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21 - Pentesting FTP

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Basic Information

The File Transfer Protocol (FTP) serves as a standard protocol for file transfer across a computer network between a server and a client. It is a plain-text protocol that uses as new line character 0x0d 0x0a so sometimes you need to connect using telnet or nc -C.

Default Port: 21

PORT   STATE SERVICE
21/tcp open  ftp

Connections Active & Passive

In Active FTP the FTP client first initiates the control connection from its port N to FTP Servers command port – port 21. The client then listens to port N+1 and sends the port N+1 to FTP Server. FTP Server then initiates the data connection, from its port M to the port N+1 of the FTP Client.

But, if the FTP Client has a firewall setup that controls the incoming data connections from outside, then active FTP may be a problem. And, a feasible solution for that is Passive FTP.

In Passive FTP, the client initiates the control connection from its port N to the port 21 of FTP Server. After this, the client issues a passv comand. The server then sends the client one of its port number M. And the client initiates the data connection from its port P to port M of the FTP Server.

Source: https://www.thesecuritybuddy.com/vulnerabilities/what-is-ftp-bounce-attack/

Connection debugging

The FTP commands debug and trace can be used to see how is the communication occurring.

Enumeration

Connect to FTP using starttls

Unauth enum

With nmap

You can us the commands HELP and FEAT to obtain some information of the FTP server:

Anonymous login

anonymous : anonymous anonymous : ftp : ftp

Brute force

Here you can find a nice list with default ftp credentials: https://github.com/danielmiessler/SecLists/blob/master/Passwords/Default-Credentials/ftp-betterdefaultpasslist.txt

Automated

Anon login and bounce FTP checks are perform by default by nmap with -sC option or:

Browser connection

You can connect to a FTP server using a browser (like Firefox) using a URL like:

Note that if a web application is sending data controlled by a user directly to a FTP server you can send double URL encode %0d%0a (in double URL encode this is %250d%250a) bytes and make the FTP server perform arbitrary actions. One of this possible arbitrary actions is to download content from a users controlled server, perform port scanning or try to talk to other plain-text based services (like http).

Download all files from FTP

If your user/password has special characters, the following command can be used:

Some FTP commands

  • USER username

  • PASS password

  • HELP The server indicates which commands are supported

  • **PORT 127,0,0,1,0,80**This will indicate the FTP server to establish a connection with the IP 127.0.0.1 in port 80 (you need to put the 5th char as "0" and the 6th as the port in decimal or use the 5th and 6th to express the port in hex).

  • **EPRT |2|127.0.0.1|80|**This will indicate the FTP server to establish a TCP connection (indicated by "2") with the IP 127.0.0.1 in port 80. This command supports IPv6.

  • LIST This will send the list of files in current folder

    • LIST -R List recursively (if allowed by the server)

  • APPE /path/something.txt This will indicate the FTP to store the data received from a passive connection or from a PORT/EPRT connection to a file. If the filename exists, it will append the data.

  • STOR /path/something.txt Like APPE but it will overwrite the files

  • STOU /path/something.txt Like APPE, but if exists it won't do anything.

  • RETR /path/to/file A passive or a port connection must be establish. Then, the FTP server will send the indicated file through that connection

  • REST 6 This will indicate the server that next time it send something using RETR it should start in the 6th byte.

  • TYPE i Set transfer to binary

  • PASV This will open a passive connection and will indicate the user were he can connects

  • PUT /tmp/file.txt Upload indicated file to the FTP

FTPBounce attack

Some FTP servers allow the command PORT. This command can be used to indicate to the server that you wants to connect to other FTP server at some port. Then, you can use this to scan which ports of a host are open through a FTP server.

Learn here how to abuse a FTP server to scan ports.

You could also abuse this behaviour to make a FTP server interact with other protocols. You could upload a file containing an HTTP request and make the vulnerable FTP server send it to an arbitrary HTTP server (maybe to add a new admin user?) or even upload a FTP request and make the vulnerable FTP server download a file for a different FTP server. The theory is easy:

  1. Upload the request (inside a text file) to the vulnerable server. Remember that if you want to talk with another HTTP or FTP server you need to change lines with 0x0d 0x0a

  2. Use REST X to avoid sending the characters you don't want to send (maybe to upload the request inside the file you needed to put some image header at the beginning)

  3. Use PORTto connect to the arbitrary server and service

  4. Use RETRto send the saved request to the server.

Its highly probably that this will throw an error like Socket not writable because the connection doesn't last enough to send the data with RETR. Suggestions to try to avoid that are:

  • If you are sending an HTTP request, put the same request one after another until ~0.5MB at least. Like this:

495KB
posts.txt
Open
posts.txt

  • Try to fill the request with "junk" data relative to the protocol (talking to FTP maybe just junk commands or repeating the RETRinstruction to get the file)

  • Just fill the request with a lot of null characters or others (divided on lines or not)

Anyway, here you have an old example about how to abuse this to make a FTP server download a file from a different FTP server.

Filezilla Server Vulnerability

FileZilla usually binds to local an Administrative service for the FileZilla-Server (port 14147). If you can create a tunnel from your machine to access this port, you can connect to it using a blank password and create a new user for the FTP service.

Config files

Post-Exploitation

The default configuration of vsFTPd can be found in /etc/vsftpd.conf. In here, you could find some dangerous settings:

  • anonymous_enable=YES

  • anon_upload_enable=YES

  • anon_mkdir_write_enable=YES

  • anon_root=/home/username/ftp - Directory for anonymous.

  • 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

  • no_anon_password=YES - Do not ask anonymous for password

  • write_enable=YES - Allow commands: STOR, DELE, RNFR, RNTO, MKD, RMD, APPE, and SITE

Shodan

  • ftp

  • port:21

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