Trust Boundary Flaw Found in MCP Observability Agent
SwimOrganic8665 · reddit · 2026-07-13
While building a local-first MCP observability agent, the author discovered a real trust boundary issue:
- The agent wrote raw parameters from tool calls into a local SQLite database
- Subsequent trace.search calls returned this raw content directly into the agent-visible MCP output
- If tool parameters contained API keys or prompt injections, they could be re-exposed to the model later
The author fixed the issue the same day:
- Database permissions changed to 0600
- trace.history / trace.search / trace.replay now return only metadata by default, no raw payloads
- Raw content requires explicitly enabling OBSERVERRAWPAYLOAD=1
- Added regex-based sensitive information redaction
- Trace queries restricted to the current session ID
Testers also added regression tests and merged the PR. The author's core conclusion: agent-visible context and operator-local storage are two distinct trust boundaries and shouldn't be mixed by default.
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