SQL Slammer Lessons

By Rainer Gerhards
Article Date: 2003-01-27

In this article, I analyze why the SQL Slammer worm was so successful in bringing down the Internet - and what can be learned from the attack. I will not analyze how the worm worked in detail. There are already a number of good analyses out; please see the links section, especially [1] and [3], if you are interested in that.

Most importantly, this paper tries to figure out why the attack was so successful.

This is not a complete or thorough analysis. I have put together the information I had at my hands from following the BugTraq mailing list, personal contacts and talks. Anyone with additional information is invited to email me any feedback at Comments are very welcome.

The Incident


On Friday afternoon, January 25th, 2003, a new worm propagated through the Internet and generated massive amounts of traffic, causing denial of service conditions at many important infrastructures. The worm spread through a several-month old vulnerability in Microsoft SQL Server 2000 and its little brother, MSDE 2000 (the MSDE is a stripped down, cost free version of Microsoft SQL Server for use by application developers). The worm spread extremely fast. Within hours, key Internet infrastructures and backbones experienced severe problems. There were reports where large areas - Korea, for example - become disconnected from the Internet for around 6 hours.

Network administrators worldwide noticed the impact of the worm rather quickly and began exchanging alerts. Fortunately, key systems became usable again relatively quickly, though it is expected that corporate environments will see service degradation for some more days. Also, as of January 27th, there is still a lot of traffic originating from this worm on the Internet (but no longer in critical amounts).

Problems Seen

The worm used a well-known security weakness in Microsoft SQL Server and spread via ca. 400-byte sized UDP packets to the MS SQL monitor port at 1434/udp. The UDP packet contained the complete worm code. Once a victim was hit, it immediately began trying to infect other servers by sending the very same UDP packet to pseudo-randomly generated IP addresses as fast as the infected machine and network allowed. This was done in a tight loop, which could only be ended by shutting down SQL Server or the host operating system.

For a detailed analysis, see [1] below.

Extremely High Network Utilization

Due to the tight loop executed by the worm, the affected server generated extremely high network utilization. To worsen things, not only were relatively large UDP packets transmitted, but also ICMP non-reachable (port and host) replies from those targets that were not hit. Postings on the BugTraq mailing list indicate that this ICMP traffic was immense.

The large amount of traffic alone caused trouble by using up available bandwidth. It alone prevented access to some sites.

Frozen Network Devices

Due to the high traffic volume, many network devices (routers, switches and firewalls) were unable to carry on normal processing. Some of them even "froze" in the middle of operation when their CPU resources were exhausted and the high-priority task of forwarding traffic prevented all other activity. I suspect that almost all devices had low buffer respective buffer overflow conditions. I had no specific reports on that yet, but it is relatively safe to assume such.

Packet Loss

When we consider the buffer overflow conditions that existed in the network devices, it's important to realize that they were forced to discard traffic. Unfortunately, not only the malicious communications, but also perfectly legitimate traffic. I expect UDP traffic was most severely hit, as UDP offers "best effort delivery" by design. As such, the spec explicitly allows an interim device to discard UDP packets should the device experience congestion and insufficient buffer space. TCP-based packets, in contrast, are guaranteed delivery and as such are only discarded when a device has absolutely no other choice. Of course, if the device is nearly frozen, it might also drop TCP connections. But I still assume the majority of discarded packets were UDP packets.

As such, UDP-based services had the least chance to operate normally. To prove this point, there were reports of disrupted voice-over-IP and streaming media, all of which are UDP-based.

Failing DNS Resolution

Please keep in mind that DNS is also UDP-based for the most part. This explains why the Internet Root DNS Servers were completely inaccessible during parts of the attack - the UDP DNS queries simply did not come through.

This DNS vulnerability illustrates an important vulnerability of the Internet as whole: if an attacker could generate even more UDP traffic than the SQL Slammer worm, and do this for an extended period of time, the name resolution and thus stability of the Internet would fail!

And the bad news is that the worm could have generated even more traffic than it has already done...

Failing ATM machines

Our society relies more and more upon the Internet. It is interesting to note that during the Slammer attack not only systems directly related to the Internet failed, but also seemingly unrelated systems, e.g. ATM machines. Can you envision shopping in a supermarket and unable to pay your food because of an Internet worm? Sound like science fiction? Well, it happened on January 25th, 2003. See [4] below (towards the end) to see that this is true.

