Dealing with malware on a fairly regular basis – its no surprise to see a subtle change in the approach used by the applications to “phone home”. A recently discovered threat – Morto – more info here targets RDP as a means of infiltrating remote machines and spreading itself. The interesting thing is that rather than the more common mechanisms of communicating with the authors of the malware Morto uses the DNS system to query TXT DNS records registered by the attackers to get information about what nasty to download next. The downloaded component is a malicious executable that is then injected into the svchost.exe service.
So the interesting issues Morto raises are
1) The use of RDP top propogate
2) Saving encrypted payload to the registry
3) Replacing some minor DLLS with its own code
4) the use of DNS TXT records to issue further commands to the remotely infected machines.
No doubt Morto is a step along the evolutionary path of Malware but we are by no means helpless.
Protecting yourself ?
- Use a firewall to block all incoming connections from the Internet to
services that should not be publicly available. By default, you should deny all
incoming connections and only allow services you explicitly want to offer to the
- Enforce a password policy. Complex passwords make it difficult to crack
password files on compromised computers. This helps to prevent or limit damage
when a computer is compromised.
- Ensure that programs and users of the computer use the lowest level of
privileges necessary to complete a task. When prompted for a root or UAC
password, ensure that the program asking for administration-level access is a
- Disable AutoPlay to prevent the automatic launching of executable files on
network and removable drives, and disconnect the drives when not required. If
write access is not required, enable read-only mode if the option is available.
- Turn off file sharing if not needed. If file sharing is required, use ACLs
and password protection to limit access. Disable anonymous access to shared
folders. Grant access only to user accounts with strong passwords to folders
that must be shared.
- Turn off and remove unnecessary services. By default, many operating systems
install auxiliary services that are not critical. These services are avenues of
attack. If they are removed, threats have less avenues of attack.
- If a threat exploits one or more network services, disable, or block access
to, those services until a patch is applied.
- Always keep your patch levels up-to-date, especially on computers that host
public services and are accessible through the firewall, such as HTTP, FTP,
mail, and DNS services.
- Configure your email server to block or remove email that contains file
attachments that are commonly used to spread threats, such as .vbs, .bat, .exe,
.pif and .scr files.
- Isolate compromised computers quickly to prevent threats from spreading
further. Perform a forensic analysis and restore the computers using trusted
- Train employees not to open attachments unless they are expecting them.
Also, do not execute software that is downloaded from the Internet unless it has
been scanned for viruses. Simply visiting a compromised Web site can cause
infection if certain browser vulnerabilities are not patched.
- If Bluetooth is not required for mobile devices, it should be turned off. If
you require its use, ensure that the device’s visibility is set to “Hidden” so
that it cannot be scanned by other Bluetooth devices. If device pairing must be
used, ensure that all devices are set to “Unauthorized”, requiring authorization
for each connection request. Do not accept applications that are unsigned or
sent from unknown sources.
One final thought for admins is to look at an IDS or similar to monitor suspicious traffic.