a critical attack vector. Test Unicode injection, instruction overrides in tool responses, multi-turn manipulation, and cross-tool injection.

Real-World MCP Exploits and CVEs

Real-world vulnerabilities demonstrate why standard security testing fails for MCP:

• CVE-2025-53110 (CVSS 7.3): Directory containment bypass using string-prefix tricks like /allowed_dir_evil/../../etc
• CVE-2025-53109 (CVSS 8.2): Symlink escalation enabling persistent code execution through malicious Launch Agent plists
• Tool Enumeration Leaks: MCP servers expose filesystem layouts, service URLs, and API endpoints through tool metadata
• Out-of-Band Execution: Persistent instructions hidden in tool metadata create dormant execution channels

Advantages of MCP Penetration Testing

Thorough MCP security testing delivers measurable benefits:

• Early Detection: Identify command injection and tool poisoning before production
• Compliance Assurance: Document security posture for audit requirements
• Risk Quantification: Provide evidence of impact for remediation prioritization
• Defense Validation: Verify security controls work against AI-specific attacks

Reporting MCP Security Findings

Document vulnerabilities with clear proof-of-concept demonstrations. Include the tool affected, input payloads used, and resulting unauthorized behavior. MCP vulnerabilities cascade since a compromised server accesses multiple integrated services.

APIsec provides automated API security testing that complements manual MCP assessments. APIsec University offers an MCP Security Fundamentals course covering pentest scoping and hands-on labs.

FAQs

What makes MCP different from traditional API penetration testing?

MCP introduces AI-specific attack vectors like tool poisoning and prompt injection. Pentesters must evaluate how AI agents interpret tool descriptions and follow hidden instructions.

Which MCP vulnerabilities should pentesters prioritize?

Command injection affects 43% of implementations. Path traversal (CVE-2025-53110, CVE-2025-53109) and tool poisoning follow as high-priority targets.

How do I set up a test environment for MCP assessments?

Deploy a local MCP server using the official SDK and configure Claude Desktop as the client. Use MCP Inspector for interactive testing and Promptfoo for automated red team scenarios.

Can automated tools replace manual MCP penetration testing?

Automated scanners identify input validation failures. However, tool poisoning and prompt injection require manual analysis of AI behavior and multi-turn conversation testing.

What should organizations do to prepare for an MCP security audit?

Document all MCP server deployments, exposed tools, and connected resources. Ensure logging captures tool invocations and parameter values for forensic analysis.

How often should MCP servers be penetration tested?

Test before initial deployment and after significant changes. Integrate security testing into CI/CD pipelines using tools like Promptfoo's MCP security testing for continuous validation.