Outline is an open source, collaborative document editor. A type confusion issue was found in ProseMirror’s rendering process that leads to a Stored Cross-Site Scripting (XSS) vulnerability in Outline. An authenticated user can create a document containing a malicious JavaScript payload. When other users view this document, the malicious Javascript can execute in the origin of Outline. Outline includes CSP rules to prevent third-party code execution, however in the case of self-hosting and having your file storage on the same domain as Outline a malicious payload can be uploaded as a file attachment and bypass those CSP restrictions. This issue has been addressed in release version 0.77.3. Users are advised to upgrade. There are no known workarounds for this vulnerability.