Microsoft SQL Server - Corrupt Backup File Heap Overflow

Credit: Brett Moore
Risk: High
Local: Yes
Remote: No

__________________________________________________________________ Insomnia Security Vulnerability Advisory: ISVA-080709.1 ___________________________________________________________________ Name: Microsoft SQL Server - Corrupt Backup File Heap Overflow Released: 09 July 2008 Vendor Link: Affected Products: MS SQL Server 2005, possibly previous versions Original Advisory: Researcher: Brett Moore, Insomnia Security ___________________________________________________________________ _______________ Description _______________ Microsoft SQL Server contains a buffer overflow that can be reached by causing the server to attempt a database restore from a corrupt back file. This can be triggered by a user with PUBLIC access through the RESTORE TSQL statement, available through the console as well as through a vulnerable SQL statement on a web server (sql injection) By default the service runs under the NETWORK SERVICE account but has the ability to impersonate through tokens, and therefore can gain full LOCAL SYSTEM account access. _______________ Details _______________ The following TSQL statement can be called by any user with PUBLIC access. RESTORE FILELISTONLY FROM DISK = 'path to file' By hosting a corrupt SQL database backup on a remote file share it is possible to force the target server to open the file, parse it, and corrupt the internal heap. Obviously the target SQL server must have egress availability to connect out through SMB or webdav functionality. The SQL backup format consists of multiple chunks of data which follow a basic structure of struct backupChunk { unsigned long nametag; unsigned long size; } The nametag describes the type of tag (ex.SCIN, SFGI, MQCI, etc). The size corresponds to the size of the complete chunk size which includes the 8byte chunk header. The parsing function goes through file using the backupChunk->size field to point to the next valid chunk. ------------------------------------------------------------------- 022E5FE2 PUSH EAX 022E5FE3 MOV EAX, [ESP+18h] 022E5FE7 PUSH EAX 022E5FE8 PUSH ESI 022E5FE9 CALL EDX ; <-- This loads the 8byte header of the block 022E5FEB PUSH EBX 022E5FEC MOV ECX,ESI ------------------------------------------------------------------- If the size field is larger than the current buffer, a check prevents the application from overflowing. ------------------------------------------------------------------- Size Check 022E6063 MOV EAX,DWORD PTR DS:[EDX+4] ;<- Load Size 022E6066 MOV ECX,DWORD PTR SS:[ESP+14] ;<- Load Buffer Size 022E606A SUB ECX,EAX 022E606C JS SHORT sqlservr.022E6076 022E606E CMP EAX,DWORD PTR DS:[EBX+C8] ;<- Compare against 0x2000 022E6074 JBE SHORT sqlservr.022E60DC ;<- Jump to continuation ------------------------------------------------------------------- After this size comparison, the size is adjusted to subtract the size of the 8byte structure header. ------------------------------------------------------------------- The size decrease 022E60DC MOV ECX,DWORD PTR SS:[ESP+10] 022E60E0 MOV EDI,DWORD PTR DS:[ECX+4] 022E60E3 SUB EDI,8 ; <- Subtract 8 bytes 022E60E6 CMP DWORD PTR DS:[EBX+40],0 022E60EA JA SHORT sqlservr.022E6107 ------------------------------------------------------------------- If the structure size was a value between 0 and 7 then an Integer Underflow/Overflow/Wraparound/Whatever will occur, and the 'negative' large value is passed directly into the StartRead function. ------------------------------------------------------------------- The write 022E6115 MOV EDX,DWORD PTR DS:[EDX+60] 022E6118 PUSH EDI ; <- Push the new 'negative' size 022E6119 PUSH EAX 022E611A MOV EAX,DWORD PTR SS:[ESP+18] 022E611E ADD EAX,8 022E6121 PUSH EAX 022E6122 PUSH ESI 022E6123 CALL EDX ; call StartRead 022E6125 PUSH EBX ------------------------------------------------------------------- StartRead, among other things, uses the size value in a call to _memcpy, that will read the available data from the file overflowing the buffer and overwriting heap memory. ------------------------------------------------------------------- 022D44BC MOV EBX,[EBP+arg_4] 022D44BF PUSH ESI ; <- The 'negative' size 022D44C0 PUSH EAX ; void * 022D44C1 PUSH EBX ; void * 022D44C2 CALL _MEMCPY ; <- Call to memcpy ------------------------------------------------------------------- The result of exploitation is that heap memory is overflowed. We have had successful exploitable 'exceptions' in the following places; ------------------------------------------------------------------- A 'write 4' 0116D35A JE sqlservr.011C6BCE 0116D360 MOV EAX,DWORD PTR DS:[EAX+C] 0116D363 MOV ECX,DWORD PTR SS:[EBP-10] 0116D366 MOV EDX,DWORD PTR DS:[ECX+C0] 0116D36C MOV DWORD PTR DS:[EDX],EAX ; <- Control of EDX and EAX ------------------------------------------------------------------- ------------------------------------------------------------------- Function Call 014AEA5F MOV EAX,DWORD PTR DS:[EDX+C] ; <- Control EDX 014AEA62 CALL EAX 014AEA64 TEST EAX,EAX 014AEA66 JE SHORT sqlservr.014AEA70 ------------------------------------------------------------------- ------------------------------------------------------------------- Function Call 011F9D08 MOV EAX,DWORD PTR DS:[EDX+58] ; <- Control EDX 011F9D0B CALL EAX 011F9D0D MOV ECX,DWORD PTR DS:[ESI+10] ------------------------------------------------------------------- SQL server appears to use its own dynamic heap management, which makes exploitation different from a standard heap overflow. Using a custom heap management routines means that the standard heap protections mechanisms are not in place. _______________ Solution _______________ Microsoft have released a security update to address this issue; _______________ Legals _______________ The information is provided for research and educational purposes only. Insomnia Security accepts no liability in any form whatsoever for any direct or indirect damages associated with the use of this information. ___________________________________________________________________ Insomnia Security Vulnerability Advisory: ISVA-080709.1 ___________________________________________________________________


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