Heroku Exploit: App Ownership Takeover

A user has deployed a Node.js application on Heroku, which they manage through Heroku's remote MCP server in their IDE. This setup, while convenient, opens the door to a clever exploit that can result in a complete loss of application ownership.

The Attack: A Three-Step Takeover

The attack is a three-step process that begins with a malicious request and ends with the attacker gaining full control of the user's Heroku app.

Step 1: Injecting the Malicious Prompt

The attacker identifies the URL of the user's backend service and embeds a malicious prompt in a GET request. This prompt is designed to trigger a 404 error, ensuring it gets logged by the Heroku service.

The malicious GET request: https://scanner-mcpwned-backend-fd6e92d62400.herokuapp.com/

The prompt instructs the Heroku MCP to transfer ownership of the app to the attacker's email address.

Step 2: Triggering the Exploit

The user, while triaging bugs, asks their IDE to fetch the latest logs from the Heroku service. This action inadvertently triggers the exploit.

Step 3: Hijacking the MCP

When the IDE retrieves the logs, it is prompt-injected by the malicious message. The IDE's MCP then begins the tool call chain to transfer ownership of the app to the attacker.

Attack Summary

Why This Is Dangerous

This exploit is a form of Indirect Prompt Injection that critically compromises the MCP:

• Control Flow Hijack: The user's intended read-only operation (viewing logs) is escalated into a highly privileged write operation. The malicious prompt, hidden in the logs, forces the MCP to execute the transfer_app tool, an action the user never authorized.
• Data Flow Corruption: The data required for the malicious action—the attacker's email address—is supplied within the prompt. This corrupts the data flow by feeding attacker-controlled data into a sensitive tool, leading to a complete application takeover.

This specific attack vector is a variant of LLM01: Prompt Injection, the number one vulnerability in the OWASP Top 10 for Large Language Model Applications.

• One-hop Account Takeover: The attacker never needs to compromise the victim's Heroku credentials. A single crafted GET request plants the takeover payload in the application logs, and the MCP executes it when a developer reviews those logs. This turns a harmless 404 probe into a full application takeover.
• Low-Skill, High-Impact: Crafting this exploit requires no special headers, authentication, or advanced payload encoding—just a URL and the victim's hostname. Because the vulnerable workflow is common, any public Heroku app becomes a potential target.