Buffer Overflows

Buffer Overflows

Table of Contents

• Generic procedure (Windows or Linux)
• Buffer Overflow Tips
• pattern_create.rb
• pattern_offset.rb
• nasm_shell.rb
• Bad characters Python code
• Windows Buffer Overflows

Generic procedure (Windows or Linux)

• Identify vulnerable input field and payload length that causes overflow (Fuzzing)
• Identify offsets
• Detect bad characters
• Identify jump instruction opcode and usable return address
• Generate shellcode
• Exploit!

Buffer Overflow Tips

• 0x00 is usually a bad character, because it often serves as a string terminator in programming languages vulnerable to buffer overflows (C/C++).
• Return Addresses CANNOT contain bad characters (which is why the Return Address is identified after detecting bad characters).
• Confusing debugger errors encountered during bad character detection likely indicate a bad character.
• Shellcode will almost always be generated with the EXITFUNC=thread option.
• Some vulnerable programs/services will always crash from buffer overflow conditions, regardless of how much preparation/compensation is done during exploit development.

pattern_create.rb

Provided by the Metasploit Framework. Create unique pattern payloads to detect register offsets.

/usr/share/metasploit-framework/tools/exploit/pattern_create.rb -l [PAYLOAD_LENGTH]

pattern_offset.rb

Provided by the Metasploit Framework. Identify exact offsets where unique patterns reside.

/usr/share/metasploit-framework/tools/exploit/pattern_offset.rb -q [VALUE] -l [PAYLOAD_LENGTH]

nasm_shell.rb

Provided by the Metasploit Framework. Convert x86 Assembly Language instructions into corresponding hexadecimal opcodes.

/usr/share/metasploit-framework/tools/exploit/nasm_shell.rb

Bad characters Python code

badchars = ("\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")

Windows Buffer Overflows

Set breakpoints in Immunity Debugger

• Goto memory address using button
• Click displayed memory address
• Press F2
• If warning message is displayed, click Yes
• Breakpoint is set when memory address is highlighted in Blue.

Search for module information in Immunity Debugger

!mona modules

Search for opcode address within module in Immunity Debugger

!mona find -s "[OPCODE]" -m [MODULE]

Convert string [STRING] to Little-Endian hexadecimal value (Python)

"[STRING]"[::-1].encode('hex').upper()

Can be useful for printing string-to-hexadecimal values in Windows buffer overflow exploits.