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The Low-Intensity, Brute-Force Zombies Are Back 203

Peter N. M. Hansteen writes "In real life, zombies feed off both weak minds and the weak passwords they choose. When the distributed brute-force attempts stopped abruptly after a couple of months of futile pounding on ssh servers, most of us thought they had seen sense and given up. Now, it seems that they have not; they are back. 'This can only mean that there were enough successful attempts at guessing people's weak passwords in the last round that our unknown perpetrators found it worthwhile to start another round. For all I know they may have been at it all along, probing other parts of the Internet ...' The article has some analysis and links to fresh log data."
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The Low-Intensity, Brute-Force Zombies Are Back

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  • by rcpitt ( 711863 ) on Sunday April 12, 2009 @06:42PM (#27551219) Homepage Journal
    None of my systems allow passwords via ssh - and I run to "3 strikes - you're out" the idiots who do the brute-forces by putting them into iptables

    Anyone with passwords turned on is not secure IMHO

    • Re: (Score:3, Informative)

      by miknix ( 1047580 )

      IMHO if the passwords are strong enough there is nothing to worry about, unless you get pissed off with flooded log files and the waste of bandwidth.

      None of my systems allow passwords via ssh

      Exactly, using public key authentication and disabling PAM/Password authentication solves the problem.

      and I run to "3 strikes - you're out" the idiots who do the brute-forces by putting them into iptables

      sshguard is nice too, they will be firewalled in no time. (watch out for DoSs though)

      However, it is not just ssh. Http and servers also suffer a lot from automated breakin attempts.
      Anyway.. I'm glad services running on port 22 are not in the same security leve

      • "IMHO if the passwords are strong enough there is nothing to worry about"

        The problem is that most users are not capable of choosing a strong password. Even when you try to enforce policies about minimum password strength, users will manage to choose weak passwords; aside from the world's most common password (password1), there are plenty of people who use their own username as a password -- and requiring non-alphanumeric symbols won't stop them: jane123 will just becomes j@ne123. Minimum password leng
    • Re: (Score:3, Interesting)

      If you only allow public key authentication then there's really no need to bother blocking them: you'll never block the entire botnet.

      Just let them try - they'll never guess the right private key.

      • The problem is that noise in the logs makes it hard to find real problems. That and the risk of a bug in openssh or openssl, though the current environment is a pretty good test for the software.
        • ... then there's no need to bruteforce it, and therefore blocking a botnet doing that is futile.

          • I disagree. Should a new bug arise in openssh, I sure feel a lot better knowing that while I do enforce key-only authentication, I also restrict access to specific IP addresses. It's pretty hard to crack a service that you can't reach on the network due to packet filtering.
          • by Z00L00K ( 682162 )

            And if you have a three try block then you may still decrease the effectiveness of a botnet unless you are the specific target of the botnet.

            But some bots are actually stupid enough to run the same list of passwords on every node, so it may work really well.

            It's more the question of making your site less attractive than other sites on the net.

            A hack for /etc/sysconfig/iptables to take care of the problem.

            :blacklist - [0:0]
            :ssh - [0:0]
            -A RH-Firewall-1-INPUT -p tcp -m tcp --dport 22 -m state

        • Re: (Score:3, Insightful)

          by BitZtream ( 692029 )

          grep -v for the win!

    • Re: (Score:3, Informative)

      I don't allow password-based logins either (SSH keys only), allow SSH only from specific IP addresses, and I use fail2ban [] across all services that involve any kind of authentication (mail, ftp, http auth, etc). I've got it set to "two strikes and you're out"; every day I still get hundreds (some days thousands) of IPs banned in the logs. It's pretty sad.
      • Re: (Score:3, Interesting)

        by dbIII ( 701233 )
        From a quick look at fail2ban it looks like one of it's features is that the blocking only lasts until the next log rotation. Considering that attacks are temporary and automatic firewall rule generation could become a script kiddies playpen this is actually a good thing. If they work out you have this and spoof a few adresses as a denial of service attack the system will recover over time without needing someone to go through all the firewall rules.
        I'm still a bit nervous about allowing malicious third p
        • From a quick look at fail2ban it looks like one of it's features is that the blocking only lasts until the next log rotation.

