A new bug in Windows Vista's built-in email program can be used by hackers to run malicious code on victims’ PCs, according to a researcher who touted an exploit-for-sale service two weeks ago.

Microsoft acknowledged the report, and said it is investigating the vulnerability.

Symantec's DeepSight network, which issued a warning about the vulnerability in Windows Mail, upped the threat rating from 6.8 to 7.5 in a follow-up alert after it confirmed that the bug was remote code exploitable. That means an attacker could introduce his or her own malware onto a compromised computer. Windows Mail is the successor to Outlook Express, the entry-level email app that's been bundled with the operating system since the Windows 95 edition.

By crafting an email message with a link to a malicious file - one hosted on a remote internet server, for example - and duping the recipient to click on the link, an attacker could infect a Vista PC with software that steals identities or with a backdoor Trojan horse.

In some cases, all that's required is that the user clicks on the link, said Symantec. "An attacker can deliver an email message containing a malicious link that references a local executable," the DeepSight alert read. "If the victim clicks on this link, the native program is executed with no further action required. For instance: an attacker could achieve the execution of the local file 'winrm.cmd’."

If run, ‘winrm.cmd’ - the Windows Remote Management command-line tool - would give an attacker complete access to a PC.

If the link points to a malicious file not on the PC, the user has one more chance to figure out the scam, added Symantec. "If the user follows [this] link, they are presented with a dialog box where they must click 'Yes' to open the file. Once the user clicks 'Yes', the file opens or executes with the privileges of that user."

Microsoft's Security Response Center (MSRC) team downplayed the potential risk. "Microsoft is not aware of any attacks attempting to use the reported vulnerability or of customer impact at this time," the MSRC said through a company spokeswoman.

It's possible that could change. The researcher who disclosed the bug and posted exploit code on the Full Disclosure security mailing list today also announced an exploit-for-sale service nearly almost two weeks ago on the same list.

On 11 March, the researcher, who goes by the name ‘Kingcope’, posted a message on Full Disclosure touting the new service. "We now have our Exploit selling site up and running ... [where] you can purchase quality advisories and exploits. Feel free to contact our sales person for getting the latest Zero-Days," Kingcope wrote.

The website referenced in Kingcope's message was unavailable today, and sported a "the system is down for maintenance" message. Kingcope did not respond to emails requesting details of the bug-for-sale service.

Both Symantec and Microsoft urged users not to click links in unsolicited email, while the former also recommended that users disable HTML within Windows Mail.

As is its practice, Microsoft said it may issue an additional advisory, or patch the problem in a future - but unspecified - security update. The next scheduled patch release date for Microsoft products, including Vista, is 10 April.

Coincidentally, it was only last week that an executive in Microsoft's security technology unit boasted about Vista and gave the new operating system an A-plus for security in its first 90 days of release.

See PC Advisor’s review of Windows Mail.