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Contents Bulletin Scripting in shell and Perl Network troubleshooting History Humor

Malware Defense History

by Dr. Nikolai Bezroukov.

Copyright: Dr. Nikolai Bezroukov 1994-2013. Unpublished notes. Version 0.80.October, 2013

Contents : Foreword : Ch01 : Ch02 : Ch03  : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 : Ch09 : Ch10 : Ch11 : Ch12 : Ch13

Chapter 7: Network worms

Introduction to Network Worms 


Look at misfortune the same way you look at success - Don't Panic! Do you best and forget the consequences.

Walt Alston



One can relish the varied idiocy of human action during  a panic to the full, for, while it is a time of great tragedy, nothing is being lost but money”

 John Kenneth Galbraith

Network worms exist for a long time. Actually two of the first worms known -- REXX-based Christmas greetings worm and Morris worm -- were network-based.  In 1988, a graduate student at Cornell University had discovered several security flaws in versions of Unix that were widely used on the Internet (mainly Solaris). Using his knowledge, the student created a program (known as a worm) that would find vulnerable computers, exploit one of these flaws, transfer a copy of itself to the compromised system, and then repeat the process. The program infected between 2,000 and 6,000 computers within hours of being released, which at the time was a substantial percentage of mail servers on the Internet. Those servers were disabled for a couple of days until the worm was disassmebled and analysed.

But those were two exotic cases as at this time networks were available only to privileged few.  Network worms got into mainstream only with emergence of high speed locan networks all connected to Internet. Corporate networks with many often unpatched and badly maintained (or not maintained at all like many home PCs) computers running Windows OS represented a perfect target for such attacks. 

Starting from the second half of  2001network worms became prominent. On July 16, 2001  Code Red started propagating on Microsoft Windows systems with ISS installed. It exploited IIS-enabled systems susceptible to the vulnerability described in CERT advisory CA-2001-13 Buffer Overflow In IIS Indexing Service DLL. Other systems not vulnerable to this exploit. Reports indicate that two variants of the "Code Red" affected more than 250,000 hosts.

Other network worms followied. Mass epidemics were pretty rate, just slightly less then a dozen for the the decade (2001-2010) and for the last six-seven years there were only a few of them, approximately one a year.  At the same time network worms were more difficult to disinfect as infections were distributed among multiple sites.

They also exposed gross blunders in design of the internal corporate networks and/or configuration of desktops (especially in case of "standard desktops" in which uniformity provide an additional attraction for network worms). 

The most affected classes of PCs are usually semi-abandoned corporate PCs such as laptop for remote users with bad connectivity, various test and regression machines that after initial one time use happily circulate air for a year or more without and single human logging to them, etc.

Contributing factor is low qualification of personnel especially if this type of activities is converted into specialized security position (security analyst ;-). Unlike regular network or desktop support personal people in such position often quickly disqualify even if at the beginning that have some level of qualification. Also such positions often attract power-hungry "good-for-nothing" type of people as anything connected with security provide an opportunity to exercise power not only over the users but also over fellow administrators.

Timeline of major network worm epidemics

Here is the (somewhat simplified) timeline of major accidents: 

SQL Slammer also brought more than 13,000 Bank of America ATMs to a halt by compromising database servers and overloading attached networks. In August, the Nachi worm that exploited RPC DCOM vulnerability and was designed to fight SQL Slammer infected Diebold ATMs at two financial institutions. A patch to close to the RPC DCOM vulnerability exploited by Nachi had been available for more than a month when that incident occurred.

Panic increased damage


Leadership has been defined as the ability to hide your panic from others

Network worms are probably the most complex type of worms to fight because they strike in short period if time and due to that often cause considerable panic in corporate environments. And the rule is that "panic kills":  in panic some absurd actions like shutting down the whole sites are completely justified. Unfortunately this if often done after the initial splash of activity of the worm after which it is just sitting more or less quietly on infected computers.

Many large corporations suffered multimillion losses due to shutdown of parts of their networks done after any real threat disappeared from the horizon and worms were just quietly sitting on infected computers: in many cases shutdown accrued hours after the peak of traffic was over.  Panic actions done by unqualified personnel amplified real damage quite nicely ;-)

Thus the major problem of mass epidemics caused by network worms is that the initial infection is often has a form of chain reaction and it occurs over very short period of time like a huge traffic splash that generates panic many times more destructive then the worm itself, especially if the fighting of the work is delegated to a completely technically incompetent bureaucrat.  The generated amount of traffic can overwhelm the network before any actions can be taken. Also if update servers are centralized (or super-centralized) and this for a while AV update cannot reach the targets even if they available.  Moreover the fact of updating of the signature in this case can became another attack on the network with AV signature distribution server as the worm Trojan horse.

