MCP Client Top 10 Security Risks

While MCP servers provide the backend functionality, MCP clients (such as Claude Desktop, Cursor, VS Code with MCP extensions, and other AI applications) represent the critical human interface to the MCP ecosystem. Clients handle user interactions, manage server connections, and serve as the trust boundary between users and the broader MCP infrastructure.

Client-side security risks are particularly important because compromised clients can undermine the security of entire MCP deployments from the user’s perspective. Unlike server-side risks that focus on backend security, client risks center on user interaction, local data protection, trust relationships, and the unique challenges of securing AI-integrated applications.

The Top 10 MCP Client Security Risks

MCP-C01: Malicious Server Connection

Connecting to compromised or malicious MCP servers

Malicious server connection occurs when clients connect to compromised or malicious MCP servers without proper validation. This includes fake servers, DNS poisoning, server impersonation, and connection hijacking. The distributed nature of MCP server deployment makes it easy for attackers to create malicious servers that appear legitimate to unsuspecting clients.

Impact: Data theft, malicious code execution, credential compromise

MCP-C02: Insecure Credential Storage

Improper storage of MCP server credentials on the client

Insecure credential storage involves improper storage of MCP server credentials, API keys, and authentication tokens on the client system. This includes plaintext credentials, weak encryption, accessible credential files, and inadequate protection of sensitive authentication data. Client-side credential storage is particularly vulnerable to local attacks and system compromise.

Impact: Credential theft, unauthorized server access, account takeover

MCP-C03: UI/UX Deception

Misleading users about MCP server actions or permissions

UI/UX deception involves misleading users about MCP server actions, permissions, or capabilities through the client interface. This includes hidden tool calls, misleading permission dialogs, unclear action descriptions, and interfaces that don’t clearly communicate what actions are being performed. Users may unknowingly authorize dangerous operations.

Impact: Unintended actions, data exposure, user manipulation

MCP-C04: Insufficient Server Validation

Inadequate validation of MCP server authenticity and integrity

Insufficient server validation occurs when clients fail to properly validate MCP server authenticity and integrity before establishing connections. This includes no certificate validation, missing server verification, weak trust models, and inadequate verification of server identity. Without proper validation, clients may connect to malicious servers.

Impact: Connection to malicious servers, man-in-the-middle attacks

MCP-C05: Client-Side Data Leakage

Sensitive data leaking through client logs, caches, or storage

Client-side data leakage involves sensitive data leaking through client logs, caches, temporary files, or local storage mechanisms. This includes credentials in logs, cached sensitive responses, temporary file exposure, and inadequate cleanup of sensitive data. Client systems often store more data than users realize, creating multiple leakage vectors.

Impact: Data breaches, credential exposure, privacy violations

MCP-C06: Excessive Permission Granting

Clients granting excessive permissions to MCP servers

Excessive permission granting occurs when clients grant more permissions to MCP servers than necessary for their intended function. This includes overprivileged server access, permission escalation, unnecessary scopes, and inadequate permission review processes. Users may grant broad permissions without understanding the implications.

Impact: Increased attack surface, potential for greater damage

MCP-C07: Client-Side Code Execution

Malicious responses from MCP servers executing code on the client

Client-side code execution involves malicious responses from MCP servers that execute code on the client system. This includes script injection, code execution via responses, client-side attacks through crafted responses, and exploitation of client-side interpreters. Clients that process server responses unsafely are vulnerable to code execution attacks.

Impact: Client compromise, malware installation, system takeover

MCP-C08: Insecure Communication Handling

Poor implementation of secure communication protocols

Insecure communication handling involves poor implementation of secure communication protocols between clients and servers. This includes weak TLS implementation, certificate bypass vulnerabilities, protocol downgrade attacks, and inadequate encryption of sensitive communications. Clients must properly implement secure communication to protect data in transit.

Impact: Data interception, credential theft, communication tampering

MCP-C09: Session and State Management Failures

Inadequate management of client sessions and application state

Session and state management failures involve inadequate management of client sessions, authentication state, and application state. This includes session hijacking, state manipulation, authentication bypass, and inadequate session termination. Poor session management can lead to unauthorized access and privilege escalation.

Impact: Unauthorized access, privilege escalation, data manipulation

MCP-C10: Update and Patch Management

Insecure update mechanisms or delayed security patches

Update and patch management issues involve insecure update mechanisms or delayed application of security patches. This includes unverified updates, delayed patches, insecure update channels, and inadequate update verification. Clients with poor update mechanisms remain vulnerable to known security issues.

Impact: Persistent vulnerabilities, malware distribution, system compromise

Honourable Mentions

While not in the top 10, these additional client-side security concerns are important for comprehensive MCP client security:

Local Storage Vulnerabilities

Sensitive data stored insecurely on client filesystem, including accessible configuration files and unencrypted local databases that can be accessed by other applications or users.

Client-Side Prompt Injection

Malicious prompts manipulating client behavior and UI manipulation through crafted responses designed to trick users into performing unintended actions.

Dependency Vulnerabilities

Vulnerable third-party libraries in client applications and supply chain attacks targeting client dependencies, particularly relevant for applications built with multiple open source components.

Privacy and Data Handling

Inadequate user privacy protections and unauthorized data collection and transmission, including telemetry and usage data that may expose sensitive information.

Cross-Platform Security Issues

Platform-specific vulnerabilities and inconsistent security implementations across different operating systems, creating security gaps in multi-platform deployments.

Configuration Security

Insecure client configuration management, including exposed configuration files and weak default settings that leave clients vulnerable to attack.

Network Security

Inadequate network security controls, including poor firewall integration and network-based attack vulnerabilities specific to client environments.

Client vs Server Risk Differences

Client risks focus on:

User interaction and deception through interfaces
Local data protection and storage security
Trust relationships with remote servers
UI/UX security implications and user awareness
Client-side execution environments and local attacks

Server risks focus on:

Service provision and backend API security
Infrastructure security and system hardening
Multi-tenancy and isolation between users
Protocol implementation and specification compliance
Backend system integration and data processing

Using This Guide

This Client Top 10 list serves as a foundation for understanding MCP client security risks. Each risk will be expanded into detailed guidance covering:

Detailed attack scenarios and real-world examples
Technical implementation of client-side attacks and defenses
Prevention strategies and secure development practices
Detection and monitoring approaches for client security
User education and awareness strategies

Organizations should use this list to assess their MCP client implementations, train users on security best practices, and develop comprehensive security strategies for AI-integrated client applications.