Discourse vulnerable to stored-dom XSS via Facebook Oneboxes

Vulnerability Description

Discourse is an open-source discussion platform. Prior to version 3.2.3 on the `stable` branch and version 3.3.0.beta3 on the `tests-passed` branch, an attacker can execute arbitrary JavaScript on users’ browsers by posting a specific URL containing maliciously crafted meta tags. This issue only affects sites with Content Security Polic (CSP) disabled. The problem has been patched in version 3.2.3 on the `stable` branch and version 3.3.0.beta3 on the `tests-passed` branch. As a workaround, ensure CSP is enabled on the forum.

References

• https://github.com/discourse/discourse/security/advisories/GHSA-5chg-hm8c-wc58
• https://github.com/discourse/discourse/commit/26aef0c288839378b9de5819e96eac8cf4ea60fd
• https://github.com/discourse/discourse/commit/311b737c910cf0a69f61e1b8bc0b78374b6619d2