Automatic tools like automatic disinfection are usually not very effective against such threat as new successful network worm is usually successful exactly because it invents a completely new attack vector.  Detection based on traffic anomalies can detect the initial attack but due to the chain reaction character of infections this detection is pretty much useless. Still it is important to have. 

The value of Microsoft patching mechanism

One of the effective and rather simple way of fighting network worms is to use automatic patching mechanism supported now by Microsoft for all major flavors of Windows. Microsoft proved to be reasonably  good in this area and historically mass infection often accrued after the patch was available. So by enabling automatic installation of patches on a large fraction of corporate desktops (for servers this is a less attractive measure and would generally be weighted against the risks) cuts the critical mass of infected computers. This measure also  helps to ensure that the initial spike will be less damaging. 

The advantage of automatic patches application is that it just work on many cases. If a problem found this particular desktop and only it to be moved into special "selected" or "security only" patches pool.  Some patches can interfere with the installed software.  But when the latter occurs, usually it is because problems with the software not patches, for example obsolete version of a popular application. Often such cases can be resolved by upgrading software.

While not all PCs in a corporate environment can use this mode, probably 80% of users can be switched to this mode. Remaining 20% still represent a problem as they need to be manually or selectively patched, but even in worst case they represent less critical mass and increase the chances that the network will survive the initial "explosive" propagation period typical for most network worms.

Again I would like to stress that usually it is not network work itself that is dangerous but unqualified and often stupid actions of sysadmins and executives that are caused by panic. One of such decitions is to use low quality third party patching software that is not only cost quite a bit of money but also is quite inferiour to Microsoft solution.

Anyway, it is important to understand that too much zeal in disinfection of network worms is usually more harmful the worms themselves.  But this is a pretty rare event as Microsoft tests its parches very well.

Some questions and suggestions

I have several suggestions that I formulated as a set of questions with brief comments. I think that enterprise customers can benefit from discussing at least some of the underling ideas and countermeasures.

  1. Why so many companies are far behind in versions of antivirus deployment ?

    Antivirus software belongs to perishable goods category. That means that we should try to prevent sliding it more then one version behind the current. Or preferably to use the current version.  Any AV software which is two version behind the current for all practical purposes cannot be considered a viable antivirus: this is a dinosaur by AV industry standards and the fact that it disinfects something is truly amazing. New viruses often require changes in virus engine and unless the engine is pluggable like in Trend Micro, the update of the software updates might be the only option to keep antivirus current and effective against new worms.

  2. Is not "selective" desktop patch policy practiced by many large enterprises questionable from the point of view of protection from viruses and worms ?
    In view of recent experience with worms continuing doing patching "an old way" looks like invitation to troubles. IMHO 60-80% of workstations (depending of the type of large enterprise) can live with automatic updates.

    The other 20-40% can be patched individually. I think that this is huge waist of resources to consider each and every workstation so special that it deserves individual patching. Many security conscious employees in large corporation voluntarily switched to automatic updates. It might be a time to institutalize this practice.

  3. Selective patching often leads to random patching of PCs when each PC has slightly different set of patches. The latter creates a permanent security vulnerability that the recent worms like Allaple as well as all previous network worms managed to exploit.

  4. Can enterprise benefit from more careful settings of IE or other "standard" browser, especially cleaning temporary files cache (this is a special setting usually not enabled by default in most browsers including IE6 and IE7).

    In enterprise environment cashing is usually duplicated by devices like CacheFlow, Squid or other proxy servers anyway. Dome network worms like Alaple exploit large number of HTML files stored in such cashes to ensure re-infected even if registry entries were cleaned.

    Among other setting that help to fight worms are making extension visible (this is a very questionable invention by Microsoft to hide extensions and several worms -- mainly mail worms exploited it to full extent putting a huge cake into the face of Microsoft software architects (or in this case pseudo-architects).

  5. Can enterprise benefit form additional independent monitoring of AV signature updates ?

    It can be done via SNMP, SMTP via a simple script or whatever.

    It looks like in large enterprises there is a stable swamp of "PCs with broken AV updates" that is a natural worm habitat for worms epidemics even if the vulnerability expolited is a year or more old.