          It's configurable, you can select any period of time for the ban to remain in effect.

          I'm still a bit nervous about allowing malicious third parties to effectively write firewall rules for me.

          That I completely understand. It's not without its potential hazards, but I think the benefits outweigh them.

          some of us don't really know where the next legitimate connection is going to come from

          I've been thinking about something like a variant on port knocking, wherein a machine would be make several connections attempts to a non-existent service port from source ports whose numbers add up to some magic number. Filtering would then be disabled for the life of that connection. Maybe someone's already done it

          • by Sancho ( 17056 ) *

            If you just want to keep the logs cleaner and avoid the bot attacks, you can just run your server on another port.

            • It's a good idea in theory, but the botnets are smarter than that these days. My server networks get portscanned multiple times a day, and it's inevitably followed up with login attempts (even with non-standard ports) on any hosts that aren't taking aggressive defense measures.
              • by Sancho ( 17056 ) *

                Interesting. I run an sshd on a non-standard port, and have yet to see this behavior.

                I'll keep an eye out, though.

              • It's still worth doing, it cuts the rate of attacks by at least one order of magnitude (if not two) and your log files will be a lot cleaner. Yes, the worms are getting more sophisticated, but they also have to leave additional footprints in the IDS logs (because of having to port scan).

                (We run public key only authentication, with SSH on a nonstandard port. The former is a bit of a PITA, but the latter is a simple change.)
    • by maeka ( 518272 )

      I only allow public key connections, and am only listening on port 2022 (I have no issues telling the world that).
      My auth.log is completely empty of password attempts. Am I missing something simple-stupid or is the bot net only going after port 22?

      • Re: (Score:2, Insightful)

        by Thantik ( 1207112 )
        I would suspect it's going after port 22 only. If your smart enough to move the port from 22, your probably smart enough to use key pairs and then what is the point of trying to brute force you? Focusing on default 22 is a good strategy because you'll find those who have completely defaulted settings, weak passwords, etc.
      • Something interesting about your usage of tcp/2022. I did the same thing recently on an occasion when I could not use tcp/22, since it seemed an obvious choice. Probable most automated attacks only concentrate on tcp/22 (the obvious target and if you can move it, you probably know enough to secure it properly), but I've been wondering if someone might start considering scanning on 2022 or any other "obvious" choice.

        Right now it seems that nobody is scanning for them, but is anyone else setting ssh servers o

    • by Sentry21 ( 8183 )

      Likewise - I use fail2ban with iptables to drop any packets from someone who fails auth about 5 times in a few minutes. I've toyed with the idea of adding them to a global blacklist for all servers in all locations, but in reality this solution works just fine.

      • Likewise - I use fail2ban with iptables to drop any packets from someone who fails auth about 5 times in a few minutes. I've toyed with the idea of adding them to a global blacklist for all servers in all locations, but in reality this solution works just fine.

        If you RTFA, they tell you that these attacks are coming from different machines, presumably so they don't trip such things as fail2ban et al.

        Looking at the logs he supplied, this is a _very_ slow attack, the attempts are many seconds, or even minutes apart. You would have to have a very guessable username / password combination for it to work.

        I would comment though that I'm not seeing anything like this attack in my logs. I personally use IPTables rules (using hashlimits) to limit 1 connection / IP per m

        • I'm using fail2ban and it configurably allows you to set how long you want to ban, and permanently isn't the default.
          You must be thinking of some other tool

    • Re: (Score:3, Insightful)

      by dbIII ( 701233 )
      Nice in the short term but giving people an easy way to add rules to your firewall may create hassles later once miscreants know that is what you are doing. Some people have scripts that implement temporary blocking so it doesn't hurt much on the day that some script kiddie decides to have fun with them by forging attacks from different addresses.
    • Re: (Score:3, Informative)

      by timmarhy ( 659436 )
      passwords are still the most portable lightweigth authentication method out there, that's why. I had a system that got caught by this when a user with shell access set a weakish password. the user was sandboxed with only a limited whitelist of applications they could execute, so no harm done, but i did log all of the bot's attempts and the IRC channel it connected to along with passwords to other bots. it was very interesting to take the attack a part, at it's core it runs a simple dictionary attack and onc
    • Re: (Score:2, Interesting)

      by al0ha ( 1262684 )
      >> Anyone with passwords turned on is not secure IMHO.

