Pierluigi Paganini March 31, 2013

Botnet author are increasing complexity of the malicious code they use and at the same time security firms are adopting more sophisticated detection methods. Between principal concerns of botmaster the need to improve the capabilities of bot agents to operate silently and necessity to masquerade traffic from bots and Command & Control servers.

Focusing on this second question in the past authors of botnet malware have misused popular platform such as Google Docs, Twitter and Facebook to masquerade malicious traffic.

Researchers at TrendMicro recently detected a malware, dubbed “BKDR_VERNOT.A” that tried to exploit popular note-taking app Evernote as Command-and-Control Server to provide instructions to the bot installed on the victim’s machine.

Authors of “BKDR_VERNOT.A” tried to send command to bot with Command-and-Control Server using Evernote, malicious code is delivered via an executable file that installs the malware as a dynamic-link library, once installed the DLL is linked to a legitimate running process to remain undetected from detection systems. 

But here’s the interesting part: BKDR_VERNOT.A retrieves its C&C server and queries its backdoor commands in the notes saved in its Evernote account. The backdoor may also use the Evernote account as a drop-off point for its stolen information.

The behavior of BKDR_VERNOT.A is typical of any backdoor code, it is able to receive commands such as downloading, executing, and renaming files. The malware collects information from the infected system, including OS version, Timezone, user name, computer name and organization.

As is possible to note in the above picture the malicious code may have used Evernote as Command Server where to store stolen data, unfortunately the researchers haven’t had during analysis access to the account used by the agent.

“Unfortunately, during our testing, it was not able to login using the credentials embedded in the malware. This is possibly a security measure imposed by Evernote following its recent hacking issue.” “Though this is a clever maneuver to avoid detection, this is not the first time that a legitimate service like Evernote was used as a method of evasion.” 

The malicious code attempts to connect to Evernote via the legitimate URL https://evernote.com/intl/zh-cn making hard detection activities.

Misusing legitimate services such as Google or Evernote is an useful technique to prevent efforts done by the security researchers, the legitimate network traffic generated is not detected by most antimalware products.

How to defend our systems from these categories of cyber threat?

Blog post provided the response:

“To avoid this threat, you must always be cautious with visiting unknown websites and opening email messages.”

Pierluigi Paganini

(Security Affairs – Malware)