# Cross-origin messaging trust failure

A message handler trusts an unauthorized sender or leaks protected data to an unintended origin.

**Candidate CWE:** [CWE-346](https://cwe.mitre.org/data/definitions/346.html)

## Test hypothesis

Observe a legitimate postMessage exchange in a local controlled application. From a researcher frame/window, send its expected shape and change only sender origin; use a harmless state marker.

## Evidence required

The unauthorized sender triggers a protected action or receives the controlled private marker. Include window/frame relationship, message schema, origin checks, and normal-delivery prerequisites.

## Rule out false positives

A wildcard target on non-sensitive messages, a handler with no reachable security-sensitive action, or console-only invocation is not a complete finding.

## Playbook and command references

- [Browser execution, cross-origin reads, and forged actions](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/modules/browser-trust.html) — Use real browser behavior to distinguish XSS, CSRF, CORS, and messaging failures from header-only leads.

[Find the associated command snippets](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/commands.html?q=Browser%20execution%2C%20cross-origin%20reads%2C%20and%20forged%20actions)

## Referenced by these target playbooks

[Web & API](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/web-api.html) · [OSS & self-hosted](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/oss.html) · [Mobile](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/mobile.html) · [Desktop & extensions](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/desktop.html)

## Report checklist

- [ ] Capture the affected version, configuration, identities, and object ownership.
- [ ] Preserve a valid baseline and a negative control.
- [ ] Reproduce the forbidden data access, action, or state change on controlled fixtures.
- [ ] Explain the intended security rule and why the observed behavior violates it.
- [ ] State prerequisites, uncertainty, cleanup, and the smallest demonstrated impact.

## Conditional ATT&CK mapping

No direct ATT&CK mapping selected. Record the weakness and demonstrated impact without forcing an adversary-behavior classification. See the [mapping guide](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/attack.html).

## Primary references

- [OWASP HTML5 security](https://cheatsheetseries.owasp.org/cheatsheets/HTML5_Security_Cheat_Sheet.html)

[Back to the complete vulnerability catalog](https://bug-bounty-field-manual.kmahshooq.chatgpt.site/vulnerabilities.html)