      I disagree. I both cases, password auth or key auth, barring any security problem with the protocol, the weakest link is the user. A passphraseless ssh key is tempting to the user for many reasons and unless you audit the passphrases selected by your users, key auth is no more secure than password auth. In fact password auth may actually be more secure if you enforce complexity on the system(s). You have no control over the passphrase on a u
      • by micheas ( 231635 )

        I think you over estimate the security of passwords.

        You admit that the weakest link is the user.

        Passwords are very dependent on the user. They are generally selected by the user and if any of your users recycled a username password pair from even though they are eight characters with a letters and numbers and mixed case they are essentially known.

        Passwords depend on users not recycling username password pairs that have have been phished.

        I will admi

  • Poor Odds (Score:5, Informative)

    by Nerdfest ( 867930 ) on Sunday April 12, 2009 @06:46PM (#27551243)
    The odds of them getting into a system like this must be quite low, but I guess they're after the low-hanging fruit. Running your services on a high port rather than the default reduces this, as does disabling password login and using 2-factor authentication. Quite easy to do, and very, very secure.
    • How-to (Score:5, Informative)

      by Nerdfest ( 867930 ) on Sunday April 12, 2009 @06:54PM (#27551269)
      Sorry, should have posted this with the original. Instructions for Linux 2 factor authentication []
    • We did remapped SSH to a higher unused port and then took it a step further blocking access to that port on the hardware firewall from every IP address except for the office. If I need to connect to the server, I first have to connect to an OpenBSD box in the office.

      We have 3 - 4 PCI scans a day (seems like every payment gateway we support for our clients scans the server daily. None of them even see SSH.

    • I had to re-install one of those low hanging fruit which was taken over via a dictionary attack about four years ago.
      For some reason the guy that was looking after what should have been a simple mail server and incredibly basic web server decided to change things so that everyone with a mailbox had shell access. Then he put a compiler on there. Then, sick of having to "su" all the time he changed the permissions of everything in /etc (system configuration files) and who knows where else so that any user o
      • I know the way to do this to allow only those specific users that need ssh access (one or two in the above case - and never root on an external facing box), use keys instead of passwords, and only accept connections from the IP addresses that legitimate users will be on. None of those things are difficult. The above example shows what can happen with a far too relaxed approach.
      • by MadAhab ( 40080 )

        "chmod 777" = justifiable homicide

  • by MichaelSmith ( 789609 ) on Sunday April 12, 2009 @06:51PM (#27551259) Homepage Journal
    ...unless they are only attacking from my existing list of blocked IP addresses.
    • by Sentry21 ( 8183 )

      A fair point. I've set up on our production servers two lists for ipset [], one each for China and Korea. Bullshit accesses to SSH and HTTP dropped way way off once I did that.

      With 719 unique CIDR blocks for China and 430 for Korea, we get a lot less garbage traffic to our servers. Worth the hour it took to set up, too.

  • Oh... (Score:5, Funny)

    by Perseid ( 660451 ) on Sunday April 12, 2009 @06:56PM (#27551293) mean zombie PROGRAMS. Damn.

    [puts shotgun down]
  • Protect yourself (Score:5, Informative)

    by Matt Perry ( 793115 ) <perry.matt54@y[ ] ['aho' in gap]> on Sunday April 12, 2009 @07:02PM (#27551329)

    Use SSH keys in addition to passwords. Disable ssh root logins. Use the AllowUsers command in sshd_config to restrict what accounts can log in with ssh. Edit /etc/hosts.deny and add IP ranges [] for where you are unlikely to login from. Use iptables rules to block people [] who are hammering your ssh server from the same address. Use tools like Fail2ban [] and DenyHosts [] to block other abusers and share abuser information with other victims.

