MCP Inspector validated hostile tool regex schemas on the UI thread

MCP Inspector validated an MCP server-provided outputSchema.pattern with AJV synchronously in the UI thread. A server could supply a catastrophic-backtracking regular expression and matching-looking structured content.

GHSA-2fw8-7jmw-r937 reports rapidly increasing validation time for longer strings and a fix in 2.14.0 (and 1.0.4 on the older branch). The demonstrated effect is client availability degradation, not data theft.

An MCP server can define a tool output schema, including a regular expression supplied in outputSchema.pattern. In the reported flaw, Inspector passed that schema into AJV and validated near-matching structured content synchronously on the UI thread. Catastrophic backtracking then made the interface unresponsive as input length increased; this is a denial-of-service mechanism rather than an exfiltration primitive. The advisory reports fixes in 2.14.0 and the older 1.0.4 branch. Length limits alone are not always enough when expression complexity is uncontrolled. Validation should be time-bounded or run away from the UI with clear rejection behavior for hostile schemas.

The input is an MCP server's schema, not a user typing a long search query. A hostile pattern combined with near-matching structured output forces synchronous validation on the browser's main thread, so a protocol feature becomes a user-interface denial-of-service route. Other Inspector advisories deal with executable import arguments and iframe tool authority; those have different impacts. Safe schema handling requires time or complexity bounds, including an explicit fallback when an untrusted pattern cannot be evaluated safely.