ASSM Ransomware (.assm File Extension)

What is Assm?

Malware can manifest in various forms, some of which are particularly perilous. Assm ransomware stands as a prime example of one of the most threatening viruses currently known. Rather than destroying your operating or file systems, it renders your files inaccessible and strategically inhibits the use of antivirus software by its victims.

This malware employs the Salsa20 encryption algorithm, whose potency is derived from the vast number of possible decryption keys. Further complicating matters, each victim is assigned a unique key stored on a remote server controlled by cybercriminals. The only recourse for retrieving the keys is to pay the ransom or hope for law enforcement to intervene successfully.

The method of infection is also noteworthy. The STOP/Djvu ransomware variants predominantly proliferate via third-party websites offering “free” versions of popular software. By tampering with the software's license verification to enable unpaid use, perpetrators can embed malware within the program's code. The type of malware you end up with—be it through a “free” version of MS Office or Anti-Malware — depends on the financial arrangements made with the malware distributors.

Assm Encryption process

The Assm uses the Salsa20 encryption algorithm. That is not the strongest method, but it still provides an overwhelming amount of possible decryption keys. To brute force the 78-digit number of keys, you need 3.5 unvigintillion years (1*10^65), even if you use the most powerful regular PC. Quantum computers can show a bit better results, but it is still too slow to get your files back while you are alive.

The exact algorithm of encryption is next: malware scans each folder for the files it is able to encrypt. Then, when it finds the target, it makes a copy of your file, removes the original one, encrypts the copy and leaves it instead of the removed original. Such a procedure is done to prevent the situation when you have already opened the file, so ransomware is not able to read it because of the Windows restrictions. To each encrypted copy, the virus adds the specific extension - ".assm". Then, ransomware creates a _readme.txt file in the folder where the encrypted file is located, and goes to the next folder.

Such an encryption method can be exploited for file recovery. Since the original file is deleted, you may try to recover it using the file recovery tools. The less time is passed - the bigger the chance to get your files back, so hurry up!

Another specific moment that can help you to use the files even after the encryption is the fact that Assm ransomware encrypts only the first 150KB of each file. Hence, you can try to run a big file, such as video or music, without the encryption. Similar feature also works with other ransomware families - Dharma, Conti and Makop encrypt the same 150KB.

Assm note:

Ransom note is the same for the whole STOP/DJVU family. In fact, it is one of the main signs of to which family the certain ransomware belongs. Here is the typical note for STOP/Djvu family:

External Links:

Indicators of Compromise(IOC)