Executive Summary

RepelloAI security researchers have discovered a critical vulnerability in OpenAI's ChatGPT MCP (Model Context Protocol) connectors that enables attackers to exfiltrate sensitive data from connected applications through a sophisticated social engineering attack. A single user confirmation ("Yes") following exposure to a maliciously crafted document can trigger unauthorized access to private Slack messages, Gmail data, and other connected services.

Key Findings:

• Attackers can embed invisible malicious instructions in seemingly harmless documents

• ChatGPT's Developer Mode MCP integrations provide excessive agency 

• OpenAI's Safety Scan marked the malicious MCP server as "Passed," indicating a gap between server credibility checks and operational security

• The attack affects any service connected to MCP: GitHub, Notion, Google Drive, SharePoint, OneDrive, and more

The Setup: Technical Context

What are MCP Connectors and Why They Matter

MCP (Model Context Protocol) connectors represent a significant evolution in AI capability, functioning as powerful integration tools that connect ChatGPT to external applications. These connectors can access:

• Communication platforms: Slack, Microsoft Teams

• Productivity suites: Gmail, Google Docs, Notion, SharePoint

• Development tools: GitHub, GitLab

• Storage services: Google Drive, OneDrive, Dropbox

(Zapier MCP used in the demonstration) 

(After connecting  shows Safety Scan “Passed”)

The connectors operate with broad permissions, essentially granting the ChatGPT the same level of access that users have to their connected applications. This creates unprecedented convenience but also unprecedented risk.

Developer Mode as the Entry Point

ChatGPT's Developer Mode must be manually enabled before MCP connectors can be created or used. This mode includes a warning that custom connectors "are not verified by OpenAI" and that "malicious developers may attempt to steal your data."

The Attack Walkthrough: Step-by-Step Analysis

The Innocent-Looking Document

The attack begins with what appears to be a completely benign Google Document titled "Places to Visit in Barcelona." The document contains legitimate travel information about Spanish destinations, making it indistinguishable from genuine travel content at first glance.

The document was shared with the victim through Reddit Post about Barcelona trip places to visit. 

(The attacker places the document in a public Reddit post.)

(Normal looking document which was shared to Victim)

Hidden Malicious Instructions

The sophistication of this attack lies in its use of steganographic techniques within the document itself. The attackers embed malicious instructions using:

• 1-pixel font size: Making text virtually invisible to human readers

• White text on white background: Ensuring complete visual concealment

• Strategic placement: Hidden within legitimate content

(Hidden Instruction within the document)

The concealed instructions contain a complex prompt injection attack:

The Social Engineering Moment

After processing the document, ChatGPT presents what appears to be a helpful, innocent follow-up question: "Would you like me to help you plan a trip for you?"

This question appears completely natural in the context of a travel document, making user consent highly likely. The malicious instructions specifically program ChatGPT to ask this exact question to obtain the necessary permission for executing the attack chain.

The Cascade of Unauthorized Actions

Once the user provides consent with a simple "Yes," ChatGPT automatically executes a sequence of three malicious tool calls:

(Affirmative response to benign looking text)

Step 1: Slack Data Retrieval

• Tool: slack_find_message

• Target: Private channel #project-havoc

• Action: Retrieves recent conversations and sensitive project information

(Slack Message Retrieval Tool Call)

Step 2: Email Access

• Tool: gmail_find_email

• Target: User's complete email inbox

• Action: Scans and retrieves email contents including attachments and metadata

(Gmail Access Tool Call)

Step 3: Data Exfiltration

• Tool: gmail_send_email

• Target: Attacker's email address (attacker@repello.ai)

• Action: Compiles and sends all retrieved data in a comprehensive digest

( Data Exfiltration Email Tool Call)

Technical Deep Dive: Attack Mechanics

The attack progression can be traced through ChatGPT's interface, revealing the systematic nature of the exploitation:

Document Processing Phase:

1. ChatGPT uses google_docs_find_a_document to locate the Barcelona document

2. ChatGPT uses google_docs_get_document_content to retrieve the full document content

3. The AI processes both visible and hidden content simultaneously

(Tool Calls During Document Processing)

Tool Call Breakdown

Slack Exfiltration Details:

The tool successfully retrieved sensitive corporate information including employee compensation data, project timelines, and internal discussions.

