Online auction site eBay has been targeted by identity thieves, who are wielding a botnet that uses brute force to uncover valid account log-in information, a Tel Aviv-based security company said Monday.

The attacks against eBay may have started as long ago as early August, said Ofer Elzam. He said that he and other researchers at Aladdin Knowledge Systems Ltd. have not been successful in notifying eBay of their weekend findings.

According to Elzam, the product manager of Aladdin's eSafe threat-protection line, the brute-force attacks are launched by a large botnet that the identity thieves have built using a sophisticated, multistage campaign that begins with compromised legitimate Web sites.

"My best estimate is that there are at least 300 compromised sites," said Elzam, who noted that they are spread worldwide and in several languages. Two sites are based in Israel, he said, including a price-comparison Web site and another operated by one of the country's largest unions. Other sites identified in a search run with information provided by Elzam included scores of real estate Web sites in Florida and Massachusetts, and a Microsoft security message forum in Italian.

Seeding genuine Web sites with malware is nothing new, but the practice has been gathering steam this year. In June, for example, hackers launched a massive bot-building attack from more than 10,000 hijacked Web sites, most of them hosted in Italy.

"These sites are compromised by SQL injection vulnerabilities, and then IFrame attack code is inserted," said Elzam, describing a common method of hacking legitimate Web sites and infecting their visitors. "The IFrame code redirects visitors to other sites which host a Trojan," he added. The Trojan horse hijacks the PC and turns it into a zombie, or bot.

"This is a very sophisticated, very complex attack," Elzam claimed, ticking off obfuscation techniques, multipart malware downloads and encryption among the tactics used by the thieves.

The resulting botnet is being used to call an eBay application programming interface (API) with pairs of possible usernames and passwords, said Elzam. The API allows the Trojan horse-infected PC -- the bot -- to communicate directly with the eBay database using XML-formatted code. If the database contains the username-password pair, it responds, which the Trojan horse notes, then later transmits to a hacker controlled server.

With enough username-password combinations -- the brute-force part of the attack -- the criminals can uncovering a limited number of real credentials.

"Each bot may be using as few as six pairs of usernames and passwords" in an attempt to come in under the security radar of eBay, said Elzam. "I don't think that eBay is even aware of the attack. The distributed nature of the attack may make it look like a merchant sending confirmations to buyers," he said.

Although Aladdin pieced together the evidence only Tuesday, Elzam said that clues indicate it might have started in early August.

It's unknown what the identity thieves have done with stolen eBay log-ons. One eBay user, however, may have offered up a possibility Tuesday in a blog post.

"I woke up this morning to a nightmare," wrote a Texas-based book collector identified on his blog only as Sam Houston. "Someone in England hacked into my personal eBay data and changed it to reflect a completely fraudulent identity with an English mailing address. That person than proceeded to send out at least 25 e-mails to individuals in the U.K. who are trying to sell Sony laptop computers on the site. He offered them more than they are asking for the laptops and wanted them mailed to him as soon as possible."

According to the blogger, the attacker has also compromised his PayPal account and tried to pay for the 25 notebooks using funds from the checking account linked to PayPal.

EBay did not reply to a request for comment Monday night.