mcp: reject deeply nested tool arguments before validation - #1179
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Tool input is unmarshaled with a JSON decoder that imposes no nesting limit of its own, and schema validation can become quadratic in the nesting depth of the input. A small adversarial payload of a few tens of kilobytes can therefore burn minutes of CPU during validation. Scan the raw input for nesting depth before unmarshaling or validating, and reject input deeper than 1000 levels with an error. The scan is linear in the length of the input and ignores string literal contents.
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Summary
Tool input is unmarshaled with a JSON decoder that imposes no nesting limit of its own, and JSON Schema validation can become quadratic in the nesting depth of the input. A small adversarial payload (a few tens of kilobytes) can therefore burn minutes of CPU during validation of
tools/callarguments.This change scans the raw input for nesting depth before unmarshaling or validating, and rejects input deeper than 1000 levels. The scan is linear in the length of the input and ignores the contents of string literals.
Tests
TestApplySchemaRejectsDeeplyNestedInputinmcp/tool_test.go, covering: