Microsoft has released a critical software update to patch a security hole in a common Windows component that could allow malicious hackers to run their own code on your machine.

The hole, in a Windows component called the ASN.1 library, affects a wide range of Windows features and software, from file sharing between Windows machines, to software applications that use digital certificates, said Microsoft and eEye Digital Security, which discovered the problem.

It is also the second critical update that Microsoft has released this month outside of its monthly download. The last one attempted to fix a vulnerability in Explorer that enabled one website to be disguised as another. There is also controversy surrounding the fact that Microsoft was informed of the problem six months ago and has taken until now to release the vital patch.

ASN, or Abstract Syntax Notation, is an international standard for representing different types of binary data such as numbers or strings of text The ASN.1 library allows different software applications running in Windows to identify the types of data they are passing back and forth, allowing each system to properly interpret the data it receives. It has been a standard Windows component since the release of Windows NT Version 4.0, said Marc Maiffret of eEye.

An unchecked buffer in the ASN.1 Library could allow remote attackers to cause a buffer overflow and take control of a vulnerable Windows system, Microsoft admitted.

The Microsoft security bulletin, MS04-007 patches the ASN.1 Library on affected Windows systems, ranging from Windows NT Workstation 4.0 Service pack 6a to the latest version of Windows Server 2003.

However, eEye has warned that Microsoft's implementation of ASN is "fraught with integer overflows" and other security flaws are likely to be found in the library. "Typically, if there's one set of vulnerabilities, there will be more found. Everything we know of was fixed today but usually where there's one, there's many," said Maiffret.

That was the case with Microsoft's implementation of the DCOM (Distributed Component Object Model), a protocol that allows software programs to communicate over a network. A security vulnerability in the DCOM interface that handled RPC (remote procedure call) traffic spawned the Blaster worm, as well as other attacks.

The ASN vulnerability could be similarly exploited and used to create a network worm, Maiffret said. Unlike DCOM, it would be difficult to simply disable ASN should such a worm appear, because it is used by so many different applications, he said.

Even more ominous, ASN is commonly used in critical infrastructure such as power grids and water supply control systems, where it allows hardware that makes up the infrastructure to send data, such as power consumption levels, to software control systems, Maiffret said.

Microsoft advised customers running affected versions of Windows to download and apply the patch immediately.