Unauthenticated Remote Compromise in MS SQL Server 2000

NGSSoftware Insight Security Research Advisory

Name: Unauthenticated Remote Compromise in MS SQL Server 2000

Systems: Microsoft SQL Server 2000, all Service Packs

Severity: Critical/Very High Risk.

Category: Remote Buffer Overrun Vulnerability

Vendor URL: http://www.microsoft.com/

Author: David Litchfield (david@ngssoftware.com)

Advisory URL: http://www.ngssoftware.com/advisories/mssql-udp.txt

Date: 25th July 2002

Advisory number: #NISR25072002

VNA reference : http://www.ngssoftware.com/vna/ms-sql.txt

This advisory covers the solution to one of the problems mentioned in the

above VNA URL.

Description

***********

Microsoft’s database server SQL Server 2000 exhibits two buffer overrun

vulnerabilities that can be exploited by a remote attacker without ever

having to authenticate to the server. What further exacerbates these issues

is that the attack is channeled over UDP. Whether the SQL Server process

runs in the security context of a domain user or the local SYSTEM account,

successful exploitation of these security holes will mean a total compromise

of the target server and its data.

Details

*******

SQL Server can be configured to listen for incoming client connections in

several different ways. It can be configured such that clients can use named

pipes over a NetBIOS session (TCP port 139/445) or sockets with clients

connecting to TCP port 1433 or both. Which ever method is used the SQL

Server will always listen on UDP port 1434. This port is designated as the

Microsoft SQL Monitor port and clients will send a message to this port to

dynamically discover how the client should connect to the Server. This

message is a single byte packet, the byte being 0×02.

There are other messages that can be sent to this port and these can be

worked out with simple experimentation.

Stack Based Buffer Overflow

*********************************

When SQL Server receives a packet on UDP port 1434 with the first byte set

to 0×04, the SQL Monitor thread takes the remaining data in the packet and

attempts to open a registry key using this user supplied information. For

example, by sending \x04\x41\x41\x41\x41 (0×04 followed by 4 upper case

‘A’s) SQL Server attempts to open

HKLM\Software\Microsoft\Microsoft SQL Server\AAAA\MSSQLServer\CurrentVersion

By appending a large number of bytes to the end of this packet, whilst

preparing the string for the registry key to open, a stack based buffer is

overflowed and the saved return address is overwritten. This allows an

attacker to gain complete control of the SQL Server process and its path of

execution. By overwriting the saved return address on the stack with an

address that contains a “jmp esp” or “call esp” instruction, when the

vulnerable procedure returns the processor will start executing code of the

attacker’s choice. At no stage does the attacker need to authenticate.

Heap Based Buffer Overflow

********************************

When SQL Server receives a packet on UDP port 1434 with the first byte set

to 0×08 followed by an overly long string, followed by a colon character (:)

and number a heap based buffer is overflowed. As this corrupts the

structures used to keep track of the heap an attacker can overwrite any

location in memory with 4 bytes of their own choosing. This can be used to

gain remote control of the processes execution. If the colon and number are

missing the SQL Server process access violates before the heap is corrupted

as the code in the SQL Monitor thread fails to handle exceptions.

For example the code calls the C function strtok(). The strtok() functions

looks for a given token in a string, in this case a colon, and if found

returns a pointer to it. If the colon is missing in the string being

searched then no pointer is returned. This is one of the reasons why the SQL

Server process access violates if the colon is missing. The code does not

check to see if a valid pointer has been returned before passing it to

another function call, atoi():

char *ptr=NULL;

int num=0;

..

ptr = strtok(string,”:”);

num = atoi(ptr); // ptr is used without being validated

Failure to check return values and handle exceptions leads to the process

dying, leading to a simple Denial of Service attack. That said, in the light

of the overflows, the DoS is the least of the problems.

Network Based Denial of Service

*************************************

When an SQL Server receives a single byte packet, 0×0A, on UDP port 1434 it

will reply to the sender with 0×0A. A problem arises as SQL Server will

respond, sending a ‘ping’ response to the source IP address and source port.

This ‘ping’ is a single byte UDP packet - 0×0A. By spoofing a packet from

one SQL Server, setting the UDP port to 1434, and sending it the a second

SQL Server, the second will respond to the first’s UDP port 1434. The first

will then reply to the second’s UDP port 1434 and so on. This causes a storm

of single byte pings between the two servers. Only when one of the servers

is disconnected from the network or its SQL service is stopped will the

storm stop. This is a simple newtork based DoS, reminiscent of the echo and

chargen DoSes discussed back in 1996

(http://www.cert.org/advisories/CA-1996-01.html). When in this state, the

load on each SQL Server is raised to c. 40 - 60 % CPU time.

Considerations for protection against these vulnerabilities

********************************

Exploitation of these security holes goes over UDP, a connection-less

communications protocol. As such it makes the task of bypassing the

protection offered by a firewall considerably easier. The spoofing of an IP

address in a UDP packet is also considerably easier.

It is trivial for an attacker to send an attack through the firewall,

setting the source IP address to that of the target’s DNS Server and the

source port to 53. Most firewalls will allow this packet through as it will

look like a response to a query to resolve a domain name.

It is strongly recommended that a rule be added to each organization’s

firewall such that any packet destined for UDP port 1434 on the ‘clean’ side

of the firewall be dropped and logged. No host, even DNS Servers, should be

allowed to send traffic to this port.

It is also recommend that firewall administrators ensure that any packet

received on the ‘dirty’ interface with a source IP address set to an address

on the clean side is also dropped and logged.

Fix Information

***************

NGSSoftware alerted Microsoft to this problem on the 17th of May 2002 and

they have produced a patch that resolves these issues. NGSSoftware urge all

customers of SQL Server 2000 to test then apply these fixes as soon as

possible.

http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security/

bulletin/MS02-039.asp

Where possible, NGSSoftware also recommend running the SQL Server as low

privileged local account and not SYSTEM or a domain account.

A check for these vulnerabilities has been added to Typhon II,

NGSSoftware’svulnerability assessment scanner, of which, more information is

available from the NGSSite, http://www.ngssoftware.com/

Further Information

********************

The author will be discussing these vulnerabilities in detail, with several

exploitation demonstrations, at the up and coming Blackhat Security

Briefings conference is Las Vegas. For those interested in attending this

conference, please see http://www.blackhat.com for more details.

For more information about buffer overflows please read

http://www.ngssoftware.com/papers/ntbufferoverflow.html

http://www.ngssoftware.com/papers/bufferoverflowpaper.rtf

http://www.ngssoftware.com/papers/unicodebo.pdf

http://www.ngssoftware.com/papers/non-stack-bo-windows.pdf