As spam filters get better at weeding out unwanted messages, spammers have started inserting their messages inside Microsoft Word documents and attaching them to e-mails.

On Thursday, content security specialists Marshal warned users to beware of Word documents attached to e-mail messages. Unlike executable files, Word documents are commonly exchanged via e-mail and are usually ignored by spam filters.

Bradley Anstis, director of product management for Marshal, told ZDNet Australia that spammers have traditionally avoided sending unwanted messages in a Word -- or similar document -- file because the message sizes are relatively large.

"Most word processors today [can open Word Documents] and historically it hasn't been like that. That is one reason. Secondly, a Word document adds a size and complication to the e-mail going out, which is probably the bigger reason why they have stayed away from this area in the past," said Anstis.

According to a Marshal advisory, the e-mails use a combination of obfuscation and social engineering to create a message that "looks like a typical business e-mail".

"Users open the document expecting to find an invoice or purchase order and instead find a spam message," the advisory said.

Since August 17, Marshal has identified more than 100 examples of this new strain of e-mail spam.