HedgeDoc is software for creating real-time collaborative markdown notes. Prior to version 1.9.9, the API of HedgeDoc 1 can be used to create notes with an alias matching the ID of existing notes. The affected existing note can then not be accessed anymore and is effectively hidden by the new one. When the freeURL feature is enabled (by setting the `allowFreeURL` config option or the `CMD_ALLOW_FREEURL` environment variable to `true`), any user with the appropriate permissions can create a note by making a POST request to the `/new/<ALIAS>` API endpoint. The `<ALIAS>` parameter can be set to the ID of an existing note. HedgeDoc did not verify whether the provided `<ALIAS>` value corresponds to a valid ID of an existing note and always allowed creation of the new note. When a visitor tried