Spammers have begun experimenting with a new file format as part of their ongoing quest to slip their tiresome messages past junk mail filters.

Following on from junk mail messages in the images of emails or in PDF attachments, users now have to contend with spam messages in the FDF (Forms Data Format). FDF files can be read by Acrobat or other PDF reader packages.

Like recent PDF spam campaigns, the FDF spam intercepted by Finnish net security firm F-Secure punts "get rich quick" stock spam.

There's never a good time to invest in response to such pump-and-dump pitches, of course, but the timing now is particularly bad, coming concert with falls in worldwide stock markets last week.

According to the latest figures from Symantec, image spam is on the wane - down to eight per cent in July from its peak of 52 per cent in January.

Meanwhile, PDF spam has continued to increase, accounting for up to eight per cent of all spam in July. Symantec said it has also blocked examples of stock and pharmaceutical spam using Microsoft Excel and Zip files. ®

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• Spammers add a new dimension to junk mail (3 September 2007)
• PDF spam tsunami hits email inboxes (9 August 2007)
• Fake e-cards signal massive DDoS attack (7 August 2007)
• Spammers dump images, switch to PDF files (23 July 2007)
• Rival malware gangs wage turf war (1 July 2007)
• Pump-and-dump scammers issue German prospectus (22 June 2007)
• FBI logs its millionth zombie address (13 June 2007)
• Anti-spam sites weather DDoS assault (11 June 2007)
• Imperfect Storm aids spammers (19 February 2007)