Mittwald mStudio MCP server breach: attacker compromises 56 customer hosting projects
Severity: high · Patch status: unknown · Since 2026-11-07
German hosting provider Mittwald CM Service says an attacker entered its mStudio platform through a flaw in the Model Context Protocol server that lets customers connect AI assistants to their accounts, and compromised 56 customer hosting projects. According to Mittwald's own post, the attacker first tested the flaw on 21 October, during the PHP2Shell patching rush, used ordinary customer accounts from 23 October, and exploited the MCP flaw on 27 October; Mittwald noticed on 4 November, about eight days later. It took the MCP server offline, which it identifies as the entry point, locked projects where traces were found, paused new registrations and suspended mStudio login and SSH access. mStudio came back on 7 November. After forensic review and repeated scans, Mittwald reports no further compromised projects and no current attacker activity on its own systems, and says the customer center and the sites managed in it were never affected. It has informed the authorities. Mittwald says the speed of the attacker's steps suggests AI tools; that is its inference and it has published no evidence for it. It is not yet known whether the flaw lies in Mittwald's own code or in open-source MCP libraries, and Mittwald has not said what data was accessed. Affected customers should check their hosted sites for unauthorised changes and review access logs for the window. Mittwald has promised a detailed account.
What to do
- Customers with projects on mStudio should check hosted sites for unauthorised changes, backdoors or modified files
- Check access logs for anomalous activity from 27 October to 4 November 2026, the window Mittwald describes
- As a precaution, change mStudio passwords and rotate API keys and secrets stored in mStudio or reachable over SSH from customer projects; Mittwald has not said which were accessed
- If you operate MCP servers exposing account management, restrict network access, enforce strong authentication, and review tool permissions
- Monitor Mittwald's promised detailed disclosure, due the week after 7 November, and apply any fixes or guidance promptly
Sources
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