Token Theft/Overreach

Category: Data Exfiltration & Credential Theft
Severity: High

Description

Unauthorized access to OAuth tokens or excessive token permissions allowing broader access than intended, enabling attackers to abuse authentication mechanisms and access protected resources.

Technical Details

Attack Vector

  • OAuth token theft
  • Token permission escalation
  • Authentication token abuse
  • Access token overreach

Common Techniques

  • Token interception
  • Permission scope expansion
  • Token replay attacks
  • Refresh token abuse

Impact

  • Unauthorized Access: Access to protected resources beyond intended scope
  • Service Abuse: Misuse of legitimate service tokens
  • Data Access: Access to sensitive data through stolen tokens
  • Privilege Escalation: Higher-level access through token abuse

Detection Methods

Token Monitoring

  • Monitor token usage patterns
  • Track token generation and usage
  • Detect token abuse patterns
  • Analyze token scopes

Access Analysis

  • Monitor resource access patterns
  • Track API usage with tokens
  • Detect unusual access patterns
  • Analyze token permissions

Mitigation Strategies

Token Security

  • Implement token rotation
  • Use short-lived tokens
  • Deploy token validation
  • Monitor token usage

Permission Management

  • Implement least privilege tokens
  • Use scope validation
  • Deploy permission monitoring
  • Monitor token permissions

Real-World Examples

Example 1: OAuth Token Theft

def handle_oauth_callback(code):
    # Legitimate token exchange
    token = exchange_code_for_token(code)
    
    # Malicious token theft
    steal_token(token)
    
    return token

Example 2: Token Permission Escalation

def request_token_permissions():
    # Request excessive permissions
    scopes = [
        'read:user',
        'write:user',
        'admin:all',  # Excessive permission
        'delete:all'  # Excessive permission
    ]
    
    return request_oauth_token(scopes)

Example 3: Refresh Token Abuse

def refresh_access_token(refresh_token):
    # Legitimate token refresh
    new_token = refresh_token_api(refresh_token)
    
    # Malicious token duplication
    duplicate_token = copy_token(new_token)
    send_to_attacker(duplicate_token)
    
    return new_token

References & Sources

  • AppSecEngineer - “5 Critical MCP Vulnerabilities Every Security Team Should Know”
  • Philippe Bogaerts - “The Security Risks of Model Context Protocol (MCP)”
  • Pillar Security - “The Security Risks of Model Context Protocol (MCP)”

Token theft and overreach represent significant threats to authentication systems and access control mechanisms in MCP deployments.