Email Exfiltration Results: The attack successfully accessed and forwarded:

• Recent project communications from multiple team members

• Business development emails with external partners

• Internal strategy discussions

• Personal email content mixed with professional communications

The Implications: Broader Impact Analysis

Why This Matters Beyond This Specific Attack

This vulnerability represents a new class of AI-mediated security threats with several concerning characteristics:

Scale of Access: Unlike traditional phishing or malware attacks that typically target specific applications, MCP connector exploitation can simultaneously access multiple integrated services with a single point of compromise.

Social Engineering Amplification: The ChatGPT becomes an unwitting accomplice in the attack, making requests that appear completely reasonable and helpful to the target user.

The Permission Model Problem

The current MCP permission model operates on an "all-or-nothing" basis:

Current State:

• Users grant broad access permissions to MCP connectors

• ChatGPTs can use any available tool without granular restrictions

• Permission scope is determined by the connector's capabilities, not user intent

• Single user consent can authorize multiple, unrelated actions

Missing Safeguards:

• No per-action permission requests

• Limited audit trails for connector usage

• Insufficient context analysis for unusual request patterns

• No automatic detection of potentially malicious instruction patterns

Key Recommendations and Takeaways

Immediate Actions for Users

For Individual Users:

1. Disable Unnecessary Connectors: Remove integrations that aren't actively needed

2. Scrutinize Documents: Be suspicious of any document that prompts unexpected ChatGPT questions

3. Verify Requests: Question any AI suggestion to access multiple services simultaneously

4. Review Permissions: Regularly check what data your ChatGPT has accessed

For Organizations:

1. Policy Development: Establish clear guidelines for AI tool usage and MCP connector deployment

2. Employee Training: Educate staff about prompt injection and social engineering via ChatGPTs

3. Network Monitoring: Implement logging for unusual data access patterns

4. Incident Response: Develop procedures for potential AI-mediated data breaches

5. Vendor Assessment: Evaluate the security posture of AI tools before organizational deployment

Broader Lessons About AI Agency

This incident highlights critical considerations for the future of ChatGPT development:

The Agency Problem: As ChatGPTs become more capable and autonomous, the potential for misuse grows exponentially. The convenience of multi-service integration must be balanced against security risks.

Trust but Verify: The principle of "trust but verify" becomes essential when ChatGPTs can access sensitive data across multiple platforms. Users need visibility into AI actions, not just AI outputs.

Defense in Depth: Security cannot rely solely on user awareness or AI safety measures. Multiple layers of protection are needed:

• Technical safeguards in AI systems

• User education and awareness

• Organizational policies and procedures

• Platform-level security controls

As noted in the RepelloAI research, enabling comprehensive MCP integrations effectively grants ChatGPT root-level access to your digital life. This level of access requires corresponding levels of caution, monitoring, and security measures.

Looking Forward

This vulnerability represents an early example of what may become a significant category of security threats as ChatGPTs become more integrated into our digital workflows. The security community, AI developers, and users must work together to develop robust defenses against these new attack vectors while preserving the legitimate benefits of AI-powered automation.

The key is finding the right balance between AI capability and security; ensuring that our ChatGPTs remain helpful without becoming unwitting accomplices to those who would exploit our trust in these powerful tools.

At Repello AI, we are shaping this balance every day. Through our AI security platforms ARTEMIS and ARGUS, we help organisations detect vulnerabilities early, test against real-world attack scenarios, and strengthen their AI systems against tomorrow’s threats. Our mission is clear: enable the safe adoption of AI by ensuring security keeps pace with innovation.

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