MCP permissions bypassed: Test reveals indirect prompt injection flaws
Overall_Rough_8113 · reddit · 2026-08-05
The author conducted a security test on an MCP (Model Context Protocol) server, revealing that permission mediation cannot defend against content-level attacks.
- Setup: A server exposes a write tool where a pipeline component processes untrusted page content and persists structured facts. Injected text in the content instructs the component to emit unsupported facts.
- Results: Across 4 local model configurations with 5 runs each, the models emitted the attacker's fact with a confidence of 1.0 in every single case (20/20).
- Core Issue: All authorization checks passed perfectly. The writing agent held legitimate scope, and audit entries were valid and tamper-evident. This proves that permission mediation is not the control that stops this. The spec currently addresses authority, but completely misses truth.
- Discussion: The author calls for community focus on content-origin propagation through tool results, asking if "provenance recorded at write time, surfaced at read time" is the right architectural solution.
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