by RSS Floser Bacurio, Joie Salvio, Rommel Joven  |  Feb 02, 2017  |  Filed in: Security Research

Sage 2.0 is the new kid on an already crowded block of ransomware, demanding hefty ransom of 2.22188 bitcoins (roughly 2000 USD) per infection. We have recently begun seeing this malware being distributed by the same malicious spam campaigns that serve better-known ransomware families, such as Cerber and Locky.

In this article we will take a closer look at some notable characteristics of this new threat, and provide some simple ways to mitigate it.

Spam Campaign

Sage ransomware has been seen spreading through the usual spam email channels with malicious attachments. Older variants didn’t have any email subjects or content, and can only be characterized by an archived attachment with filenames containing the recipient of the email. Newer variants have become more sophisticated with the addition of a fake bank notification for social engineering.

Fig. 1 - New and old variants of the Sage/Cerber spam campaign

Archived attachments contain either Javascript or Microsoft Word documents with malicious macros.

Like we see so often in malicious documents, this malware uses a macro to execute a Powershell script, which in turn downloads the ransomware to the system as %TMP%\Roaming.exe. We have also seen variants using random filenames.

Fig. 2 - Malicious documents that deliver Sage 2.0 and Cerber

 

Fig. 3 - Executed Powershell scripts from the macros

The more recent Javascript attachments serve as the first layer of obfuscation for the actual Javascript downloader, which is dropped and executed in the directory %TMP%\.js.

Fig. 4 - Javascript packer of the Javascript downloader

Fig. 5 - Decrypted Javascript with Sage 2.0 download URL

A Locky variant was also found to be hosted on the same site (hxxp[:]//affections[.]top/dd/15.exe) waiting to be served by the campaign.

Persistence

In case of interruption or discovery, Sage 2.0 comes with several techniques designed to keep executing in the user’s system.

To prevent its execution from being terminated, it always runs two processes of itself, each with a watchdog thread looping indefinitely. This routine waits for an event that terminates its other process. When that happens it spawns another process of itself, keeping the execution going.

Fig. 6 - Sage 2.0 process spawns back after terminating

It also creates a copy of itself in %appdata%/<8-characters>.exe. All filenames used by the malware are created using the base64-encoded custom hash of the Machine GUID, which means they are almost always unique per system.

In Windows 7 and higher versions where the UAC feature exists, it tries to execute its copy with elevated privilege. In default Windows settings, this action prompts UAC notification. Since this malware does not proceed until the user permits the request, it keeps on prompting until the user allows the execution.  

Fig. 7 - Sage’s execution triggers Windows UAC

Just in case the infected system is rebooted in the middle of execution, it also creates a shortcut file (LNK) in the startup folder and adds a scheduled task to execute itself during startup.

Fig. 8 - Adds startup shortcut and scheduled task

Encryption

Before file encryption, Sage 2.0 checks for the installed locale identifiers in the system and cross-checks them with a hard-coded list. If any of these identifiers are found in the list (see image below), the malware terminates and deletes itself from the system without performing any file encryption.

Fig. 9 - Function to determine if the system is a target for infection

Interestingly, it also skips any file encryption if the file “C:\\Temp\lol.txt” is found in the system. It’s possible that this is just a way to prevent accidental executions on the author’s machine.

Fig. 10 - Exits the function if “lol.txt” was found

In encrypting files and its network traffic, Sage 2.0 uses the ChaCha20 cipher, which is very much similar to Salsa20 which was used by Petya, another infamous ransom malware. The algorithm was determined mainly due to the operators and round values that were used during encryption.

Fig. 11 - Characteristic xor-rot-add operators and constants of ChaCha20

As a basic summary, a 20-hex byte master key is generated and passed to the ChaCha20 algorithm. From that, 40-hex bytes are generated and used in a simple XOR operation to encrypt the file content 40-hex bytes at a time. Another 40-hex bytes will be generated for the next set of data and so on.

The master key is unique for each file, and is created using Elliptic Curve Cryptography on a randomly generated 20-hex value. In our analysis, it appears that this function is done twice, encrypting the random value by two levels. To further complicate the decryption of the files, it only writes the first level of the encrypted key on the file. So to get the actual key, the value from the files needs to go through that same function again.

Fig. 12 - Structure of file encrypted by Sage 2.0

Since the encryption is symmetric and the key is generated locally, this malware can encrypt the system without C&C contact or an internet connection.

The list of extensions that it encrypts can be found in the Appendix of this article.

Network Traffic

As previously mentioned, the keys to encrypt the files are generated locally and stored in the encrypted files. This way it is no longer necessary to send they keys it to its C&C.

The sample that we analysed sends the data to “mbfce24rgn65bx3g” with either “er29sl[.]in or “rzunt3u2[.]com” as its domain.  These domains can be found encrypted at the overlay of the malware executable using the same ChaCha20 cipher. The malware ID and the key used to decrypt the domains are also hard-coded in the overlay.