What made the Worm so successful?

Some sources have called this worm more destructive than "CodeRed" or "Nimda." I tend to agree. There are a number of factors that made it so successful. Unlike Nimda and CodeRed, it is not simply a case of the "lazy admin," at least not in my opinion. In fact, this was a major motivation in writing this paper. We need to be aware of all the things that worked together to allow this worm to cause its damage.

Usage of UDP

A major "advantage" of the worm was its ability to spread via UDP. Unlike TCP, there are no timeouts for session setup, thus messages can be sent extremely fast. UDP will also send the complete packet up to a top-level router, at least, before the packet is discarded, thus causing many of the congestion problems (TCP will discard packets more quickly, such as when no session can be established due to either an invalid host or a host not listening to the port targeted).

Unpatched Servers in Internet Data Centers

Internet Data Centers housing customer machines were among the most severely hit. Typically, those facilities provide network management, power, and climate for customer machines. The customer, however, is responsible for administration of the machine. The housing provider typically does not even have administrative access to customer systems. Internet data centers are typically very well connected to the Internet.

For this reason, a few unpatched SQL servers inside an Internet data center can lead to massive amounts of traffic, both inside the providers' facilities as well as on the backbones the data center connects to. While the network operators are able to detect this condition, they typically cannot patch or even shut down the customers' machines, as they do not have admin access to them.

To make things even worse, there are many Internet data centers that lease machines (often called root-servers) to their customers. In this business model, the customer is again solely responsible for server administration. The worst part is that those leased machines are typically single-machine setups that do not make use of any kind of firewalling. Some data centers do not support multiple machine configurations with a firewall in front (or offer firewall functionality for lease). Those who do offer firewalling do not sell those packages very often - customers trying to save money skimp on security, and choose the cheaper technique of just a single machine.

MSDE Integration

Pivotal to the worm's success was the widespread use of MS SQL Server. One might wonder how it happens that so many servers sitting directly in the Internet are not even protected by a minimal firewall rule set.

In my opinion, the fact that MSDE was vulnerable, too, is highly important. In contrast to the "real" SQL server, which typically (hopefully) is set up and administrated by a skilled administrator, the MSDE is often used on desktops. Also, people simply do not realize they are running SQL server but get it unknowingly when a third party application installs MSDE. So even the (somewhat) caring admin is not really aware that he needs to monitor SQL Server patches.

In fact, MSDE is installed as part of Microsoft Visual Studio.NET as well as a number of other Microsoft products, including Sharepoint Services plus Project 2002 Server. Also, a growing number of third-party applications install MSDE, some of them silently.

From our developer and admin point of view, it appears that Microsoft is making deployment and integration of MSDE much harder than the full SQL engine (be warned: I might be wrong here, it is my own personal impression). In any case, it is definitely harder to patch MSDE; some of the current patches require a SP2 version that comes only on CD and cannot be downloaded. The vulnerability exploited by the worm required such a patch. I guess the unpatched state of some MSDEs can be attributed to this fact.

There have been security issues with the MSDE setup since it appeared in the industry. For example, many products install MSDE with the default admin account of "sa" without a password. Some of these problems were cleaned up with MSDE 2000, but again, there are still many glitches around that make the setup vulnerable. Another example is that it is relatively hard to change the default setup directory.

The fact that many apps bring MSDE with them is a key problem, along with the fact that the end user does not necessarily know he is running a database server on his machine. Some of those applications are also in wide use at desktops - just think about the number of Visual Studio.NET installations that have potentially installed an (unpatched) MSDE.

Unpatched Home User System

Unpatched home systems are the never-ending story of the insecurity of the Internet. I am sure (but have no definite evidence right now) that home desktops running MSDE versions have contributed to the worm traffic. It goes without a saying that many home workstations are still unprotected (insufficiently protected, to say the least). And broadband is fueling them with more and more power.

Outgoing UDP Firewalling

Traditionally, many organizations do not take equal care to set up firewall rules up for traffic flowing from inside the network to the external side. While attack traffic coming in from the Internet is closely scrutinized, admins tend to be more lax with traffic that originates from their networks. Just keep in mind how many setups allow spoofed traffic, not related to the internal network, to be transmitted to the Internet.

