completeWebAuthnAssertionWithId
Complete a WebAuthn authentication ceremony by validating the signature against the previously generated challenge without logging the user in
This record as markdown: /tools/io-fusionauth-mcp-api/completewebauthnassertionwithid.md
What completeWebAuthnAssertionWithId does on Fusionauth
AI agents invoke completeWebAuthnAssertionWithId to trigger actions in Fusionauth. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call: builds kicked off, notifications sent, workflows started.
Why completeWebAuthnAssertionWithId is rated High
This tool performs an external cryptographic validation operation (WebAuthn assertion/signature verification) against a stored challenge. It triggers an authentication ceremony execution rather than simply reading data or writing a record. The 'without logging the user in' clause suggests it may be a partial flow, but it still executes a verification operation with side effects (consuming/validating a challenge).
From the tool's definition 'Complete a WebAuthn authentication ceremony by validating the signature against the previously generated challenge without logging the user in'
Attacks that exploit this kind of access
The rule that runs completeWebAuthnAssertionWithId safely
PolicyLayer is an MCP gateway: it sits between your AI agents and Fusionauth, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For completeWebAuthnAssertionWithId, this is the rule to start with:
completeWebAuthnAssertionWithId stays usable, but rate-capped: a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
The button opens the PolicyLayer dashboard: create your workspace, connect Fusionauth, apply this rule, and every completeWebAuthnAssertionWithId call is checked against it from then on.
Questions about completeWebAuthnAssertionWithId
Complete a WebAuthn authentication ceremony by validating the signature against the previously generated challenge without logging the user in. It is categorised as a Execute tool in the Fusionauth MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Fusionauth MCP server in PolicyLayer and add a rule for completeWebAuthnAssertionWithId: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Fusionauth. Nothing to install.
completeWebAuthnAssertionWithId is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the completeWebAuthnAssertionWithId rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.
Set action: deny in the PolicyLayer policy for completeWebAuthnAssertionWithId. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.
completeWebAuthnAssertionWithId is provided by the Fusionauth MCP server (@fusionauth/mcp-api). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
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