Anti-virus company Symantec warned its customers about a number of critical holes in Windows that surfaced late Thursday and that could make their systems vulnerable to compromise by remote attackers.

Symantec acted after security researchers published the details of the heap overflow vulnerabilities in messages posted to online security news groups Thursday. The flaws affect most supported versions of Windows, but Microsoft has not yet issued a patch for the newly disclosed holes. Windows users are vulnerable to Internet based attacks until patches are issued, Symantec said.

In overflow vulnerabilities, storage areas in a computer's memory are exceeded, allowing random data or malicious code to be placed on the computer.

In one instance, researchers at Venustech Security Labs described a vulnerability in a component of Windows, winhlp32.exe, that processes Help files. Attackers could launch attacks using a Help file created to trigger the overflow vulnerability, though victims would have to be tricked into downloading and opening the malicious file on their computers for it to be compromised, Symantec said.

Symantec has also warned about a second vulnerability in a Windows component called "LoadImage" that is used to load desktop icons, cursors, or bitmap images. A flaw in the way LoadImage processes image files could allow malicious hackers to use specially crafted images to trigger an overflow and place their own code on vulnerable machines. Images that trigger the flaw could be sent in e-mail messages or downloaded from Web pages controlled by the hackers, Symantec said.

As with the Help file vulnerability, most supported versions of Windows are affected by the LoadImage flaw, including versions of Windows NT, Windows XP, Windows 2000 and Windows Server 2003, Symantec said.

While no active exploits targeting the vulnerabilities have been discovered, proof of concept code showing how both vulnerabilities work have been published on the Internet.

Symantec recommended that Windows users exercise caution when receiving and opening files from unknown sources. Organisations can defend their networks from attacks by limiting user privileges and deploying intrusion detection software to spot attacks, Symantec said.