Technical Analysis of Crytox Ransomware

Crytox is a multi-stage ransomware that encrypts local and network drives using per-file AES-256 keys protected by a locally generated RSA key, while dropping the uTox messenger to enable victim-actor communication. It employs anti-analysis techniques (packing, API hashing, encrypted configs), a Run key for startup ransom note display, and a notable weakness in its weak random generator that could enable brute-force decryptions. #Crytox #RTL #ThreatLabz #uTox #Zscaler #Win64.Ransom.Crytox

Keypoints

Crytox is a ransomware family with multiple stages of encrypted code, first observed in 2020.
It encrypts files on connected drives and on network drives, then drops the uTox messenger to facilitate attacker-victim communication.
The ransomware uses AES-CBC with a per-file 256-bit key, protected by a locally generated RSA public key.
Decryption may be possible via a known-plaintext brute-force attack due to weaknesses in the key generation method.
First-stage config is decrypted and stored in the registry; a Run key ensures the ransom note runs at startup.
Second-stage payload deletes shadow copies and clears logs; final stage decrypts the main config and encrypts files with per-file keys.
Crytox employs several anti-analysis techniques (packing, API hashing, encrypted configs/shellcode, remote thread injection) to hinder analysis.

MITRE Techniques

[T1027] Software Packing – Crytox is packed with UPX and uses anti-analysis techniques including API hashing, encrypted configurations and shellcode. Quote: ‘Crytox uses different techniques to thwart static analysis including the following: API hashing; Encrypted configurations; Encrypted shellcode; Remote thread injection’
[T1055] Process Injection – The malware locates a target process and uses remote thread injection to run its shellcode. Quote: ‘The code proceeds to locate a process to inject… A remote thread is created to execute the first piece of shellcode’
[T1112] Modify Registry – The malware stores configuration in the registry (e.g., under HKCR.waitingshellopencommand). Quote: ‘Under the sub-key HKCR.waitingshellopencommand, the ransomware stores the following value-data pair…’
[T1547.001] Boot or Logon Autostart: Registry Run Keys/Startup Folder – It creates Run entries to display the ransom note at startup. Quote: ‘open’ registry value and data ‘C:ReadMe.hta’ are created under HKLM…Run
[T1486] Data Encrypted for Impact – The final stage decrypts the main configuration and encrypts files with per-file AES keys, then appends the victim ID to filenames. Quote: ‘The final stage decrypts the content from the resource… This final stage decrypts the main configuration containing the following information’
[T1059.003] Windows Command Shell – The second stage executes batch-like commands to delete shadow copies and clear logs (e.g., wevtutil, vssadmin, diskshadow). Quote: ‘for /F “tokens=*” %%1 in (‘wevtutil.exe el’) DO wevtutil.exe cl “%%1” vssadmin.exe Delete Shadows /All /Quiet diskshadow.exe /s ../pghdn.txt’
[T1070.001] Clear Windows Event Logs – The batch commands clear event logs as part of cleanup. Quote: ‘remove events from the logs’
[T1135] Network Share Discovery – The malware enumerates connected drives and network drives to extend encryption scope. Quote: ‘The code proceeds to retrieve connected drives and for each drive found…’
[T1083] File and Directory Discovery – The ransomware enumerates drives and encrypts files, including per-file operations. Quote: ‘The ransomware encrypts files on connected drives…’

Indicators of Compromise

[Hash] Hashes – 1c0bf0c2e7d0c34ec038a8b717bb19d9c4cf3382ada1412f055a9786d3069d78, 2115c4c859d497eec163ca33798c389649543d8a6e4db5806a791c6186722b71, and 7 more hashes
[File] Files written – C:ReadMe.hta, and files with “.waiting” extension (e.g., YOUR ID.waiting)
[Registry] Registry keys – HKCR.waitingshellopencommand (used to store ransom payload/configs)
[Process] Targeted processes in key generation – explorer.exe (hash check uses a value corresponding to explorer.exe) and svchost.exe