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Sloppy Linux Admins Enable Slow Brute-Force Attacks

Posted by kdawson on from the time-lapse-intrusion-monitering dept.

badger.foo passes on the report of Peter N. M. Hansteen that a third round of low-intensity, distributed brute-force attacks is now in progress — we earlier discussed the first and second rounds — and that sloppy admin practice on Linux systems is the main enabler. As before, the article links to log data (this time 770 apparently already compromised Linux hosts are involved), and further references. "The fact that your rig runs Linux does not mean you're home free. You need to keep paying attention. When your spam washer has been hijacked and tries to break into other people's systems, you urgently need to get your act together, right now."

11 of 391 comments (clear)

  1. Outward facing systems ... by taniwha · · Score: 5, Informative

    That system you have with SSH facing outwards - right now: PermitRootLogin no, PubkeyAuthentication yes, PasswordAuthentication no, Allowusers one-guy-only

    1. Re:Outward facing systems ... by quintus_horatius · · Score: 5, Insightful

      And perhaps set your SSH port to a non-standard port, where possible? Brute-force attackers seem to ignore high (> 1023) ports.

    2. Re:Outward facing systems ... by icydog · · Score: 5, Insightful

      I agree with your post if only one person needs access to the box (and i agree with PermitRootLogin no always). But while public key auth is great, it just isn't feasible for many applications. For example, imagine you're a cheap webhost that provides ssh, scp, sftp access to your users, Do you require them all to use public keys auth? 90% of them don't even know what that means. What a support headache.

      And public keys aren't always that secure either. There are probably still plenty of servers with weak keys from the Debian debacle. What do you do with those users if password authentication is disallowed? Just lock them out and make them call you for a key reset?

    3. Re:Outward facing systems ... by mysidia · · Score: 5, Interesting

      Don't set 'PasswordAuthentication no' * out the password for the SSH-only allowed users. Or even better yet, run ssh on a non-standard port, and do a fake SSHD that always denies and connection tarpitting on port 22.

      That way the 'brute forcers' will have no idea your system is more secure. While they're wasting time trying to break security on your uber locked down systems, they're leaving some other systems alone. If they're trying to brute force X hosts at a time, and some of them are secure, it will be longer before they move along to possibly more insecure hosts.

      This reduces the rate of expansion of these annoying brute forcers

    4. Re:Outward facing systems ... by cetialphav · · Score: 5, Insightful

      Port knocking is a good way to conceal that ssh is available.

      I guess it depends on what type of attacker you are trying to protect against. For attackers that are trolling around looking for easy targets, then things like this that add obscurity probably make sense. On the other hand, if I were in charge of a high value target, then I probably wouldn't bother. A high value target will have knowledgeable attackers who are very focused on exploiting you. In those cases, things like this are only mild inconveniences that will not make them give up. The port knocking sequence needed to open up ssh is not exactly a secret. It gets exposed in the clear to the network on every ssh connection. For high value targets, I would actually want the system as simple as possible to reduce the possiblity that a bug in one of the obscurity features actually becomes the attack vector.

      Using port knocking is like locking my car door. It makes it harder for lazy, stupid thieves to get into my car, but it does absolutely nothing for someone who really, really wants to steal my car because a good thief can bypass it in a trivial amount of time.

    5. Re:Outward facing systems ... by IMightB · · Score: 5, Informative

      I don't agree with setting the SSH port to non-standard, it is trivial for any determined attacker to figure out which one you've changed it to. Use one of the port/log monitoring daemons that are mentioned further down the page.

      That being said I used to work for a hosting company with a few thousand linux servers, most of them running cPanel (cPanel is a hunk of insecure crap). We'd get a few script kiddie break ins a week. Our solution with dramatically reduced the amount of break-ins (In addition to the SSH mods by the grand-parent) were:

      1) put /tmp as a separate partition and mount it as noexec, nosuid. Make sure your programs php/httpd use /tmp for temporary files, caches and session info. This simple step stopped 80% of attacks.
      2) host allow/deny is your friend
      3) rpm -V is your friend, most script kiddies/attackers are not bright enough to alter the rpm db, they will simply replace system binaries.

      there are a few more but I can't seem to remember them.

  2. Re:learn to.... by Nested · · Score: 5, Funny

    Obviously it's only relevant by outing parent as a random Windows admin.

  3. Re:learn to....denyhosts by nairb774 · · Score: 5, Informative

    Ah, but things like denyhosts [1] with distributed reporting can and does catch these attacks. [1] http://denyhosts.sourceforge.net/

  4. A REALLY SLOW attack ... by Jerry · · Score: 5, Insightful

    This attack was first reported last November, eleven months ago, and again in April of this year, 180 days ago.

    IF the bad guys have been able to capture only 770 Linux boxes since April that is only slightly more than 4 boxes per day. At that rate it would take them 833 years to create a Linux bot farm equal in size to the 1.3 Million Windows bot farm recently reported. Out of the millions of Linux boxes in use 770 represents a vanishingly small threat.

    Using this "threat" as an excuse NOT to move from Windows to Linux, or to move from Linux back to Windows, would be similar to playing Russian roulette with a fully loaded revolver and hoping to survive.

    --

    Running with Linux for over 20 years!

  5. Re:If it's SSH it's really easy to rate limit atta by Bigjeff5 · · Score: 5, Interesting

    Obviously, you didn't RTFA, or even the summary.

    These attacks completely avoid the problem, you'd have to drop the IP for several days to mitigate this attack. It is hundreds of linux boxes tagging a target and waiting a while before hitting it again. It's a slow brute force attack because no individual bot attacks a particular target more than once or twice in a given time period, maybe several minutes, maybe even several hours. The frequency of this attack was about 1500 attacks per day total, which is only two attacks per machine in the 770 bot network in a single day.

    Implimenting your strategy to prevent these attacks would also mean you would be locking out legitimate users who mis-type a password for a day or more. That is not going to work in any environment I am aware of.

    The brilliance of this attack is that while a bot is only attacking a particular machine once or twice a day, there is nothing stopping it from attacking other machines in the mean time. A bot can still send out thousands of attacks per day, they are just sending them to thousands of machines instead of one. Well coordinated it certainly has the same potential for building a large botnet as normal brute force methods. The downside of course is your odds of getting a particular machine are terrible, you're playing statistics to get a large botnet.

    --
    Security is mostly a superstition... Avoiding danger is no safer in the long run than outright exposure. - Helen Keller
  6. Re:Ask Slashdot by cetialphav · · Score: 5, Interesting

    My server just mails me its daily security run, and most days there is a couple of brute force attempts.

    Of course if the server were compromised, would you expect it to mail you a log that showed that it was compromised? If someone gets in with root access (and they know what they are doing), they could just modify the logs to not show what just happened. As long as you keep getting the same type of security summary, you will be happy.

    It reminds me of a time I was in an airport going through the TSA security line to go into the terminal. The agent checked my ID and boarding pass and then got distracted by a bunch of flight attendants she had to let through. She then turned back around and asked me if she had checked my ID. I gave her a hard time because in this system I am assumed to be untrustworthy until she says otherwise so she shouldn't trust anything I tell her.

    The point is that if something is a potential attack vector, then you must assume that any information it gives you might be a lie.