In my experience, firewalling UDP ports is an even worse story. For example, even otherwise (partly) caring admins tend to open up UDP port above 1024 to make their DNS responses work. Of course, this is only necessary for DNS servers, but it is often applied to all servers as a general policy. In general, traffic originating internally is more likely to pass through most existing firewall setups than traffic originating from the external side.

Some organizations experienced the worm even when firewalls prevented it from entering via the "normal" Internet gateway. When vulnerable home users, or laptops on the road (developer machines, desktop engine for replication on mobile machines) got infected, they were likely to infect the organizational network when they dialed in. One such machine could infect an internal SQL server when the worm tried to infect the rest of the Internet (and the Intranet as well) from the internal side of the firewall. Because outgoing traffic is not carefully monitored or closely restricted, the SQL server would in turn have been able to congest the Internet. If the firewall was a low-powered one, chances are good that at some point the firewall would be monopolized by the malicious traffic, effectively denying service to legitimate traffic. Bear in mind that I do not have any report of things going that way, but I bet it happened at least once.

In-Band Adminstrative Data

In the past, the discussion of in-band vs. out-of-band network management was much more active than in recent times, now that everyone does everything via the Internet and VPN.

I do not have yet any authoritative sources saying that they received alerts either too late or not at all because of network congestion. However, many protocols used for such systems are UDP-based. An example is syslog. For this reason, I would expect that network administration and alerting was at least not as efficient as it should have been during the attack. While we ourselves were fortunate enough not to receive an amount of traffic that lead us into real trouble, this also means I can not confirm the effectiveness of the network management over here.

I would appreciate any feedback on this issue.

At least, I doubt that the current in-band management approach can facilitate malware like SQL Slammer.

Lessons Learned

Remember, this is a first and quick effort to analyze the effects of the SQL Slammer worm. In fact, I expect that we will learn more lessons than I describe here and I also suspect that I will need to change some of my conclusions after they have undergone peer review. Anyhow, I hope they are helpful - if nothing else, they hopefully start discussions.

Reconsider Firewalling

One of the main lessons learned is that main firewall configuration must pay more attention to outgoing traffic. This is not really new news. But I think it is worth reiterating.

Deny all ICMP

If you don't do this already, it is a very good idea to drop all ICMP packets other than those generated by the firewall. This might have some implications on day-by-day operations, but it definitely helps under such attacks. It also makes it much harder for an attacker to detect which services are running at a given IP address.

Block all outgoing UDP

The need for outgoing UDP should very seriously be considered. Only those machines with a definite need should be allowed to send UDP traffic to the Internet. For the same reason, services requiring UDP should - if possible - placed on a dedicated machine and not mixed together with other services like SQL server. This might not be easy for some small shops. It should be highly affordable for the medium and large organizations. If you need a good argument to justify the cost to your management: count the traffic generated by the worm and calculate the expense of it. Then, calculate the cost for a dedicated UDP services machine...

Be Careful with MSDE


I don't want to bash Microsoft here. One thing that I think requires immediate action is providing downloadable patches for some of the MSDE versions. Offering them only on CD is unacceptable and is for sure responsible for at least some of the unpatched versions out there. This should never again happen.

Other Vendors

Vendors shipping MSDE as part of their product should very prominently state the fact that they install a full-blown database engine on the customer's machine. Vendors shipping MSDE as an integral part should also take responsibility for it and notify their customers when Microsoft releases an important security bulletin.

End Users

Take care of what you install. Read the spec for your applications and make sure that you know when you install a database engine.

Internet Data Centers

Internet data centers should provide a way for their customers to easily - and without additional cost - to have some router filters assigned. Even some very generic filter classes would be helpful.

Out-of-Band Administration

The community as whole should consider out-of-band administration for critical resources. Of course, this cannot be accomplished for all resources. Protocol designers and implementers should consider careful use of Quality of Service (QoS) to ensure that alerts have a better chance of surviving congested routers and networks. In doing so, they should take great care not to overdo this effort: if any single probe warning message is flagged as high priority, QoS packets will probably flood the network, too.

What is next?

Could it have been become Worse?

Definitely, yes. All in all, the worm did not generate as much traffic as it could have. I am not sure if the additional traffic would have made it past the already-failing network devices, but there is a chance it could have.

