CVE-2026-48774: ProxySQL MCP run_sql_readonly executes side-effecting MySQL multi-statements despite read-only contract

ProxySQL is a proxy for MySQL and its forks, as well as PostgreSQL. In versions 3.0.0 through 3.0.8, ProxySQL's GenAI/MCP runsqlreadonly tool violates its documented read-only contract for MySQL targets. The tool validates only the full input string with a substring blacklist and first-keyword allowlist, but then executes the entire SQL string on a backend connection created with CLIENTMULTISTATEMENTS. As a result, a caller can submit a read-only first statement followed by a side-effecting second statement, such as SELECT 1; RENAME TABLE .... The validator accepts the payload because it starts with SELECT and because side-effecting MySQL statements such as RENAME TABLE, SET, RESET, LOCK TABLES, and KILL are not rejected by the blacklist. In a live MCP runtime test, the /mcp/query endpoint accepted a runsqlreadonly request. The MCP response reported success for the first SELECT, and direct backend verification showed that the table had actually been renamed. This violates the endpoint's read-only security contract and lets an MCP caller perform backend writes or administrative SQL, limited by the configured MCP target account's database privileges. Version 3.0.9 contains a fix. Other operator mitigations include: keeping MCP disabled unless required; setting a non-empty mcp-queryendpointauth token before exposing /mcp/query; restricting MCP listener network exposure; configuring MCP backend target credentials as database-level read-only users; and adding temporary MCP query rules to block obvious multi-statement patterns.