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Running (X)Ubuntu 10.04.2 LTS behind a router.

I just received an email from my root account on that machine, with subject: *** SECURITY information for <hostname> ***

<hostname> : jun 1 22:15:17 : <username> : 3 incorrect password attempts ; TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/bin/sh /tmp/tmpPHBmTO

I can see no /tmp/tmpPHBmTO file, though there is a file named /tmp/tmpwoSrWW with a timestamp dating from 2011-06-01 22:14, so just before the mentioned date/time. It's a binary file, and the content doesn't look familiar to me. Also, that file only has -rw------- permissions.

As I read it, this means that someone (or something) has (had) access to my machine. Apparently not root access (yet), but still, enough to write files to my /tmp directory at the very least.

Does someone have any pointers as to where I could look for more information: who could have done this, and how they could have done this?

My router is configured to allow access to forward traffic for SSH, HTTP (nginx acting as reverse proxy for one of several other services), SMTP, POP (postfix) and IMAP (dovecot), and also port 51413 (Transmission).

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I just got an email with the same message. The only thing exposed to the internet is Deluge and nginx, so if it's not Deluge, I have no idea what it can be. – Stavros Jan 16 at 13:36

migrated from superuser.com Jun 4 '11 at 6:16

2 Answers

up vote 9 down vote

If you have SSH opened to the Internet you WILL see hack attempts where scripties will try to crack that password.

Possible mitigation steps:

  • Do not allow root login via SSH (su after login if needed)
  • Have a VERY strong password (think passphrase - 10 or more characters)
  • Use key authentication for SSH and turn off password auth
  • Install fail2ban to block login attempts after n login attempts
  • Move ssh off of the default port (as recommended by @voithos)

Note: if your machine is already compromised, none of the above will help.

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Also, if you can, change the SSH port to some obscure high number, but one that you can remember. By simply doing that, I get exactly 0 break-in attempts each month. But, don't rely on it, as your SSH server should be secure anyway (e.g. public key auth, disable password auth, etc.). – voithos Jun 2 '11 at 17:11
It's actually the first time I've seen a message like this -- but doesn't the COMMAND=/bin/sh /tmp/tmpPHBmTO indicate that they already had access? – Martijn Jun 2 '11 at 17:14
@Martijn: Good question. Maybe, maybe not. Anyone can write to the /tmp directory. It's probably easy enough to trick one of the other services into writing a file to /tmp without requiring authentication. Often times /tmp is mounted with noexec (specified in fstab) to prevent exactly this. Who is the owner of the file? It might be fun to analyse the file to see what it does. – Code Bling Jun 2 '11 at 17:34
@CodeBling: the owner of the file is my 'regular' user name; the one that's also mentioned in the e-mail message. – Martijn Jun 2 '11 at 17:45
2  
It sounds more like someone already has access.. – Code Bling Jun 2 '11 at 19:06
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up vote 1 down vote

This answer refers also to comments made on @uSlakr's post.

Your computer is at least partly compromised. Being able to write to your /tmp with your username indicates the ability to work under your user name. Being able to write under your username = able to do what you are currently doing.

If you have services that run under your username, mostly likely that particular service has been compromised. (1) If you do not have services that run under your username, then your account has been compromised totally (i.e. with interactive shell access). But this is less likely, see (1).

Your best bet is to (1) backup the current system to a virtual machine for inspection + restore from old backup (do inspect for files like that) + stop network access (2) change password of all account (3) disable all services (4) update all services to newest version (5) install intrusion detection program.

(1): As you can see the intruder is trying to run some program in order to obtain root access. It does not sound very much like they have interactive shell access -- if they had interactive shell they should be grabbing the hash of your password and crack it with some manner // installing some sort of program to your interactive shell to capture your root password // use sudo (these are generally better idea than having wrong password 3 times, well, if they are THAT stupid... nevermind)

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