Hackers tried to trick users into downloading the Storm bot Trojan on Friday by unleashing a flood of Fourth of July spam bearing links to malicious sites.

The spam campaign, anticipated last week by MX Logic, used messages with subject headings ranging from 'Amazing firework 2008' and 'Celebrating Fourth of July' to 'Light up the sky' and 'Spectacular fireworks show', said UK-based Sophos in an alert on Friday.

Links in the spam led to hacker-controlled sites that trumpeted a video clip worth downloading. "Colorful Independence Day events have already started throughout the country," the malicious sites claimed. "The largest firework happens on the last weekday before the Fourth of July. Unprecedented sum of money was spent on this fabulous show. If you want to see the best Independence Day firework just click on the video and run it."

The file pitched to users was an executable: 'fireworks.exe'.

Users who agreed to the download didn't receive a video, but instead infected their PCs with the Storm Trojan horse, which hijacked the system and added it to the existing collection of compromised computers making up the Storm botnet.

"You're not going to be feeling in the mood for celebrations if this malware infects your PC," said Graham Cluley, a Sophos senior technology consultant.

Security researchers at F-Secure, the SANS Institute's Internet Storm Center (ISC) and Trend Micro also reported the Storm spam and infection attempts.

Storm's backers have regularly used holiday-themed spam to dupe users into downloading the Trojan and self-infecting their PCs. Last year, the bot was behind a massive surge in spam during July, and it has been linked to campaigns around Christmas and New Year's.

Earlier this year, Microsoft researchers said that their company's Malicious Software Removal Tool (MSRT) had beaten Storm into submission, a claim contested by third-party security experts.

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