Security experts have warned of a malware offensive being spread by spam posing as a 4 July greeting card.

Sophos said that the emails, which are hitting inboxes worldwide, claim that the recipient has been sent an e-card greeting by a friend and tells the user to click on a link to view the card.

Subject lines used in the malicious spam campaign include:

America the Beautiful
God Bless America
Happy Fourth of July
Independence Day Celebration
July 4th Fireworks Show
Your Nations Birthday

"Cyber-criminals have no qualms about taking advantage of celebrations like 4 July to infect innocent people's computers and potentially steal their identities," said Graham Cluley, senior technology consultant at Sophos.

"This is not just an American problem. These kind of attacks strike around the world, and are designed to abuse PCs on a global scale."

Clicking on the link inthe email, which is in the form of a numeric IP address, takes surfers to a compromised zombie computer hosting the JSecard-A Trojan.

This malware then tries to download additional code from the internet which Sophos intercepts as Mal/Dorf-C.

"Rather than being sent to a real e-card website when you click on the link, you are visiting someone else's compromised computer which is hosting malicious code designed to infect your Windows PC," explained Cluley.

"It is these same computers, based all around the world, which are spewing out spam.

"A real e-card company is unlikely to send you emails which contain links that are a set of four numbers in the format xxx.xxx.xxx.xxx, so that should set alarm bells ringing instantly."