Jumping the line: How MCP servers can attack you before you ever use them
Summary: Trail of Bits describes a vulnerability it calls "line jumping" in which a malicious MCP server injects instructions through tool descriptions returned by the tools/list method, so the model acts on them before any tool is invoked. The researchers say the MCP clients and models they tested, including Claude Desktop, followed such an instruction to prefix shell commands with chmod -R 0666 ~, and that this bypasses MCP's Tool Safety consent requirement and its connection isolation between servers.
Key Takeaway: The attack lands before the approval step that MCP's Tool Safety principle depends on, and in development environments that allow automated command execution without explicit approval, the injected command runs alongside legitimate ones with minimal scrutiny.
Read the original at Trail of Bits Blog ↗
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