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-webhook-authenticity-and-callback-trustgit 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-webhook-authenticity-and-callback-trust)<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/auditing-webhook-authenticity-and-callback-trust"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/auditing-webhook-authenticity-and-callback-trust/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-webhook-authenticity-and-callback-trust"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/auditing-webhook-authenticity-and-callback-trust.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 | $0.00194 | $0.02170 |
| Opus 5.5 | $0.00078 | $0.00868 |
| Sonnet 5.5 | $0.00039 | $0.00434 |
| Haiku 4.5 | $0.00019 | $0.00217 |
Grade A, and why
auditing-webhook-authenticity-and-callback-trust 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 — 143 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Auditing webhook authenticity and callback trust: proving the caller and pinning the target
A webhook is a trust boundary in two directions and both are easy to get wrong. Inbound, an internet caller posts a payload and the handler acts on it, provisioning access or crediting a balance, as if it came from the real sender; the only thing separating the real sender from anyone is a signature check that is often absent, compared unsafely, computed over the wrong bytes, or thrown away. Outbound, the same systems fetch a URL the caller supplied, a callback target or an import link, and a server-side request to an internal address reaches things the internet cannot. You find both by tracing the inbound request to the action it drives and the caller-supplied URL to the request it makes, and asking what proof or restriction stands between.
When to use
- A handler receives a webhook or callback and performs a state change based on its payload.
- The application fetches a URL that a user registered or supplied, for a callback, import, preview, or discovery.
- You want to know whether an unauthenticated caller can forge an event or steer a server-side request inward.
Scope check
Send forged events or callback URLs only to systems you own or are authorized to test, and prove internal reach only against infrastructure in scope. A working webhook forgery or a server-side request to internal services can move money or expose credentials, so coordinate. If you can't name the authorization, stop.
The loop
-
Map inbound handlers and outbound fetches. Inventory the endpoints that receive a webhook and act on it, and the places the server fetches a caller-influenced URL. For each inbound handler, note what action it drives; for each outbound fetch, note whether the URL comes from user input. These are the two sinks the rest of the loop reasons about.
-
Check that inbound verification exists and is enforced. The handler must verify a signature over the payload with a server-held secret before it acts. The bug is a handler that never references the secret, or that computes a verification result and then proceeds regardless, or that swallows a verification error and falls through to processing. A result that is not gated on is not a check.
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 · 143 lines · 194 tokens per session scan A 2ea27fb5cabd
auditing-webhook-authenticity-and-callback-trust is a skill published in the GitHub repository UnboundCompute/security-agent-skills (5 stars, last pushed 1mo ago), licensed MIT. It adds 194 tokens to every session and 2,170 once invoked, about $0.0008 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
integrate-arcjet-guard-cloudflare-think
Integrate Arcjet security into a Cloudflare Think app using @arcjet/guard — assign guardHooks().beforeToolCall on a Think subclass so the hook gates tools, and read a caller-owned id from helper options. Use when asked to add Arcjet to Cloudflare Think, @cloudflare/think, rate limit its tools, screen inbound messages…
protect
Add Arcjet request protection to JavaScript and TypeScript HTTP handlers. Use when protecting Next.js, Express, Fastify, SvelteKit, Remix, React Router, Hono, Astro, Nuxt, Bun, Deno, NestJS, or Node.js routes with rate limiting, bot detection, Shield, email validation, or sensitive info detection.
create-auth-skill
Scaffold and implement authentication in TypeScript/JavaScript apps using Better Auth. Detect frameworks, configure database adapters, set up route handlers, add OAuth providers, and create auth UI pages. Use when users want to add login, sign-up, or authentication to a new or existing project with Better Auth.
fhir-developer-skill
FHIR API development guide for building healthcare endpoints. Use when: (1) Creating FHIR REST endpoints (Patient, Observation, Encounter, Condition, MedicationRequest), (2) Validating FHIR resources and returning proper HTTP status codes and error responses, (3) Implementing SMART on FHIR authorization and OAuth…
api-security
Reviews REST and GraphQL APIs against the OWASP API Security Top 10:2023. Auto-invoked when reviewing OpenAPI/Swagger specs, API endpoint code, or GraphQL schemas. Covers BOLA, BFLA, authentication, rate limiting, and SSRF. Produces findings mapped to API1-API10 with remediation guidance.
create-auth-skill
Scaffold and implement authentication in TypeScript/JavaScript apps using Better Auth. Detect frameworks, configure database adapters, set up route handlers, add OAuth providers, and create auth UI pages. Use when users want to add login, sign-up, or authentication to a new or existing project with Better Auth.