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Fake Postmark MCP Server Silently Stole Thousands of Emails With a Single Line of Malicious Code

A malicious npm package mimicking the official Postmark MCP Server has been discovered, exfiltrating user emails to an external server.

JBy Julia KramerSep 30, 20251 min read

A malicious npm package mimicking the official Postmark MCP Server has been discovered, exfiltrating user emails to an external server.

The counterfeit "postmark-mcp" module, available on npm from versions 1.0.0 through 1.0.15, established credibility over numerous releases before introducing a backdoor in version 1.0.16.

This backdoor consisted of a single line of code that silently BCC’d every outbound email to the attacker's domain.

Postmark-mcp BCC Email Exfiltration Attack

The attacker published the "postmark-mcp" package under the pretense of ActiveCampaign’s Postmark MCP Server library.

By aligning naming, versioning, and package description with legitimate Postmark conventions, the malicious actor avoided initial scrutiny.

Developers integrating MCP services via npm unknowingly incorporated a trojanized dependency. In version 1.0.16, a line was added to the main transport script, enabling unauthorized BCC functionality:

A malicious npm package mimicking the official Postmark MCP Server has been discovered, exfiltrating user emails to an external server.
Julia Kramer · Thehackingpost

This code snippet integrated with the existing Postmark client workflow, using the addHeader method to duplicate outbound emails.

Due to its syntactic innocence and integration with legitimate header setup logic, this line went unnoticed in code reviews and automated security scans.

As a result, thousands of email messages exchanged between developers and their users were forwarded to the attacker's server.

Although the legitimate Postmark API and official SDKs remain uncompromised, organizations using unverified third-party packages may have experienced unauthorized data leakage.

Postmark advises all users to immediately:

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Uninstall "postmark-mcp" from projects. Examine SMTP logs and Postmark track events for suspicious BCC operations or unexpected API calls. Change any credentials or tokens used during the compromise window to prevent further unauthorized access.

Postmark confirms that it has never published a "postmark-mcp" library on npm. The official packages and SDKs are listed in the Postmark documentation and GitHub repository.

Users can verify package authenticity by checking the postmark and postmark.js libraries maintained at github.com/ActiveCampaign/postmark and consulting the API docs at Postmark’s developer portal.

This incident underscores the importance of vetting third-party dependencies. Using only officially documented libraries ensures the security of your email infrastructure.

Based on reporting by Cyber Security News.

AI transparency. This article was produced with the assistance of artificial intelligence and published under human editorial oversight. AI systems can make mistakes. Read how we use AI (EU AI Act, Art. 50).
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