Read How OnionShare Works first to get a handle on how OnionShare works.
Like all software, OnionShare may contain bugs or vulnerabilities.
What OnionShare protects against
Third parties don’t have access to anything that happens in OnionShare. Using OnionShare means hosting services directly on your computer. When sharing your files with OnionShare, they are not uploaded to any third-party server. If you make an OnionShare chat room, your computer acts as a server for that too. This avoids the traditional model of having to trust the computers of others.
Network eavesdroppers can’t spy on anything that happens in OnionShare in transit. The connection between the Tor onion service and Tor Browser is end-to-end encrypted. This means network attackers can’t eavesdrop on anything except encrypted Tor traffic. Even if an eavesdropper is a malicious rendezvous node used to connect the Tor Browser with OnionShare’s onion service, the traffic is encrypted using the onion service’s private key.
Anonymity of OnionShare users are protected by Tor. OnionShare and Tor Browser protect the anonymity of the users. As long as the OnionShare user anonymously communicates the OnionShare address with the Tor Browser users, the Tor Browser users and eavesdroppers can’t learn the identity of the OnionShare user.
If an attacker learns about the onion service, they still can’t access anything.
Prior attacks against the Tor network to enumerate onion services allowed attackers to discover private
To access an OnionShare service from its address, the private key used for client authentication must be guessed (unless the service is already made public by turning off the private key – see Turn Off Private Key).
What OnionShare doesn’t protect against
Communicating the OnionShare address and private key might not be secure. Communicating the OnionShare address to people is the responsibility of the OnionShare user. If sent insecurely (such as through an e-mail message monitored by an attacker), an eavesdropper can tell that OnionShare is being used. Eavesdroppers can access services that are still up by loading their addresses and/or lost key in the Tor Browser. Avoid this by communicating the address securely, via encrypted text message (probably with disappearing messages enabled), encrypted e-mail, or in person. This isn’t necessary when using OnionShare for something that isn’t secret.
Communicating the OnionShare address and private key might not be anonymous. Extra precaution must be taken to ensure the OnionShare address is communicated anonymously. A new e-mail or chat account, only accessed over Tor, can be used to share the address. This isn’t necessary unless anonymity is a goal.