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Wednesday, May 31, 2023

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Learning Web Pentesting With DVWA Part 1: Installation

In this tutorial series I'm going to walk you through the damn vulnerable web application (DVWA) which is damn vulnerable. Its main goal according to the creators is "to aid security professionals to test thier skills and tools in a legal environment, help web developers better understand the process of securing web applications and to aid both students & teachers to learn about web application security in a controlled class room environment."

I am going to install DVWA in docker so the prerequisite for this tutorial will be an installation of docker (Docker is not the only way to install DVWA but if you have docker already installed then it may be the easiest way to install DVWA).

To install DVWA in docker run your docker deamon if it's not running already and open a terminal or powershell and type:

docker rum --rm -it -p 8080:80 vulnerables/web-dvwa

It will take some time to pull the image from docker hub depending on your internet speed and after it is complete it will start the dvwa application. In the command we have mapped the image instance's port 80 to our hosts port 8080 so we should be able to access the web application from our host at http://localhost:8080

Now open your favorite web browser and go to http://localhost:8080
You should be prompted with a login screen like this:

login with these creds:
username: admin
password: password

After login you'll see a database setup page since this is our first run. Click on Create / Reset Database button at the bottom. It will setup database and redirect you to login page. Now login again and you'll see a welcome page.

Now click on DVWA Security link at the bottom of the page navigation and make sure the security level is set to Low. If it is not click on the dropdown, select Low and then click submit.

Now our setup is complete, so lets try a simple SQL attack to get a taste of whats about to come.

Click on SQL Injection in navigation menu.
You'll be presented with a small form which accepts User ID.
Enter a single quote (') in the User ID input field and click Submit.
You'll see an SQL error like this:

From the error message we can determine that the server has a MariaDB database and we can see the point of injection.
Since there are many quotes we are not able to determine the exact location of our injection. Lets add some text after our single quote to see exactly where our injection point is.
Now I am going to enter 'khan in the User ID field and click Submit.

Now we can see exactly where the point of injection is. Determining the point of injection is very important for a successful SQL injection and is sometimes very hard too, though it might not be that much useful here in this exercise.

Now lets try the very basic SQL Injection attack.
In the User ID field enter ' or 1=1-- - and click Submit.

We will explain what is going on here in the next article.

References:-
1. DVWA Official Website: http://www.dvwa.co.uk/

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Reversing Pascal String Object

There are many goodware and malware developed in pascal, and we will see that the binary generated by the pascal compilers is fascinating, not only because the small and clean generated binaries, or the clarity of the pascal code, but also the good performance. In Linux we have Lazarus which is a good free IDE like Delphi and Kylix the free pascal IDE for windows.

The program:

program strtest;

var
cstr: array[0..10] of char;
s, s2: ShortString;

begin
cstr := 'hello world';
s := cstr;
s2 := 'test';

WriteLn(cstr + ' ' + s + ' ' + s2);
end.

We are going to compile it with freepascal and lazarus, and just the binary size differs a lot:

lazarus 242,176 btytes 845 functions
freepascal 32,256 bytes 233 functions
turbopascal 2,928 bytes 80 functions (wow)

And surprisingly turbopascal binaries are extremely light.
Lets start with lazarus:

Logically it imports from user32.dll some display functions, it also import the kernel32.dll functions and suspiciously the string operations of oleaut32.dll

And our starting point is a function called entry that calls the console initialization and retrieve some console configurations, and then start a labyrinth of function calls.

On functions 10000e8e0 there is the function that calls the main function.

I named execute_param2 because the second param is a function pointer that is gonna be executed without parameters, it sounds like main calling typical strategy.
And here we are, it's clearly the user code pascal main function.

What it seems is that function 100001800 returns an string object, then is called its constructor to initialize the string, then the string is passed to other functions that prints it to the screen.

This function executes the method 0x1c0 of the object until the byte 0x89 is a null byte.
What the hell is doing here?
First of all let's create the function main:

Simply right button create function:

After a bit of work on Ghidra here we have the main:

Note that the struct member so high like 0x1b0 are not created by default, we should import a .h file with an struct or class definition, and locate the constructor just on that position.

The mysterious function was printing byte a byte until null byte, the algorithm the compiler implemented in asm is not as optimized as turbopascal's.

In Windbg we can see the string object in eax after being created but before being initialized:

Just before executing the print function, the RCX parameter is the string object and it still identical:

Let's see the constructor code.
The constructor address can be guessed on static walking the reverse-cross-references to main, but I located it in debugging it in dynamic analysis.

The constructor reads only a pointer stored on the string object on the position 0x98.

And we have that the pointer at 0x98 is compared with the address of the literal, so now we know that this pointer points to the string.
The sentence *string_x98 = literal confirms it, and there is not memory copy, it only points reusing the literal.

Freepascal

The starting labyrinth is bigger than Lazarus so I had to begin the maze from the end, searching the string "hello world" and then finding the string references:

There are two ways to follow the references in Ghidra, one is [ctrl] + [shift] + F but there is other trick which is simply clicking the green references texts on the disassembly.

At the beginning I doubted and put the name possible_main, but it's clearly the pascal user code main function.

The char array initialization Is converted by freepascal compiler to an runtime initialization using mov instructions.

Reducing the coverage on dynamic we arrive to the writeln function:

EAX helds a pointer to a struct, and the member 0x24 performs the printing. In this cases the function can be tracked easily in dynamic executing the sample.

And lands at 0x004059b0 where we see the WriteFile, the stdout descriptor, the text and the size supplied by parameter.

there is an interesting logic of what happens if WriteFile() couldn't write all the bytes, but this is other scope.
Lets see how this functions is called and how text and size are supplied to figure out the string object.

EBX helds the string object and there are two pointers, a pointer to the string on 0x18 and the length in 0x18, lets verify it on windbg.

And here we have the string object, 0x0000001e is the length, and 0x001de8a68 is the pointer.

Thanks @capi_x for the pascal samples.

Continue reading

Prêt et crédit

Wednesday, May 31, 2023

Hackerhubb.blogspot.com

Learning Web Pentesting With DVWA Part 1: Installation

Reversing Pascal String Object

Freepascal

Tuesday, May 30, 2023

Hackerhubb.blogspot.com

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