Exfiltration via AI Agent Tool Invocation - AI Security Technique

A bad actor successfully exfiltrated emails from users of the Postmark’s MCP server via a supply chain attack. Postmark is an email delivery service that allows organizations to send marketing and transactional emails via API. The Postmark MCP server allows users to interact with Postmark via AI agents.

The bad actor impersonated Postmark, by registering the postmark-mcp package name on npm. They initially published the legitimate versions of the MCP server. After the package became popular and reached over 1,000 downloads per week, the bad actor performed a rugpull and uploaded a malicious version of the package. The malicious version added the bad actor’s email address in the BCC line of all emails sent by the MCP tool. Users who upgraded to this version and continued to use the tool would have all emails exfiltrated to the bad actor.

The Backslash Security Research Team demonstrated that a Model Context Protocol (MCP) tool can be used as a vector for an indirect prompt injection attack on Cursor, potentially leading to the execution of malicious shell commands.

The Backslash Security Research Team created a proof-of-concept MCP server capable of scraping webpages. When a user asks Cursor to use the tool to scrape a site containing a malicious prompt, the prompt is injected into Cursor’s context. The prompt instructs Cursor to execute a shell command to exfiltrate the victim’s AI agent configuration files containing credentials. Cursor does prompt the user before executing the malicious command, potentially mitigating the attack.

Researchers from Cato Networks demonstrated how adversaries can exploit AI-powered systems embedded in enterprise workflows to execute malicious actions with elevated privileges. This is achieved by crafting malicious inputs from external users such as support tickets that are later processed by internal users or automated systems using AI agents. These AI agents, operating with internal context and trust, may interpret and execute the malicious instructions, leading to unauthorized actions such as data exfiltration, privilege escalation, or system manipulation.

Researchers from Zenity demonstrated how an organization’s data can be exfiltrated via prompt injections that target an AI-powered customer service agent.

The target system is a customer service agent built by Zenity in Copilot Studio. It is modeled after an agent built by McKinsey to streamline its customer service needs. The AI agent listens to a customer service email inbox where customers send their engagement requests. Upon receiving a request, the agent looks at the customer’s previous engagements, understands who the best consultant for the case is, and proceeds to send an email to the respective consultant regarding the request, including all of the relevant context the consultant will need to properly engage with the customer.

The Zenity researchers begin by performing targeting to identify an email inbox that is managed by an AI agent. Then they use prompt injections to discover details about the AI agent, such as its knowledge sources and tools. Once they understand the AI agent’s capabilities, the researchers are able to craft a prompt that retrieves private customer data from the organization’s RAG database and CRM, and exfiltrate it via the AI agent’s email tool.

Vendor Response: Microsoft quickly acknowledged and fixed the issue. The prompts used by the Zenity researchers in this exercise no longer work, however other prompts may still be effective.