The VORACLE attack vulnerability
Description
Security researcher Ahamed Nafeez has presented a new attack vector which targets VPN tunnels which utilize compression, named VORACLE. The attack vector bears similarities to the CRIME and BREACH attacks, which hit especially HTTPS based connections.
It has been discovered that it is possible to gain information about an encrypted VPN tunnel’s contents in very specific circumstances, if an attacker has the ability to capture the encrypted data packets while a certain type of data is transferred through the VPN tunnel. By that we mean that if a VPN user visits for example an unencrypted HTTP website through an encrypted VPN tunnel, and this information is being compressed and encrypted through the VPN tunnel, certain clues about the contents of this information can still be gathered if the encrypted packets can be captured and analysed, and data can be fed through the VPN tunnel by the attacker. To explain this better, following below is a simplified example. The example mentioned here is not the only possible attack against encryption combined with compression, and it is also very simplified, but it is useful to explain the principle behind attacks like VORACLE, CRIME and BEAST.
Let’s say Alice has setup a login page. To check passwords entered there, Alice sends a message like “tell me if <the password entered> matches <secret password>” to Bob. This information between Alice and Bob is sent through an encrypted VPN tunnel that also uses compression. The more similar the <the password entered> is to <secret password> the better this message compresses. If the attacker Eve can ask Alice to verify passwords and can see the length of the encrypted VPN messages, she gets a pretty good idea how close her guesses are, since the encrypted messages get shorter when her guesses get better.
Without compression the length of the encrypted packets does not change, so Eve cannot gain any information from this. Strictly speaking the length changes if Eve’s password length changes but that gives no additional information. The real world attacks are more complicated and need to take in account the specific circumstances (for example HTTPS or VPN) but they rely on the same principle as demonstrated in this simple example.