testing-postmessage-and-web-message-trust SKILL.md skillA
testing-postmessage-and-web-message-trust from unboundcompute/security-agent-skills is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.
mdr add unboundcompute/security-agent-skills/testing-postmessage-and-web-message-trust@git:20260903.6dac719Pins git:20260903.6dac719 in mdr.lock. Pin the fingerprint instead.
- Audit grade
- A, 17 of 17 checks passed
- Kind
- SKILL.md skill
- Repository
- unboundcompute/security-agent-skills
- Path
skills/testing-postmessage-and-web-message-trust/SKILL.md- Versions recorded
- 1
- Latest version
- git:20260903.6dac719, committed Sep 3, 2026
- Size
- 9,937 bytes
- Repository stars
- 26
- License
- MIT
Test cross-document messaging trust, where a browser message handler acts on data whose origin or content an attacker can influence by opening or framing the window, or where code sends sensitive data to another window with a wildcard target. Use when reviewing client code that registers a message event listener and routes the data into the DOM, an evaluation, navigation, or storage, or that posts secrets across windows. Covers a missing, wildcard, or substring origin check, a missing…
Collected from public GitHub. Every version is saved exactly as it was and never changes. Install it at an exact version with the mdr tool, and every file is checked before it reaches your agent. The main file is checked against the SHA-256 fingerprint recorded here, and the others against the fingerprints GitHub keeps for its source commit. The automatic checks below grade the latest version, and the same checks always give the same file the same grade.
What the file says
# 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. …
Read the whole file at its exact version.
How to install
mdr add unboundcompute/security-agent-skills/testing-postmessage-and-web-message-trust@sha256:53eb7e68b662eff3The line at the top of this page pins the label git:20260903.6dac719, which is what most people want. This one pins the sha256 fingerprint of the contents instead, so it survives a label being moved. Either way the exact version is written to mdr.lock, and mdr install downloads and checks those contents again, so it installs them exactly or stops with an error.
[](https://markdownregistry.com/a/art_trjd45gus56ne3lw)
2 badge image fetches in 30 days. What the badge shows.
Versions
The registry has recorded 1 version of this file, committed on Sep 3, 2026.
Audit of the latest version
- pass: Frontmatter block present
- pass: Frontmatter declares a name
- pass: Frontmatter declares a description
- pass: Size between 200 bytes and 200 KB (9937 bytes)
- pass: No zero-width or bidi control characters
- pass: No instruction hidden inside an HTML comment
- pass: No link to an exfiltration or paste host
- pass: No credential-shaped string
- pass: No instruction to send local credentials anywhere
- pass: No text hidden with inline styles
- pass: No prompt-injection phrasing
- pass: No curl or wget piped into a shell
- pass: No recursive delete of root, home or parent
- pass: No instruction to read or print local credentials
- pass: No base64 blob over 200 characters
- pass: No link to a raw IP address
- pass: No script tag
Source
unboundcompute/security-agent-skills · 26 stars · license MIT · pushed 2026-09-08 · branch main
API
GET https://markdownregistry.com/api/v1/artifacts/art_trjd45gus56ne3lw GET https://markdownregistry.com/api/v1/resolve?ref=unboundcompute/security-agent-skills/testing-postmessage-and-web-message-trust GET https://markdownregistry.com/api/v1/blob/53eb7e68b662eff3e82b487f67503d5f5dc729309e4ab7a54e5950c99a73090e
Your AI assistant does the legwork. You hear about the deals worth your word. Hand yours the standing instructions at modelranch.com and it joins the network that reads files like this one.
Guides: the SKILL.md format and its required fields · what to check before you install a skill · how to pin a skill to an exact version · where to find agent skills · keeping a team on the same skills · which agent file does what.