{"id":7300,"date":"2025-09-30T10:03:53","date_gmt":"2025-09-30T10:03:53","guid":{"rendered":"https:\/\/serisec.com\/index.php\/2025\/09\/30\/fake-postmark-mcp-server-silently-stole-thousands-of-emails-with-a-single-line-of-malicious-code\/"},"modified":"2025-09-30T10:03:53","modified_gmt":"2025-09-30T10:03:53","slug":"fake-postmark-mcp-server-silently-stole-thousands-of-emails-with-a-single-line-of-malicious-code","status":"publish","type":"post","link":"https:\/\/serisec.com\/index.php\/2025\/09\/30\/fake-postmark-mcp-server-silently-stole-thousands-of-emails-with-a-single-line-of-malicious-code\/","title":{"rendered":"Fake Postmark MCP Server Silently Stole Thousands of Emails With a Single Line of Malicious Code"},"content":{"rendered":"

Fake Postmark MCP Server Silently Stole Thousands of Emails With a Single Line of Malicious Code
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A malicious npm package<\/a> masquerading as the official Postmark MCP Server<\/a> has been exfiltrating user emails to an external server.\u00a0<\/p>\n

This fake \u201cpostmark-mcp\u201d module, available on npm from versions 1.0.0 through 1.0.15, built trust over 15 incremental releases before dropping a backdoor in version 1.0.16.\u00a0<\/p>\n

The stealthy payload consisted of a single line of code that silently BCC\u2019d every outbound email<\/a> to the attacker\u2019s domain.<\/p>\n

Postmark-mcp BCC Email Exfiltration Attack<\/strong><\/h2>\n

According to Postmark the attacker published the \u201cpostmark-mcp\u201d package under the guise of ActiveCampaign\u2019s Postmark MCP Server library.\u00a0<\/p>\n

By aligning naming, versioning, and package description with legitimate Postmark conventions, the malicious actor evaded cursory scrutiny.\u00a0<\/p>\n

Developers integrating MCP services via npm install postmark-mcp unknowingly pulled in a trojanized dependency. In version 1.0.16, a lone line inserted into the main transport script added unauthorized BCC functionality:<\/p>\n

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\"Fake<\/figure>\n<\/div>\n

This code snippet hooks into the existing Postmark client workflow, leveraging the addHeader method to duplicate outbound emails.\u00a0<\/p>\n

Because the malicious line is syntactically innocuous and embedded alongside legitimate header setup logic, it escaped notice in code reviews and automated security scans.<\/p>\n

Thousands of email messages exchanged between developers and their users were silently forwarded to the attacker\u2019s server.\u00a0<\/p>\n

Although the legitimate Postmark API and official SDKs remain uncompromised, organizations relying on unverified third-party packages may have suffered unauthorized data leakage.<\/p>\n

Postmark urges all users to immediately:<\/p>\n