Microsoft Windows may be dominating the headlines for security-related breaches but the open-source Linux server operating system remains the biggest target of overt intruder attacks, according to a study by U.K.-based Mi2g.

Mi2g, which provides digital risk management research, said 67 percent of all successful overt digital attacks was done against the Linux OS, which has exploded in popularity but in the U.S. and abroad.

Attacks against Linux were three times higher than against Microsoft Windows, which accounted for 23.2 percent, Mi2g reported.

In August alone, the company found that 12,892 Linux online servers running eBusiness and information sites were successfully breached. During the same period, 4,626 Windows servers were victims of successful intrusion.

The BSD (define) operating system fared much better, with just 360 successful breaches reported in August.

"Over the last twelve months, Linux remained the most attacked operating system online with 51 percent of all successful overt digital attacks," Mi2g said, noting that within the government environment, the tables were reversed with Windows being the biggest target, counting 51.4 percent of all successful attacks. Linux was in second place with 14.3 percent.

The research firm said the economic damage from attacks in August -- measured in terms of lost productivity and recovery costs -- was estimated at $707 million.

"The overall economic damage in August from overt and covert attacks as well as malware - viruses and worms - damage stood at an all time high of $28.2 billion. The Sobig and MSBlast malware that afflict Microsoft platforms contributed significantly to the record estimate," the company said.