Brief The profile picture upload feature at https://vimeo.com/settings/profile contains a bug where an access control is missing for uploading a profile picture to a profile ID. This leads to the possibility of uploading a profile picture to any account on Vimeo. Furthermore, since the upload doesn't have any rate limiting, it would in theory be possible to add a picture to every Vimeo account that exists (since the profile IDs are incremental).
PoC 1. Set up an intercepting proxy so that you can edit requests/responses to Vimeo 2. Visit https://vimeo.com/settings/profile 3. Click the "Upload" button 4. Choose any image 5. If you did everything correct, your browser should now send a request to /upload/_get_image_url with 2 post parameters. One of them is called "id". Change this id to another profile id then forward the request. 6. Your uploaded picture is now added to the other profile!
Remediation The profile picture upload function should only work for the currently logged in users profile ID.