    • It's security theater.
      There are good reason for allowing (private key only) root login, allong with autorized_keys command= directives.
      Furthermore password+ssh keys is rather pointless.

      • Is that good reason: You're an idiot?

      • While there might be good reasons to allow root access with a restricted key, that's hardly wise and there usually there is no need to do so.

        As for the pointlessness of keys AND password, I think you're rather uncreative. There are a few uses of that scheme, the first one to come to my mind is a random password tightly controlled by the IT staff and periodically changed (and the user can do nothing about that) plus a key, under user control. The password allows enforcement of the security rules and insures

        • by MadAhab ( 40080 )

          Unless the root access is restricted to a single command - e.g. rsync for backups, scripts to generate checksums on bin and lib dirs, etc.

          Needless to say, this should be run only from a secure host that has no inbound services except, if necessary, ssh.

      • Furthermore password+ssh keys is rather pointless.

        Until your account is compromised and the attacker can now ssh into your other accounts without having to enter a password. I'd rather have to enter the password when the key is used. Better safe than sorry.

        • Wouldn't a passphrase on the key itself be better in that case?

        • it's compromised.
          And having to type a password too often is not added security, because it only adds minimal, easily circumvented security in the rare case where you're already fucked, but it annoys the hell out of users *all* the time, causing them to have unsafe practices.

      • by Rennt ( 582550 )

        No its not. Disabling root logins means the attacker has to brute force the username as well as the password, thus increasing the chances of the attack failing by a power of two. In actuality it is even more effective, as the attacker will usually try "root" and move on.

        Same rationale exists on Windows. If you don't rename the "Administrator" account on setup, (or create a new admin account with a different name and delete the old one if you have a pre-installed box) you are asking for a world of hurt.


          • by Rennt ( 582550 )

            Look up defense-in-depth and the reasons it is used. Encrypted keys may be the strongest link in the chain, but if they are the only defense you have then you are SOL the instant a vulnerability is discovered.

            Why would you sit back after implementing private key access without any further consideration to the other techniques for making ssh even safer? For home servers this is probably sufficient, but if it is your job to secure systems then this would be negligence - possibly criminally so, depending on th

    • Re: (Score:3, Insightful)

      by anothy ( 83176 )
      mostly good advice. you might consider using ssh keys instead of passwords, depending on your environment. the only thing i'd outright disagree with is pre-denying IP ranges based on a guess of where you're likely to log in from. i've had to leave the country on business unexpectedly on very short notice; it'd suck to have been locked out when i landed.
      • Re: (Score:3, Informative)

        by GaryOlson ( 737642 )
        Use VPN. Although it may seem redundant, SSH thru a VPN tunnel does provide a secondary access method which is secure.
      • you might consider using ssh keys instead of passwords

        If you don't have a password as well then if your account is compromised the attacker will be able to access your accounts on other servers without entering a password. I know generating a ssh key without a password is convenient but it also creates risk.

        • by Ash-Fox ( 726320 )

          If you don't have a password as well then if your account is compromised the attacker will be able to access your accounts on other servers without entering a password.

          I don't see how that's possible if he doesn't have the private key which is held by the client only.

    • by mcelrath ( 8027 )

      By the way, you can look at the traffic statistics from DenyHosts [], you can clearly see that ssh password-guessing traffic increased about 10 fold on Apr 6. (And since I configured DenyHosts to email me every time it blocks an IP, I've been very aware of this attack)

      I've always wondered why someone doesn't do something with the DenyHosts IP list. It should be impossible to forge IP's for ssh, due to handshaking and key exchange. So doesn't DenyHosts have a pretty good map of somebody's botnet? Is any

      • Same here with denyhosts and emails. I noticed immediately that three different systems were being slowly plucked away with ssh attempts. mainly to 'admin', which I suppose is typical of home routers.

        you bring up a very good point about the denyhosts data being a good map of the botnet. I think it would be up to the private sector to initiate something before law enforcement would bother though. It seems most of the blackhat stuff is reported/stopped by a few whitehats instead of by government entities.

