BambooFox CTF 2021 – Emoji [Web]
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# BambooFox CTF 2021 – Emoji [Web]

In this web challenge we have a short PHP code where it is possible to execute arbitrary code after bypassing two input sanitization filters.

First, we have an input length limitation protection so all inputs exceeding the maximum of 9 characters will be denied.

1 strlen($![🐱](https://s.w.org/images/core/emoji/13.0.1/svg/1f431.svg)=$_GET['ヽ(#Д´)ﾉ'])<0x0A 

The second check is a regex that denies any string that contains any alphanumeric character. Thankfully, this check is case insensitive.

1 preg_match('/[a-z0-9]/i',\$![🐱](https://s.w.org/images/core/emoji/13.0.1/svg/1f431.svg)) 

strlen and preg_match functions are exclusive for strings so if we input the string as an array, we will be able to bypass these two protections. Fortunately, our input is printed with print_r instead of print so the array will be dumped and evaluated by eval function.

With this technique we can bypass all checks so we can execute code using system. Listing directories with ls we see the file flag_de42537a7dd854f4ce27234a103d4362 that contains the flag.

By introducing this payload in the GET parameters we can print the flag:

1 /?ヽ(%23Д´)ﾉ[]=system('cat /flag_de42537a7dd854f4ce27234a103d4362'))?> `