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Date: Tue, 4 Mar 2014 00:38:21 -0500 (EST)
Subject: Re: CVE Request?: konqueror - https uses all ciphers, even weak ones

Hash: SHA1

>> We received this bugreport for the KDE default webbrowser
>> Konqueror:

> So the question becomes where do we draw the line?

An example in which a CVE ID could be assigned is a product that sends
a Client Hello message with an unintended omission of one strong
cipher suite. For example, the product's documentation says that it
supports all of the widely accepted 256-bit cipher suites, but an
off-by-one error causes it to omit the very strongest one. This
behavior is still a vulnerability, even though the product cannot use
anything other than 256 bits. There's no "draw the line" number.

In this Konqueror case, the question of whether it's a vulnerability
or an opportunity for security hardening depends on the author's
intention in writing the code that way, and on the threat model.

The information sent to this list so far doesn't really clarify the
root cause of the problem. One message perhaps suggested that, because
of lack of developer time, the product doesn't use any non-default
features of OpenSSL even if they would be security improvements.
Another message mentioned "failed to find a module to configure the
ciphers in the KDE configuration module jungle." So, there might be
various possibilities, including:

  - the source code doesn't call SSL_CTX_set_cipher_list anywhere

  - the source code does call SSL_CTX_set_cipher_list if a cipher suite
    list is available, but the product doesn't actually look for that
    list in any part of the configuration

  - the product looks for a cipher suite list, but it is not parsed
    correctly, and therefore valid data is effectively ignored

> 40/56 can be broken in single digit seconds

We think this means your threat model is that there's an established
session using 40-bit or 56-bit encryption, and then someone sniffs the
network and does a successful brute-force search of the complete key
space. Certainly this isn't a desirable outcome, but it doesn't seem
to imply a vulnerability in Konqueror. If, for example, 40-bit
encryption is in use, this means the server selected a 40-bit cipher
suite. Four of the possible scenarios are:

  - It's an https server that's simply not capable of using anything
    other than 40 bits. Suppose that the server also supports http,
    which is very realistic. The Konqueror user needs to use this
    server for some reason, and would prefer not to have data
    intercepted. Here, it seems better for Konqueror to include 40-bit
    cipher suites in its Client Hello message, because the alternative
    is that the handshake will fail and the user will visit the
    analogous http:// URL instead. Use of 40-bit cipher suites is the
    right choice. It is not a vulnerability in Konqueror.

  - The server can support strong cipher suites, but is misconfigured
    to select only 40-bit cipher suites. This is a similar situation.
    If the user must use the server immediately (i.e., he doesn't have
    time to contact the server operator and ask for a
    reconfiguration), a 40-bit cipher suite is the right choice.

  - It's a rogue server that wants to launch an "attack" against
    Konqueror by making Konqueror use a 40-bit cipher suite.
    Ultimately, this attack has no real value. The server has access
    to the plaintext and can redistribute the plaintext to anyone. (We
    can probably ignore the possibility that a 40-bit session makes
    plaintext data available to an attacker on the client-side local
    network, and it might be expensive or risky for the server
    operator to transport the data there.) Similarly, it's not
    especially relevant that this has made it easy for another client
    to hijack a session and send different data. The server could
    simply pretend to receive the different data.

  - The server didn't intentionally select a 40-bit cipher suite, but
    (because a different security issue was exploited during the
    handshake) the communication channel ends up using 40-bit
    encryption. This is not ultimately the fault of Konqueror's use of
    40-bit cipher suites.

At the level of a policy decision designed to improve the entire https
ecosystem, it may be preferable for 40-bit cipher suites to be dropped
from all clients. This would, for example, encourage any remaining
40-bit-only server operator to upgrade. However, from the perspective
of an individual user wishing to use Konqueror once to connect to one
web server, it's not necessarily preferable. Because of this, the
prospect of easily attackable 40-bit sessions doesn't, by itself, make
this a Konqueror vulnerability.

(Certainly it could be a security improvement if Konqueror clearly
indicated to the user that the current https session is a 40-bit
session. This would mainly be applicable in a case where KDE decided
to intentionally support 40-bit sessions forever, ignoring the
"improve the entire ecosystem" principle.)

- -- 
CVE assignment team, MITRE CVE Numbering Authority
M/S M300
202 Burlington Road, Bedford, MA 01730 USA
[ PGP key available through ]
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