ScanAlert is, or was, privately owned and claims that over 75,000 websites show its mark. It is based in Napa, California, and employs 130 people.

McAfee will pay $51m in cash with a possible bonus of $24m if targets are hit. ScanAlert audits websites before granting its kitemark - sites are checked daily to ensure they remain secure. It also claims to be the only kitemark to be shown within the major price comparison sites.

The service also ensures that websites are complying with regulations set by card issuers.

ScanAlert will be bundled into McAfee's SiteAdvisor - a website, or browser plug-in, which certifies that websites are dealing properly with data and not full of spamware. SiteAdvisor was bought by McAfee last year.

The deal gives McAfee the ability to sell ecommerce solutions alongside its anti-virus, anti-intrusion, and other security products. Strategically, it also aims the company more squarely at the consumer market as well as the business and enterprise market. The deal looks to fit neatly with the earlier acquisition of SiteAdvisor.

The two companies will operate separately until the deal closes.