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A number of TLS vulnerabilities are listed by Wikipedia, including attacks with brand names such as FREAK, DROWN, CRIME, BREACH, and POODLE. My lay reading of their description makes me think these vulnerabilities are reliant on an attacker being able to either 1) influence a victim's browser's behaviour, or 2) occupy a man-in-the-middle position on the network and actively manipulate traffic.

Are there any TLS vulnerabilities which can be exploited using only passive observation of traffic, and which are practical enough to be a concern for ordinary systems administrators?

Some definitions:

  1. By "practical" I mean not reliant on a theoretical mathematical breakthrough, or on excessive compute resources or time (say, no more than $100,000 worth of commodity x86 server could do if working on its hypothetical decryption algorithm for a year).

  2. By "ordinary systems administrators" I mean someone working at a Fortune 500 corporation wanting to protect innocuous customer data, not a nation state agency protecting ultra-secret launch codes.

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  • RC4, as far as I know, is a purely statistical attack and just relies on a lot of traffic being available – MechMK1 Jan 18 at 16:37
  • The Logjam researchers speculated that an IPSEC/IKE break described in the documents leaked by Snowden were actually of a widely-used DH-1024 group; the same method applies to TLS (and SSH) and is completely passive using openly known techniques, although it would be (or was) a lot more than $100k. Many (most?) TLS implementations if they still use classic-DHE (FFDHE) at all now default to minimum 2048 bits, but users or admins may be able to force smaller and potentially vulnerable sizes. – dave_thompson_085 Jan 19 at 5:43
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There are no practical feasible attacks for a purely passive attacker with severely limited resources (i.e. no government or similar) against currently used TLS standards (TLS 1.2 and better, but even TLS 1.0 is still sufficient). This does not mean though that specific implementations of these standards might be a problem, for example due to bugs or deliberately weakened or backdoored random generators. Though at the moment there are no publicly known cases for such weaknesses.

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