CDXX Ransomware (.cdxx File Extension)

What is Cdxx?

Discovering files with the .cdxx extension and _readme.txt notes scattered across various folders is a telltale sign of infection with the Cdxx ransomware. This malware is designed to encrypt files on the victim's computer before demanding a ransom. Decrypting these files without specific tools is generally considered impossible.

The Cdxx virus, like others in the STOP/Djvu ransomware family, employs the Salsa20 encryption algorithm, which boasts an astronomical number of potential decryption keys, making brute-force attempts futile. The use of quantum computing wouldn't significantly reduce the decryption time, possibly outlasting the Earth's lifespan. However, there's a glimmer of hope as certain specialized decryption tools can potentially restore your files within hours.

Cdxx ransomware is primarily distributed through third-party websites that tempt users with free access to paid programs or through the use of various dubious tools. Hackers who circumvent program licensing inject malicious code—or even the entire virus—into their "products," with malware developers compensating them for these actions.

Dubious tools, designed for illegal activities such as generating license keys for software or activating Windows unlawfully, are particularly susceptible to misuse for distributing malware. Since antivirus solutions typically flag such tools, perpetrators often advise disabling security software or whitelisting the hacking tool. Regrettably, the malicious intent becomes apparent only after the damage is done.

Cdxx Encryption process

The Cdxx 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 - ".cdxx". 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 Cdxx 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.

Cdxx 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)