New Cridex variant from drive-by blackhole exploit

August 17, 2012

Dell Sonicwall Threats research team have discovered a new variant of the Cridex Banking Trojan. The Trojan utilizes the blackhole exploit kit for drive-by system infection. The purpose of this Trojan is to steal online banking credentials. It has the ability to intercept encrypted communications between banking websites and their customers.

The infection occurs when visiting a webpage with a malicious blackhole script [Detected as GAV: BlacoleRef.W_8 (Exploit)]:

The Trojan creates the following file on the filesystem:

  • %APPDATA%KB01217753.exe [Detected as GAV: Cridex.E_4 (Trojan)]

The Trojan creates the following key in the Windows registry to enable startup after reboot:

  • HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionRun KB01217753.exe "%APPDATA%KB01217753.exe"

Upon infection the Trojan injects code into explorer.exe, causing it to send the following sensitive data, encrypted, to a remote C&C server:

In the same manner as in a previous variant, it hooks various Winsock internet communication API's to intercept incoming and outgoing browser traffic.

It also hooks crypto API's such as CryptDecrypt, CryptEncrypt, EncryptMessage and DecryptMessage. This is for the purposes of intercepting SSL communication between banking websites and thier customers and injecting html code into webpages to steal credentials. Below is a sample of a decrypted configuration file that we recieved from a remote C&C server:

SonicWALL Gateway AntiVirus provides protection against this threat via the following signatures:

  • GAV: BlacoleRef.W_8 (Exploit)
  • GAV: Cridex.E_4 (Trojan)