21 - Pentesting FTP
Basic Information
The File Transfer Protocol (FTP) is a standard network protocol used for the transfer of computer files between a client and server on a computer network.
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
Enumeration
Banner Grabbing
Connect to FTP using starttls
Unauth enum
You can us the commands HELP and FEAT to obtain some information of the FTP server:
Connections
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.
Anonymous login
anonymous : anonymous anonymous : ftp : ftp
Automated
Anon login and bounce FTP checks are perform by default by nmap with -sC option or:
Shodan
ftpport:21
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
Some FTP commands
USER usernamePASS passwordHELPThe server indicates which commands are supportedPORT 127,0,0,1,0,80This 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.LISTThis will send the list of files in current folderAPPE /path/something.txtThis 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.txtLikeAPPEbut it will overwrite the filesSTOU /path/something.txtLikeAPPE, but if exists it won't do anything.RETR /path/to/fileA passive or a port connection must be establish. Then, the FTP server will send the indicated file through that connectionREST 6This will indicate the server that next time it send something usingRETRit should start in the 6th byte.TYPE iSet transfer to binaryPASVThis will open a passive connection and will indicate the user were he can connects
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.
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:
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 0x0aUse
REST Xto 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 begging)Use
PORTto connect to the arbitrary server and serviceUse
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:
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)
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
HackTricks Automatic Commands
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