[CentOS] an actual hacked machine, in a preserved state

Johnny Hughes johnny at centos.org
Thu Jan 5 23:34:08 UTC 2012

On 01/05/2012 02:51 PM, Bennett Haselton wrote:
> On 1/5/2012 6:53 AM, Johnny Hughes wrote:
>> On 01/04/2012 07:47 PM, Bennett Haselton wrote:
>>> On 1/4/2012 1:59 PM, Lamar Owen wrote:
>>>> [Distilling to the core matter; everything else is peripheral.]
>>>> On Jan 4, 2012, at 2:58 PM, Bennett Haselton wrote:
>>>>> To be absolutely clear: Do you, personally, believe there is more than a
>>>>> 1 in a million chance that the attacker who got into my machine, got it
>>>>> by brute-forcing the password?  As opposed to, say, using an underground
>>>>> exploit?
>>>> Here's how I see it breaking down:
>>>> 1.) Attacker uses apache remote exploit (or other means) to obtain
>>>> your /etc/shadow file (not a remote shell, just GET the file without
>>>> that fact being logged);
>>>> 2.) Attacker runs cloud-based (and/or CUDA accelerated) brute-forcer
>>>> on 10,000,000 machines against your /etc/shadow file without your
>>>> knowledge;
>>>> 3.) Some time passes;
>>>> 4.) Attacker obtains your password using distributed brute forcing of
>>>> the hash in the window of time prior to you resetting it;
>>>> 5.) Attacker logs in since you allow password login.  You're pwned by
>>>> a non-login brute-force attack.
>>>> In contrast, with ssh keys and no password logins allowed:
>>>> 1.) Attacker obtains /etc/shadow and cracks your password after some
>>>> time;
>>>> 2.) Attacker additionally obtains /root/.ssh/*
>>>> 3.) Attacker now has your public key.  Good for them; public keys
>>>> don't have to be kept secure since it is vastly more difficult to
>>>> reverse known plaintext, known ciphertext, and the public key into a
>>>> working private key than it is to brute-force the /etc/shadow hash
>>>> (part of the difficulty is getting all three required components to
>>>> successfully reverse your private key; the other part boils down to
>>>> factoring and hash brute-forcing);
>>>> 4.) Attacker also has root's public and private keys, if there is a
>>>> pair in root's ~/.ssh, which may or may not help them.  If there's a
>>>> passphrase on the private key, it's quite difficult to obtain that
>>>> from the key;
>>>> 5.) Attacker can't leverage either your public key or root's key pair
>>>> (or the machine key; even if they can leverage that to do MitM (which
>>>> they can and likely will) that doesn't help them obtain your private
>>>> key for authentication;
>>>> 6.) Attacker still can't get in because you don't allow password
>>>> login, even though attacker has root's password.
>>>> This only requires an apache httpd exploit that allows reading of any
>>>> file; no files have to be modified and no shells have to be acquired
>>>> through any exploits.  Those make it faster, for sure; but even then
>>>> the attacker is going to acquire your /etc/shadow as one of the first
>>>> things they do; the next thing they're going to do is install a
>>>> rootkit with a backdoor password.
>>>> Brute-forcing by hash-cracking, not by attempting to login over ssh,
>>>> is what I'm talking about.
>>> I acknowledged that the first time I replied to someone's post saying a
>>> 12-char password wasn't secure enough.  I hypothesized an attacker with
>>> the fastest GPU-driven password cracker in the world (even allowing for
>>> 100-factor improvements in coming years) and it would still take
>>> centuries to break.  I understand about brute-forcing the hash vs.
>>> brute-forcing the login, but some others had posted about brute-forcing
>>> the login specifically and I was commenting on how ridiculous that was.
>>>> This is what I mean when I say 'multilayer metasploit-driven attacks.'
>>>> The weakest link is the security of /etc/shadow on the server for
>>>> password auth (unless you use a different auth method on your server,
>>>> like LDAP or other, but that just adds a layer, making the attacker
>>>> work harder to get that all-import password).  Key based auth is
>>>> superior, since the attacker reading any file on your server cannot
>>>> compromise the security.
>>>> Kerberos is better still.
>>>> Now, the weakest link for key auth is the private key itself.  But
>>>> it's better protected than any password is (if someone can swipe your
>>>> private key off of your workstation you have bigger problems, and they
>>>> will have your /etc/shadow for your workstation, and probably a
>>>> backdoor.....).  The passphrase is also better protected than the
