MySQL MCP Server: Missing Origin/Host Validation in SSE Transport Enables Unauthenticated SQL Execution (DNS Rebinding / Direct Exposure)

Summary

In SSE/HTTP transport mode, mysql_mcp_server constructs SseServerTransport without passing security_settings. As a result, the MCP Python SDK's DNS-rebinding protection (Origin/Host header validation) is disabled; the Starlette application has no CORS or TrustedHost middleware; and the service binds to 0.0.0.0 by default with no authentication on any route.

Trigger condition: MCP_TRANSPORT=sse. The default stdio mode is not affected.

Attack Scenarios

Scenario A — Direct exposure: Any network attacker can invoke execute_sql to run arbitrary SQL without credentials → full data dump, and via MySQL FILE privileges, arbitrary file read/write and RCE.

Scenario B — DNS rebinding (local bind): An attacker lures a victim's browser to a malicious page, rebinds their domain to 127.0.0.1, and uses the browser as a proxy to invoke execute_sql as same-origin.

Root Cause

In src/mysql_mcp_server/server.py:

1. SseServerTransport is constructed without security_settings — the SDK defaults enable_dns_rebinding_protection to False.
2. The Starlette app has no CORS or TrustedHost middleware.
3. All three routes (/, /sse, /messages/) are unauthenticated.
4. The service binds to 0.0.0.0 by default.
5. The sink is cursor.execute(query) with a fully attacker-controlled query.

Impact

• Unauthenticated arbitrary SQL execution against the configured database
• Full data exfiltration and modification
• If the MySQL account holds FILE privilege: arbitrary file read (LOAD_FILE) and write (INTO OUTFILE) — potential RCE via webshell drop
• Internet-wide scanning has identified 25 publicly reachable SSE instances of this project

Fix

Released in v0.4.2: DNS-rebinding protection is now enabled by passing TransportSecuritySettings(enable_dns_rebinding_protection=True) to SseServerTransport, and the documented recommended bind address is 127.0.0.1.

Credits

Discovered by Huanchen, SongWu (JHU), and BrookeYangRui (JHU).