Improper Access Control in Onionshare
Between September 26, 2021 and October 8, 2021, [Radically Open Security](https://www.radicallyopensecurity.com/) conducted a penetration test of OnionShare 2.4, funded by the Open Technology Fund's [Red Team lab](https://www.opentech.fund/labs/red-team-lab/). This is an issue from that penetration test.Vulnerability ID: OTF-009Vulnerability type: Improper Access ControlThreat level: LowDescription:Authenticated users (or unauthenticated in public mode) can send messages without being visible in the list of chat participants.Technical description:Prerequisites:Existing chatroomAccess to the chatroom (Public or known Private Key)Either a modified frontend client or manual requests from burp/curlIf a user opens the chatroom without emitting the join message he will not be present in session.users[x] list. Therefore there is no listing in the frontend and no chat participant knows another party joined the chat. It is still possible to send messages in the chatroom.If a user decides to abuse OTF-003 (page 22) he can impersonate messages from existing users; others would not be able to distinguish original and faked messages. This is also a prerequisite for OTF-004 (page 19).Impact:An adversary with access to the chat environment can send messages to the chat without being visible in the list of chat participants.Recommendation:Allow chat access only after emission of the join event.Implement proper session handling.