Improper Access Control in Onionshare

Between September 26, 2021 and October 8, 2021, Radically Open Security conducted a penetration test of OnionShare 2.4, funded by the Open Technology Fund's Red Team lab. This is an issue from that penetration test.

• Vulnerability ID: OTF-009
• Vulnerability type: Improper Access Control
• Threat level: Low

Description:

Authenticated users (or unauthenticated in public mode) can send messages without being visible in the list of chat participants.

Technical description:

Prerequisites:

• Existing chatroom
• Access to the chatroom (Public or known Private Key)
• Either a modified frontend client or manual requests from burp/curl

If 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.