A customer recently experienced a virus on one of their PCs which resulted in their WAN IP address being blacklisted by Spamhaus and CBL.
The virus Trojpig was transmitting from inside the network and this was being picked up by the block list providers. The virus was cleaned from an internal server once, and the blacklist cleared. However we then received another warning saying the site had been blacklisted again.
Below is the message from the CBL website report (the 81. IP address is the customer’s external WAN address). The warning was issued due to the customer having already been delisted once previously:
WARNING: If you continually delist 81.x.x.x without fixing the problem, the CBL will eventually stop allowing the delisting of 81.x.x.x.
The second time this occurred the original server was checked and seen to be no longer infected, thus causing a problem in that we had an unknown infected machine on the company’s internal network. The question is how do you find the machine in order to remove the virus and delist the company, thus returning their email functionality?
The answer was straightforward. The company thankfully uses ISA 2006 as it’s main firewall. In ISA we can create rules to allow or deny all types of traffic and then produce filtered reports for that specific rule, which is what we did.
We created a rule in ISA for all outgoing traffic to block delivery to one of Trojpig’s known management server addresses on the internet (See CBL blacklist report below). We then configured ISA to report on the specific rule we had configured. Within 30 seconds we had the internal IP Address of the problem client which was then isolated and later fixed.
We were then able to whitelist the customer and get them back up and running in the shortest time possible.
Below is the Blacklist report from CBL:
CBL Lookup Utility:
Automated/scripted bulk lookups are forbidden. Upon detection, automated scripts will be denied access, and the source IP may be listed in the CBL.
IP Address 81.x.x.x is listed in the CBL. It appears to be infected with a spam sending trojan or proxy.
It was last detected at 2011-06-09 06:00 GMT (+/- 30 minutes), approximately 5 hours, 30 minutes ago.
It has been relisted following a previous removal at 2011-06-06 08:33 GMT (3 days, 3 hours, 10 minutes ago)
This IP is infected with, or is NATting for a machine infected with Torpig, also known by Symantec as Anserin.
This was detected by observing this IP attempting to make contact to a Torpig Command and Control server at 91.19.44.118, with contents unique to Torpig C&C command protocols.
Torpig is a banking trojan, specializing in stealing personal information (passwords, account information, etc) from interactions with banking sites.
Torpig is normally dropped by Mebroot. Mebroot is a Rootkit that installs itself into the MBR (Master Boot Record).
With Mebroot or any other rootkit that installs itself into the MBR, you will either have to use a “MBR cleaner” or reformat the drive completely – even if you manage to remove Torpig, the MBR infection will cause it to be reinfected again.
The best way to find the machine responsible is to look for connections to the Torpig C&C server. This detection was made through a connection to 91.19.44.118, but this changes periodically. To find these infections, we suggest you search for TCP/IP connections to the range 91.19.0.0/16 and 91.20.0.0/16 (in other words: 91.19.0.0-91.20.255.255) usually destination port 80 or 443, but you should look for all ports. This detection corresponds to a connection at 2011-06-09 06:14:31 (GMT – this timestamp is believed accurate to within one second).
These infections are rated as a “severe threat” by Microsoft. It is a trojan downloader, and can download and execute ANY software on the infected computer.
You will need to find and eradicate the infection before delisting the IP address.
We strongly recommend that you DO NOT simply firewall off connections to the sinkhole IP addresses given above. Those IP addresses are of sinkholes operated by malware researchers. In other words, it’s a “sensor” (only) run by “the good guys”. The bot “thinks” it’s a command and control server run by the spambot operators but it isn’t. It DOES NOT actually download anything, and is not a threat. If you firewall it, your IPs will remain infected, and they will still be able to download from real command & control servers run by the bot operators.
If you do choose to firewall these IPs, PLEASE instrument your firewall to tell you which internal machine is connecting to them so that you can identify the infected machine yourself and fix it.
We are enhancing the instructions on how to find these infections, and more information will be given here as it becomes available.
Virtually all detections made by the CBL are of infections that do NOT leave any “tracks” for you to find in your mail server logs. This is even more important for the viruses described here – these detections are made on network-level detections of malicious behaviour and may NOT involve malicious email being sent.
This means: if you have port 25 blocking enabled, do not take this as indication that your port 25 blocking isn’t working.
The links above may help you find this infection. You can also consult Advanced Techniques for other options and alternatives.
Pay very close attention: Most of these trojans have extremely poor detection rates in current Anti-Virus software. For example, Ponmocup is only detected by 3 out of 49 AV tools queried at Virus Total.
Thus: having your anti-virus software doesn’t find anything doesn’t prove that you’re not infected.
While we regret having to say this, downloaders will generally download many different malicious payloads. Even if an Anti-Virus product finds and removes the direct threat, they will not have detected or removed the other malicious payloads. For that reason, we recommend recloning the machine – meaning: reformatting the disks on the infected machine, and re-installing all software from known-good sources.
WARNING: If you continually delist 81.137.208.239 without fixing the problem, the CBL will eventually stop allowing the delisting of 81.137.208.239.
If you have resolved the problem shown above and delisted the IP yourself, there is no need to contact us.