I won't try to explain exactly how the worm could be made more destructive, but consider just these few points:

  • using 1024 or 1514 byte sized packets instead of 400 byte packets
  • specifically addressing multicast and broadcast addresses
  • combining the current exploited together with other known vulnerabilities

Game Server Attacks

Exploiting known vulnerabilities in Game Servers could generate even more traffic than the SQL Slammer worm. Be sure to read the PivX Advisory in [5] - it describes pretty much what already has happened on SQL server with SQL Slammer. From the description and the increasing number of game servers, added to the fact that they are mostly hosted in well-connected data centers, I see an even greater potential for damage.


By writing the notes on QoS in the out-of-band administration section above, I had the idea that a worm using UDP together with the "proper" QoS bits could eventually cause routers to even drop TCP traffic in favor of the malicious one. I do not know enough about QoS to say this is really the case. But it is interesting to think about. Is anybody out there with a more educated comment?


Related Software

The MonitorWare product family provides near real-time monitoring and alerting. It can be used to detect unusal system activity. It also allows to gather router and firewall logs for later analysis or near-real-time alert generation. With MonitorWare, incidents like SQL Slammer can be detected based on the warning messages generated by network devices.

Revision History

2003-02-18 english language brush up (no substance changed)
2003-01-27 Initial version created.

Author's Address

Rainer Gerhards
Adiscon GmbH
rgerhards @

UDP Port List

An incomplete list of UDP ports:


domain 53/udp+tcp Domain Name Server
portmap 111/udp+tcp
snmp 161/udp SNMP
snmptrap 162/udp SNMPTRAP
syslog 514/udp 
syslog-conn 601/udp+tcp Reliable Syslog Service

kerberos 750/udp+tcp kerberos_master 751/udp+tcp nfs 2049/udp

synotics-relay 391/udp+tcp SynOptics SNMP Relay Port snmp-tcp-port 1993/udp+tcp cisco SNMP UDP port oce-snmp-trap 2697/udp+tcp Oce SNMP Trap Port websphere-snmp 3427/udp+tcp WebSphere SNMP apc-snmptrap 7845/udp+tcp APC SNMP Trap Proxy apc-snmp 7846/udp+tcp APC SNMP Proxy patrol-snmp 8161/udp+tcp Patrol SNMP


imgames 1077/udp+tcp IMGames
iberiagames 1726/udp+tcp IBERIAGAMES
gamegen1 1738/udp+tcp GameGen1
egs 1926/udp+tcp Evolution Game Server
redstorm_join 2346/udp+tcp Game Connection Port
redstorm_find 2347/udp+tcp Game Announcement and Location
redstorm_info 2348/udp+tcp Information to query for game status
gamelobby 2914/udp+tcp Game Lobby
xbox 3074/udp+tcp Xbox game port
nm-game-admin 3148/udp+tcp NetMike Game Administrator
nm-game-server 3149/udp+tcp NetMike Game Server
nm-asses-admin 3150/udp+tcp NetMike Assessor Administrator
nm-assessor 3151/udp+tcp NetMike Assessor
ironstorm 3504/udp+tcp IronStorm game server
sphidia-port 3737/udp+tcp Sphidia Game Port
parsec-game 6582/udp+tcp Parsec Gameserver
gamesmith-port 31765/udp+tcp GameSmith Port

vdmplay 1707/udp+tcp vdmplay directplay 2234/udp+tcp DirectPlay msi-selectplay 2871/udp+tcp MSI Select Play netplay-port1 3640/udp+tcp Netplay Port 1 netplay-port2 3641/udp+tcp Netplay Port 2 ps-ams 3658/udp+tcp PlayStation AMS (Secure) directplay8 6073/udp+tcp DirectPlay8 nta-ds 7544/udp+tcp FlowAnalyzer DisplayServer nta-us 7545/udp+tcp FlowAnalyzer UtilityServer directplaysrvr 47624/udp+tcp Direct Play Server


nmsp 537/udp+tcp Networked Media Streaming Protocol
h263-video 2979/udp+tcp H.263 Video Streaming 
ms-slipstream 3132/udp+tcp MS-Slipstream

rtsp 554/udp+tcp Real Time Stream Control Protocol

Messenging Services

wimd 2980/udp+tcp Instant Messaging Service
icq 4000/udp+tcp ICQ <>
ircu 6665-6669/tcp+udp IRCU


The information within this paper may change without notice. Use of this information constitutes acceptance for use in an AS IS condition. There are NO warranties with regard to this information. In no event shall the author be liable for any damages whatsoever arising out of or in connection with the use or spread of this information. Any use of this information is at the user's own risk.

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