    This swamp usually includes many remote PCs, some lab PCs and some second desktops. IMHO unless large enterprize make conscious efforts to drain at least a part of this swamp they will be always ready for a ride.

  6. Is not typical password policy too weak from the point of view of preventing worms propagation ?

    IMHO the idea of 7 letters minimum in password length that is used by most large enterprises truly belongs to the last century. I am convinced that with current scope of networks all  large enterprises should stick to AOL scheme on Windows and do it fast unless they really want to pay the price.

    That means increasing the minimal length of password to 10 or 12 and making mandatory for user using two word concatenation with the second word being the last 4 difits of his phone extension or cubicle number (I think now everybody have phones, but just in case). Friendly-8392 or, better, FriendlY-"8392". This is not that much more difficult for a user to remember but it is impossible for any worm to crack. 

    There are just too many people who will never learn how to create good password so the increase in the mandatory length to 12 characters might be the best "idiot-proof" way to solve the problem in large enterprize environment, the problem that the latest worms like  probably Allaple.B  so successfully exploited.

    It also creates another huge security vulnerability that the recent worms like Allaple.B  and future worms will manage to exploit.

Recommended Articles

How to use the RestrictAnonymous registry value in Windows 2000

This article describes how administrators can use the RestrictAnonymous registry value on a Windows 2000-based computer to restrict access over anonymous connections.

Local Security Policy MMC snap-in

  1. Click Start, point to Programs, point to Administrative Tools, and then click Local Security Policy.

    Note If you cannot perform this step because "Administrative Tools" does not show up in the Program list, then click Start, point to Settings, point to Control Panel, click Administrative Tools, and then click Local Security Policy. Then proceed to step two.

  2. Under Security Settings, double-click Local Policies, and then click Security Options.
  3. Double-click Additional restrictions for anonymous connections, and then click No access without explicit anonymous permissions under Local policy setting.
  4. Restart the member computer or domain controller for the change to take effect.

RestrictAnonymous registry value

Use Registry Editor to view the following registry key, and then add the following value to this key, or modify it if the value already exists:


Value: RestrictAnonymous
Value Type: REG_DWORD
Value Data: 0x2 (Hex)

Restart the computer after any change to the RestrictAnonymous key in the registry.

When the RestrictAnonymous registry value is set to 2, the access token built for non-authenticated users does not include the Everyone group, and because of this, the access token no longer has access to those resources which grant permissions to the Everyone group. This could cause undesired behavior because many Windows 2000 services, as well as third-party programs, rely on anonymous access capabilities to perform legitimate tasks.

For example, when an administrator in a trusting domain wants to grant local access to a user in a trusted domain, there may be a need to enumerate the users in the trusted domain. Because the trusted domain cannot authenticate the administrator in the trusting domain, an anonymous enumeration may be used. The benefits of restricting the capabilities of anonymous users from a security perspective should be weighed against the corresponding requirements of services and programs that rely on anonymous access for complete functionality.

The following tasks are restricted when the RestrictAnonymous registry value is set to 2 on a Windows 2000-based domain controller:

Down-level member workstations or servers are not able to set up a netlogon secure channel.
Down-level domain controllers in trusting domains are not be able to set up a netlogon secure channel.
Microsoft Windows NT users are not able to change their passwords after they expire. Also, Macintosh users are not able to change their passwords at all.
The Browser service is not able to retrieve domain lists or server lists from backup browsers, master browsers or domain master browsers that are running on computers with the RestrictAnonymous registry value set to 2. Because of this, any program that relies on the Browser service does not function properly.

Because of these results, it is not recommended that you set the RestrictAnonymous registry value to 2 in mixed-mode environments that include down-level clients.

Setting the RestrictAnonymous registry value to 2 should only be considered in Windows 2000 environments only, and after sufficient quality assurance tests have verified that appropriate service levels and program functionality is maintained.

Note Pre-defined "High Secure" security templates set the RestrictAnonymous registry value to 2, and because of this, caution should be used when using these templates. For more information about the RestrictAnonymous registry value, click the following article number to view the article in the Microsoft Knowledge Base:

178640 ( Could not find domain controller when establishing a trust

RestrictAnonymous is set by changing the registry key to 0 or 1 for Windows NT 4.0 or to 0, 1, or 2 for Windows 2000. These numbers correspond to the following settings:



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