Saw this first shoot out on Twitter at the end of my workday, but without any details, I simply made a mental note to keep an eye out. Sooner than expected, further details on this TLS MITM attack have surfaced.

Is this a big deal? Possibly. Certainly big enough to keep on the *front* burner, especially since initial details are pretty technical.

Does this allow an attacker to intercept and sniff TLS-encrypted traffic? It doesn’t sound like it so far. If I’m reading this correctly, an attacker can inject data into the stream and influence what the browser (in a web client->server scenario) renders, with no visible warning to the client that bad data has been introduced. That or I’m seeing that the client can influence what the server sees in the requests being made, in which case this is an attack on the server? Either way, this appears to be an MITM injection attack and not necessarily a MITM sniffing.

I’m also unsure how this stands with TLS negotiations without client certificates, such as most people I imagine are familiar with in their web environs.

I wonder if this might be very important for anything using TLS and client certs for authentication, such as the smart cards mentioned in the advisory. Would it be possible for someone to usurp that authentication and re-use it such that the attacker can then access/view those protected areas on the server?

When I find out more, I’ll post a follow-up!