A few weeks ago a new TLS and SSLv3 renegotiation vulnerability was disclosed, and there’s been a fair bit of confusion around it. When the first reports of the bug hit the wire, my initial impression was that the exploit was too complex to be practical, but as more information comes to light I’m starting to think it’s worth paying attention to. Since every web browser and most other kinds of encrypted Internet connections – such as between mail servers – use TLS or SSLv3 to protect traffic, the potential scope for this is massive.

The problem is that TLS and SSLv3 allow renegotiation outside of an established TLS connection, creating a small window of opportunity for an attacker to sit in the middle and, at a particular phase of a connection, inject arbitrary data. The key bits are that the attacker must be in the middle, and there’s only a specific window for data injection. The encryption itself isn’t cracked, and the attacker can’t read the encrypted data, but the attacker now has a hole to inject something which could allow unanticipated actions, such as sending a command to a web application a user is connected to.

A lot of people are comparing this to Cross Site Request Forgery (CSRF), where a malicious website tricks the browser into doing something on a trusted site the user is logged into, like changing their password. This is a bit similar because we’re injecting something into a trusted connection, but the main differentiator is where the problem lies. CSRF happens way up at the application layer, and to hit it all we need to do is trick the user (or their browser) to get access. This new flaw is at a networking layer, so we have a lot less context or feedback.

For the TLS/SSL attack to work, the attacker has to be within the same local network (broadcast domain) as the victim, because the exploit is at the “transport” layer. This alone decreases the risk significantly right out of the gate.

Is this a viable exploit tactic? Absolutely, but within the bounds of a local network, and within the limits of what you can do with injection. This attack vector is most useful in situations where there is easy access to networks: unsecured WiFi and large network segments that aren’t protected from man in the middle (MITM) attacks. The more significant cause for concern is if you are running an Internet facing web application that is:

  • Vulnerable to the TLS/SSL renegotiation vulnerability as described and either…
    • Running a web app that doesn’t have any built in application layer protections (anti-CSRF, session state, etc.).
    • Running a web app that allows users to store and retrieve things using simple POST requests (such as Twitter).
  • Or using TLS/SSLv3 as transport security for something else, such as IMAP/SSL, POP/SSL, or SMTP/TLS…

In those cases, if an attacker can get on the same network as one of your users, they can inject data and potentially cause bad things to happen, possibly even redirecting your user to a new, malicious site. One recent example (since fixed) showed how an attacker could trick Twitter into posting the user’s account credentials.

Currently the draft of the fix binds a renegotiation handshake to a particular already established TLS channel, which closes the hole. Unfortunately, since SSLv3 does not support extensions there is no possible way for a secure renegotiation to happen; thus the death of SSL is nigh, and long live (a fixed) TLS.

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