Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add UnboundCompute/security-agent-skills --skill auditing-webauthn-and-passkey-flowsgit clone --depth 1 https://github.com/UnboundCompute/security-agent-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/unboundcompute/security-agent-skills/auditing-webauthn-and-passkey-flows)<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/auditing-webauthn-and-passkey-flows"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/auditing-webauthn-and-passkey-flows/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/auditing-webauthn-and-passkey-flows"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/auditing-webauthn-and-passkey-flows.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00173 | $0.02129 |
| Opus 5.5 | $0.00069 | $0.00852 |
| Sonnet 5.5 | $0.00035 | $0.00426 |
| Haiku 4.5 | $0.00017 | $0.00213 |
Grade A, and why
auditing-webauthn-and-passkey-flows scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 142 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Auditing passkey and WebAuthn ceremonies: when a verified signature seats the wrong user
A passkey ceremony proves that some authenticator signed a challenge. It does not, by itself, prove who is signing in. Every WebAuthn bug worth finding lives in the gap between "this signature is valid" and "therefore user U is logged in," and that gap is closed only by a specific list of server-side checks. When one of those checks is missing, homegrown, or bypassable, attacker-shaped bytes from the ceremony response flow straight into an authenticated session. You find these by reading the verification path and asking, for each required check, whether it runs and whether the value it checks is the one the attacker cannot forge.
When to use
- Code verifies a WebAuthn registration or authentication response and then creates a credential or a session.
- The application implements the ceremony itself, or calls a library but supplies the policy and expected values.
- You want to know whether a valid signature can be replayed, phished, or bound to the wrong account.
Scope check
Test passkey flows only on systems you own or are authorized to assess, with test accounts and test authenticators. A ceremony bypass is an authentication bypass, so treat any confirmed finding as account-takeover-grade and coordinate disclosure. If you can't name the authorization, stop.
The loop
-
Map the verification path and whether a library owns the ceremony. Find the registration and authentication endpoints and the function that decides success. Determine whether the ceremony is delegated to a conformant verification library or hand-rolled. If a library owns it, the bug moves from "missing check" to "wrong arguments": read what expected-challenge, expected-origin, expected relying-party identifier, and required-user-verification values are passed in. A library called with defaults verifies only what you gave it.
-
Check challenge binding. The challenge in the response must equal a value the server generated, stored against this session, has not used before, and has not let expire. The bug is a challenge compared against a client-sent field, against a constant, or not at all, or a challenge that is never invalidated after use so the same response replays. If nothing reads a server-side challenge store before the compare, the ceremony is replayable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 1mo ago First seen · 142 lines · 173 tokens per session scan A 2ffc7910341c
auditing-webauthn-and-passkey-flows is a skill published in the GitHub repository UnboundCompute/security-agent-skills (5 stars, last pushed 1mo ago), licensed MIT. It adds 173 tokens to every session and 2,129 once invoked, about $0.0007 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-05.
Other skills, from other repositories
prodcheck-review
Review this codebase against the prodcheck pre-production checklists — security, performance, scale, integrations, post-launch readiness and people. Use when asked to check whether a project is ready to ship, to audit an area before launch, or to work through a specific checklist. Produces evidence with file:line…
integrate-arcjet-guard-genkit
Integrate Arcjet security into a Genkit JS agent using @arcjet/guard — wrap ai.defineTool, put guardMiddleware on generate({ use }) for unwrapped / MCP / filesystem tools, and read a caller-owned id from generate({ context }). Use when asked to add Arcjet to genkit, rate limit its tools, screen inbound messages, or…
integrate-arcjet-guard-tanstack-ai
Integrate Arcjet security into a TanStack AI chat() app using @arcjet/guard — put guardMiddleware first on chat({ middleware }) so onBeforeToolCall gates tools, and read a caller-owned id from chat({ context }). Use when asked to add Arcjet to TanStack AI, rate limit its tools, screen inbound messages, or block prompt…
integrate-arcjet-guard-claude-managed-agents
Integrate Arcjet security into Claude Managed Agents (hosted REST+SSE, beta managed-agents-2026-04-01) using @arcjet/guard — screen user.message / initialevents before sessions.events.send, and gate custom tools on agent.customtooluse. Use when asked to add Arcjet to Claude Managed Agents, rate limit custom tools, or…
linear-claude-skill
Manage Linear issues, projects, and teams.
varlock-claude-skill
Secure environment variable management ensuring secrets are never exposed in Claude sessions, terminals, logs, or git commits.