Security experts last week warned that hackers are preparing Christmas card emails that appear to lead to innocent images, but in fact trick users with Windows systems into downloading viruses.

To avoid difficulties, firms should check their mail filtering systems to ensure they handle emailed images in the same way as other HTML traffic, and should also educate users about this issue.

Security specialist ISS said contributors to hacker mailing lists have recently been discussing new techniques to bypass firewalls by mislabelling general HTML files as JPEGs. Steven Darrall, senior consultant at ISS X-Force Security Assessment Services, said the problem is caused by Microsoft's Internet Explorer (IE) web browser automatically opening files labelled with .jpg or .gif extensions. "If the file does not contain an image, IE will try and guess the content," Darrall said.

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