  • by myspace-cn ( 1094627 ) on Sunday April 12, 2009 @07:06PM (#27551341)

    Roll out SPA / Port knocking, their IP shouldn't be touching your sensitive ports without a rule, table, or chain specifically allowing access. FORGET THE PASSWORD!

  • Another solution (Score:3, Insightful)

    by IceCreamGuy ( 904648 ) on Sunday April 12, 2009 @07:06PM (#27551347) Homepage
    Use a script like denyhosts, and I'm sure there are a ton of others out there that are just as good if not better. Unless your password is weak enough to be guessed in five attempts and the attacker isn't already in the denyhosts list, you shouldn't have to worry about too much. And, most importantly, just peruse your auth logs every now and then, it's not really that big of a chore.
  • by value_added ( 719364 ) on Sunday April 12, 2009 @07:20PM (#27551425)

    For those already familiar with Peter Hansteen's website [], I'll offer a Thumbs Up recommendation for his Book of PF [].

    There's already been several stories on Slashdot either submitted by or about him, and I don't recall any mention of his book. I'd say his efforts if not his humility deserve some kind of reward, and the reduced sale price of $19.77 is a bargain.

  • by the eric conspiracy ( 20178 ) on Sunday April 12, 2009 @07:21PM (#27551429)

    I've used a script to block servers that failed a certain number of attempts along with AllowUsers. That worked well for a couple of years, but was annoying in that you could see the attempts being made and knowing that if you made a config error you could be vulnerable. It seems to me that even after I got several hundred systems in my block list it wasn't making a difference since the pool of zombies was so large.

    Now I just use key only access and AuthUsers and feel a lot more secure. I'm thinking I may add a white list of IP addresses as well. That would really lock things down pretty well.

  • In all seriousness (Score:5, Informative)

    by actionbastard ( 1206160 ) on Sunday April 12, 2009 @07:35PM (#27551497)
    This has been going on for years. Really. I've been seeing this crap in my logs since we started running an Internet-facing SSH host nearly ten years ago. It's always the same password based login attempts with the same dictionary/script used in the attacks. This is probably just some training exercise for Chinese hackers at some state-run school to see who can break into the running-dog Yankee Imperialist's computers the fastest.
  • I'm safe... (Score:5, Funny)

    by hoytak ( 1148181 ) on Sunday April 12, 2009 @07:35PM (#27551503) Homepage
    I've now changed my password from Thomas to ThomasX, where X is a digit that I'm not telling.
  • Just checked my auth logs and I'm seeing hundreds of various IPs, some of which are connecting up to 20 times. Definitely a new twist. I'll have to do some poking around to see what kind of machines are doing the probing. ( Is it compromised windows boxes?)

  • by Anonymous Coward

    Be proactive on port 22 as well. At the advice of another comment I saw on /. a year or so ago I'm running a honeypot, with three static ports (one of them 22) and 4 roving ports. Establishing a TCP connection to any of them causes your IP to be instantly added to an iptables blacklist. It's worked pretty well; I've got about 2-3 unique addresses trying per day, and about 294 have been blocked since mid-December 2008. It takes care of both port scanners and bots deliberately connecting in order to try and g

    • Is there any difference with respect to a PKCS#11 token?
      I've been thinking of using one of these tokens as a "road warrior" SSH key, but then realised that since they need drivers to be useable, that wouldn't be practical to use on machines not owned by me.

      Also, why not S/KEY [] instead of one of those yubikeys (or at least the random password)?

