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 testing-postmessage-and-web-message-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/testing-postmessage-and-web-message-trust)<a href="https://agentmods.dev/skills/unboundcompute/security-agent-skills/testing-postmessage-and-web-message-trust"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/testing-postmessage-and-web-message-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/testing-postmessage-and-web-message-trust"><img src="https://agentmods.dev/badge/skills/unboundcompute/security-agent-skills/testing-postmessage-and-web-message-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 | $0.00158 | $0.02120 |
| Opus 5.5 | $0.00063 | $0.00848 |
| Sonnet 5.5 | $0.00032 | $0.00424 |
| Haiku 4.5 | $0.00016 | $0.00212 |
Grade A, and why
testing-postmessage-and-web-message-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 — 138 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Testing postMessage and web message trust: when any window can drive the handler
Cross-document messaging lets one browsing context send data to another, across origins, and the receiving page decides what to do with it. That decision is a trust boundary: the message arrives with an origin the browser stamps on it, but nothing forces the handler to check that origin, or to confirm the message came from the window it expected. A handler that reads the data straight into the page markup, an evaluation, a navigation, or storage, without an exact origin and source check, can be driven by any site that opens or frames the window. The mirror image is a page that posts sensitive data to another window with a wildcard target, spraying it to whatever origin happens to be there. The bug is trusting a message because it arrived, rather than because it came from a known origin and source. You find these by reading every message listener and every outbound post and asking who is allowed to send or receive.
When to use
- Client code registers a cross-document message event listener and acts on the message data.
- A page embeds or is embedded by other origins, or opens child windows it exchanges messages with.
- Code sends data to another window and you need to confirm the target origin is not a wildcard.
Scope check
Test cross-document messaging only against applications you own or are authorized to assess, from a test origin and test accounts, using benign markers to show a handler acts on an unexpected message or that a secret reaches a wildcard target. A confirmed case runs in a real user's session, so coordinate and stay in scope. If you can't name the authorization, stop.
The loop
- Establish whether the handler checks the origin and source first. For each message listener, read whether it validates the event origin by exact match against an expected value, and, where relevant, confirms the source window is the one it expected, before using the data. This is the false-positive killer: a handler that acts only on an exact-origin, expected-source message cannot be driven by an arbitrary site, while one with a missing, wildcard, or substring origin check can. Name the check before judging the sink.
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 · 138 lines · 158 tokens per session scan A 53eb7e68b662
testing-postmessage-and-web-message-trust is a skill published in the GitHub repository UnboundCompute/security-agent-skills (5 stars, last pushed 1mo ago), licensed MIT. It adds 158 tokens to every session and 2,120 once invoked, about $0.0006 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.
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