ScanAlert runs the Hacker Safe security service which provides a certification programme for online retailers which meet certain security standards. Qualifying sites are awarded a 'Hacker Safe' logo.
The company claims that the certification boosts consumer confidence, and can result in increased sales.
Under the terms of the deal, McAfee will pay $51m up front and a further $24m based on certain performance targets.
McAfee plans to integrate Hacker Safe into its SiteAdvisor service to provide e-commerce ratings alongside its anti-phishing, anti-malware and anti-spam alerts.
"Through this acquisition, we will enhance McAfee's triple-play protection," said chief executive Dave DeWalt.
"This will create the industry's only service that helps consumers stay safe as they search, surf and shop."
The ScanAlert deal is the latest chapter in McAfee's quest to shift from an antivirus vendor into a major player in the broader security market.
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