  • quickest fix (Score:2, Interesting)

    by capn_nemo ( 667943 )
    While it's true there are a variety of techniques that can increase security, I've found simply moving to a new high-numbered port eliminates all random login attempts. Yes, security through obscurity is all it's cracked up to be, but for now, I've eliminated the problem with a pretty quick fix.
  • Goodness. (Score:5, Informative)

    by geekboy642 ( 799087 ) on Sunday April 12, 2009 @08:27PM (#27551823) Journal

    There sure are a lot of people who didn't bother to read the article.
    The point of these attacks are that it's a coordinated botnet attack. Meaning if you block any single IP, or even a large subnet, you've cost the attacker nothing. Fail2ban, denyhosts, all of these won't even slow these attacks down.

  • iptables goodness (Score:5, Informative)

    by grasshoppa ( 657393 ) <skennedy AT tpno-co DOT org> on Sunday April 12, 2009 @08:48PM (#27551963) Homepage

    Once again, we have a built in linux goody which helps us out;
    -A INPUT -p tcp -m tcp --dport 22 --tcp-flags SYN,RST,ACK SYN -m recent --set --name sshattack --rsource
    -A INPUT -p tcp -m tcp --dport 22 --tcp-flags SYN,RST,ACK SYN -m recent --rcheck --seconds 300 --hitcount 3 --name sshattack --rsource -j LOG --log-prefix "SSH Drop: "
    -A INPUT -p tcp -m tcp --dport 22 --tcp-flags SYN,RST,ACK SYN -m recent --rcheck --seconds 300 --hitcount 3 --name sshattack --rsource -j REJECT --reject-with tcp-reset
    -A INPUT -p tcp -m tcp --dport 22 -j ACCEPT

    The above allows three connections in a 5 minute period to port 22. After that it rejects any further connection attempts until the 5 minute timer is up.

  • Post your banlist (Score:2, Informative)

    by UnRDJ ( 712762 )

    I get about 5 of these a day, on a relatively small site. I wrote a small shell script out of sheer boredom that parses hosts.deny and gives me country and hostname info. Here's the output from the past week or so. It seems to confirm that most of these are from public isps overseas. CN, China KR, Korea, Republic of KR, Korea, Republic of BR, Brazil CL, Chile BR,

  • My Solution (Score:4, Informative)

    by ironicsky ( 569792 ) on Sunday April 12, 2009 @09:20PM (#27552153) Journal

    I was having similar brute force attacks.
    I've made some alterations to protect my server from brute force SSH attempts.

    1) Moved SSH to another random port
    2) Bound the SSHD to an IP address that is not used for Web/Mail/FTP, etc.. So the IP should generally see less traffic
    3) Disable Password Authentication, Users who are given SSH access must use a password protected key file
    4) Disabled Root SSH Login
    5) Setup the system that 3 failed logins add the entire IP Subnet(X.X.X.0-X.X.X.255) for 15 minutes, 5 failed attempts 1 week, anything else is a never ending ban. (iptables and hosts.deny, just in case)

    • by Ash-Fox ( 726320 )

      4) Disabled Root SSH Login

      Woha, there are still Linux distributions out there that have root logins enabled on SSH by default?

  • Obviously ssh should be off by default. Many people use a different port for ssh, as long as you understand that that is security trough obscurity, that is fine. It is not a real option if your system is a multi user system. Imagine if each webserver on the internet ran on a random port. Not very nice.

    So then you must ask yourself if everybody in the whole world is allowed to run ssh to you or just a few.
    You can already use your hosts.allow to block many and allow other IP adresses.
    Then you can ask yourself

  • Maybe that's redundant, but "james" has tried 7500 times in the last 5 days to login to a machine that neither allows password authentication, nor has a user named "james".

    Something is broken on their end.

  • Some years ago, I worked for a partner of a then-enormous national ISP that shall remain unnamed, but its initials are AOL. At one point, I had access to a large list of usernames and passwords of their users. Out of curiosity, I performed a statistical analysis of the data and discovered, to my not very large surprise, that about 20% -- I forget the exact figure -- had passwords that were either "password", "secret", or their username. In other words, if you know one of their usernames, you have a roughly

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