Fig. 13 - Encrypted Domains and hard-coded key in the overlay

Network activity observed with this malware is only for system identification. The collected system information is stored in a “packed” structure.

Fig. 14 - Packed system information

Later on, the information is encrypted using the ChaCha20 cipher with the same key used to decrypt the domains. The malware identifier is also prepended to the data before it is sent to the C&C. We suspect that the C&C contains pairs of malware identifier and keys to decrypt the encrypted network traffic.

Fig. 15 - Malware encrypts collected system information

Conveniently, it also executes a function to convert the “packed” information to much more readable data. In the following structure, we can easily identify the information that this malware gathers to identify the system.

Ransom Note

Aside from creating HTML ransom notes (!Recovery_<3-chars>.html) on infected directories, it also changes the user’s desktop wallpaper. As usual, the notes contain the typical message to intimidate the user about the encrypted files along with instructions on how the payment can be made.

Fig. 16 - HTML ransom note

Fig. 17 - Desktop Wallpaper set by Sage 2.0

Conclusion

Explicitly requesting higher privilege from the user and using symmetric encryption demonstrate that Sage 2.0 has not reached a level where it can stand beside Locky or Cerber with respect to capabilities. However, it is still a ransom malware capable of causing considerable damage on a system, and must not be taken lightly.

The cyber criminals behind these attacks appear to be juggling between relatively old and new ransomware families to increase their chances of infecting systems. However, since their infection vectors have not changed much, there are simple ways to mitigate such attacks.

  1. Set UAC to “Always notify” – Doing this prevents any executable from gaining higher system privilege without notifying the user. If the system is suspected of Sage 2.0 infection, a simple way to stop the execution is to reboot in Safe Mode and remove the autostart elements
  2. Be suspicious of attachments - An email with an executable or script attachment, even when archived, are good signs of malicious intent. In such cases, always try to check the files before opening.
  3. Disable Macro – Document macros are still one of the major distributors of malware in the wild. If this feature is not needed or used on a frequent regular basis, it is advised to always have it disabled.

-=FortiGuard Lion Team=-

Sage 2.0 samples (SHA256) ­

20dd2391367c65bd47f4f077f4cfff979086b0306829695348d37bc0fc66456b - W32/Kryptik.FNGP!tr

714faf1c72f7b738799d99a3c7595134eeadd4b61f80e4071370e176864f77e7 - W32/Kryptik.FNGP!tr

770bcc42029c73542ca3d9388bafb2c85e456dd8ef950abdeb7b0e1fd93f6eae - W32/Kryptik.FNGP!tr

8aabf4fd450226f00b19c74d9dc75c773537a9f96ca1d53132109b9b4b68d34b - W32/Kryptik.FNGP!tr

9e03f1775731e13c59a8513638929f6ddd39dd2f00814f45e489b52b0328203d - W32/Kryptik.FNGP!tr

b71167636e00ed97a10e0bf63270709d1dd32dac9001db1892bd9178382afd7d - W32/Kryptik.FNGP!tr

bc811f6a4717c6ff4b1d312694fc01bf59de1738dae13cf4bda43e7154e2ae03 - W32/Kryptik.FNGP!tr

7c9a19c0e7e53383be796dc51615dc80835dd40365f40e5d4c47c43625671f5d - W32/SageCrypt.SGE!tr

c90e7fdbdcdc44ed8e9f66323be4c74dc516357bbcfa78f24c9e224ec040fa08 - W32/SageCrypt.SGE!tr

3b36355d10aa53d20e580892da8f18db16e01808e0d8145628a6eb482defad43 - W32/SageCrypt.AC!tr

678c11809db9de8d3e25dd76cf403e971e99dccf92010bbd0219f890c57a9a7b - W32/SageCrypt.AC!tr

fe9fb9330c1f075715eab6dbc10d50baaeb26393f6dce10bb6aaa8f66faddc91 - W32/SageCrypt.BS!tr

 

IOC

Dropped Files:

%AppData%\{8 alphanumeric char}.exe –  main executable

%AppData%\{8 alphanumeric char}.tmp – encrypted component

%Startup%\{8 alphanumeric char}.lnk – shortcut file

%SystemRoot%\Windows\System32\Tasks\{8 alphanumeric char} – scheduled task

%Temp%\__config{random numbers}.bat – delete-me script

%Temp%\{3 char}.bmp – desktop wallpaper image file

{infected directory}\!Recovery_{3 char}.html – ransom note

 

C&C

mbfce24rgn65bx3g[.]er29sl[.]in

mbfce24rgn65bx3g[.]rzunt3u2[.]com

7gie6ffnkrjykggd[.]er29sl[.]in

7gie6ffnkrjykggd[.]rzunt3u2[.]com

 

Appendix

Targeted file extensions:

by RSS Floser Bacurio, Joie Salvio, Rommel Joven  |  Feb 02, 2017  |  Filed in: Security Research