>>>> typical MD5 hash password, too.
>>>> It is the consensus of the security community that key-based
>>>> authentication with strong private key passphrases is better than any
>>>> password-only authentication, and that consensus is based on facts
>>>> derived from evidence of actual break-ins.
>>> Well yes, on average, password-authentication is going to be worse
>>> because it includes people in the sample who are using passwords like
>>> "Patricia".  Did they compare the break-in rate for systems with 12-char
>>> passwords vs. systems with keys?
>>> I have nothing in particular against ssh keys - how could anybody be
>>> "against ssh keys"? :)  My point was that when I asked "How did
>>> attackers probably get in, given that the password was a random
>>> 12-character string?" people pounced on the fact that I was using a
>>> password at all, and kept insisting that that had a non-trivial
>>> likelihood of being the cause (rather than the
>>> less-than-one-in-a-billion it actually was), even to the point of making
>>> ridiculous statements like Mark saying that an attacker trying
>>> "thousands of times per hour" would get in "sooner or later".  This was
>>> to the exclusion of what was vastly more likely to be the correct
>>> answer, which was "Apache, sshd, and CentOS have enough exploits that
>>> it's far more likely an attacker got in by finding one of those (and
>>> tools like SELinux help mitigate that)."
>>> Again, if I hadn't stood behind the math on the issue of long passwords
>>> vs. keys, I probably never would have gotten the answer that was
>>> actually useful.
>>> Do you think it's possible that people focused so much on the use of a
>>> "password" as a possible cause, vs. the existence of exploits, despite
>>> the former being literally about 1 billion times less probably than the
>>> latter, because the former puts more of the blame on the user?  (Not
>>> that anyone is "to blame" that CentOS and Apache have bugs -- everything
>>> does -- but that password security would be an issue even with a perfect
>>> operating system.)
>>>> While login-based brute-forcing of a password that is long-enough
>>>> (based upon sshd/login/hashing speed) is impractical for passwords of
>>>> sufficient strength, login-based brute forcing is not the 'state of
>>>> the art' in brute-forcing of passwords.  Key-based auth with a
>>>> passphrase is still not the ultimate, but it is better than only a
>>>> password, regardless of the strength of that password.
>>>> If your password was brute-forced, it really doesn't matter how the
>>>> attacker did it; you're pwned either way.
>>>> It is a safe assumption that there are httpd exploits in the wild,
>>>> that are not known by the apache project, that specifically attempt to
>>>> grab /etc/shadow and send to the attacker.  It's also a safe
>>>> assumption that the attacker will have sufficient horsepower to crack
>>>> your password from /etc/shadow in a 'reasonable' timeframe for an MD5
>>>> hash.
>>> Well I disagree that that's a "safe assumption".  If you think that
>>> 12-character passwords are within striking distance, try 20-char
>>> passwords -- 1^36 possible values to search, so with a botnet of over 1
>>> billion infected computers each checking 10 billion passwords per second
>>> (both orders of magnitude beyond what's in play today, and
>>> unrealistically assuming that every resource in the world is focused on
>>> this one password), it would take on the order of 10 billion years to crack.
>> Then we get back to rainbow tables and hashes that have been generated
>> by someone else (with super computer access) and published for "X" sized
>> passwords (you pick the size: 8,12,20,24,etc).  Then they don't need to
>> calculate anything, just do a sql lookup against a database with what
>> they get from the shadow file.  Someone else already cracked all "X"
>> size logins for all possible iterations.
> But if the system adds a salt to the password before taking the hash, 
> then the size of the precomputed rainbow table grows exponentially, 
> because it has to store the hash of all possible passwords to test, with 
> all possible salts.  If a 12-char random password (72 bits of 
> randomness) is salted with a 32-bit salt you now have to precompute 
> 10^31 values in your rainbow table instead of "only" 10^21.

OK ... you continue to use passwords on your servers.  I'll use keys and
require a vpn to access mine.

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