Individual skill
web-postmessage
Exploit insecure cross-window messaging (`window.postMessage`) — handlers that trust `event.data` without checking `event.origin`, leading to XSS, token theft, or state change. Load when the app uses iframes/popups/SSO widgets, on "postMessage", or when JS registers a `message` listener. Signals: `addEventListener("message", ...)`, embedded third-party frames, SSO/login popups.
skills/web/web-postmessage/SKILL.md
Source commit ee0e0c50d2f6d941a14b73dd85a2b470d0ab976e
What was checked
- Matches the observed repository HEAD
- Yes
- Scanner rules are current
- No
- Repository scan complete
- No
- Skill scan complete
- Yes
- Observed behavior
- Not checked yet
- Last checked
- Nov 7, 2026
These are observations at the recorded commit, not a guarantee of safety. Review the source before running it.
Install this skill
Choose an agent in your Stack or copy a command below. Commands fetch the pinned commit and copy only this skill directory. Existing destinations are refused. Copying a command does not install anything.
An install command is unavailable: current rules, complete repository and skill scans, an eligible behavior level and a permitted license are required.
Repository context
- Repository stars (not skill ratings)
- 27
- Repository last updated
- Oct 14, 2026
- License
- MIT
Observed agent integration: No specific integration evidence. Standard SKILL.md installation does not imply verified compatibility.
- agent-skills Repository topic
- ai-agents Repository topic
- ai-security Repository topic
- application-security Repository topic
- blue-team Repository topic
- bug-bounty Repository topic
- claude-code Repository topic
- codex Repository topic
- detection-engineering Repository topic
- gemini Repository topic
- data Derived category
- security Derived category
- security-application Derived category
Files and excerpt
File metadata and permitted, redacted excerpts come from the recorded scan. Source links are pinned to that commit.
Browse files
SKILL.mdcore · 2,756 bytes
Read excerpt
# postMessage abuse
## When it applies
The target sends or receives `postMessage` between windows/iframes (SSO, payment widgets, chat,
embeds). A receiver that doesn't validate the sender's origin will act on a message from *your*
page.
## Why it works
`postMessage` is cross-origin by design; safety depends entirely on the receiver checking
`event.origin` and the sender setting a specific `targetOrigin`. Miss the origin check and any site
can drive the handler; use `targetOrigin="*"` and any embedder can read the message. Then it's just
a matter of what the handler does with `event.data` — often `innerHTML`, `eval`, `location`, or a
relayed token.
