Username spoofing 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-005
• Vulnerability type: Improper Input Sanitization
• Threat level: Low

Description:

It is possible to change the username to that of another chat participant with an additional space character at the end of the name string.

Technical description:

Assumed users in Chat:

• Alice
• Bob
• Mallory

1. Mallory renames to Alice .
2. Mallory sends message as Alice .
3. Alice and Bob receive a message from Mallory disguised as Alice , which is hard to distinguish from the Alice
   in the web interface.

Other (invisible) whitespace characters were found to be working as well.

Impact:

An adversary with access to the chat environment can use the rename feature to impersonate other participants by adding whitespace characters at the end of the username.

Recommendation:

• Remove non-visible characters from the username