Microsoft Plugs 'Critical' Windows Hole

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

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

Microsoft security bulletin MS04-007, posted Tuesday, 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.

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

Buffer Overflow Danger

ASN, or Abstract Syntax Notation, is an international standard for representing different types of binary data such as numbers or strings of text, according to security experts at EEye.

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, says Marc Maiffret, EEye chief hacking officer.

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 says.

In buffer overflow attacks, hackers use flaws in a software program's underlying code to overwrite areas of the computer's memory, replacing legitimate computer instructions with bad data or other instructions.

The patch fixes vulnerabilities EEye discovered in the ASN.1 Library and informed Microsoft about in July 2003. However, the company warned that Microsoft's implementation of ASN is "fraught with integer overflows." Speaking Tuesday, Maiffret said it is likely that other security flaws will 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," he says.

Related Problems

That was the case with Microsoft's implementation of the Distributed Component Object Model (DCOM), a protocol that allows software programs to communicate over a network. A security vulnerability in the DCOM interface that handled 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 says. 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 adds.

Even more ominous, ASN is commonly used in critical infrastructure such as power grids and water supply control systems. It enables the systems that make up the infrastructure to send data such as power consumption levels to software control systems, Maiffret says.