Tens of thousands of Web sites have been compromised by an automated SQL injection attack, and although some have been cleaned, others continue to serve visitors a malicious script that tries to hijack their PCs using multiple exploits, security experts said this weekend.

Roger Thompson, the chief research officer at Grisoft, pointed out that the hacked sites could be found via a simple Google search for the domain that hosted the malicious JavaScript. On Saturday, said Thompson, the number of sites that had fallen victim to the attack numbered more than 70,000. "This was a pretty good mass hack," said Thompson, in a post to his blog. "It wasn't just that they got into a server farm, as the victims were quite diverse, with presumably the only common point being whatever vulnerability they all shared."

Symantec cited reports by other researchers -- including one identified only as "websmithrob" -- that fingered a SQL vulnerability as the common thread. "The sites [were] hacked by hacking robot by means of a SQL injection attack, which executes an iterative SQL loop [that] finds every normal table in the database by looking in the sysobjects table and then appends every text column with the harmful script," said websmithrob in a blog post. "It's possible that only Microsoft SQL Server databases were hacked with this particular version of the robot since the script relies on the sysobjects table that this database contains."

According to websmithrob, the attack appends a JavaScript tag to every piece of text in the SQL database; the tag instructs any browser that reaches the site to execute the script hosted on the malicious server.

Hacked sites included both .edu and .gov domains, the SANS Institute's Internet Storm Center (ISC) reported in a warning posted last Friday. The ISC also reported that several pages of security vendor CA's Web site had been infected.

Grisoft's Thompson said that his research had identified a 15-month-old vulnerability as one of those exploited by the attack code. The exploit, he said, targeted the MDAC (Microsoft Data Access Components) bug patched in April 2006 with the MS06-014 security update. "They went to the trouble of preparing a good Web site exploit, and a good mass hack but then used a moldy old client exploit. It's almost a dichotomy," said Thompson.

Other researchers, including websmithrob and Symantec, said that the JavaScript also launched an exploit targeting a much more recent vulnerability: a RealPlayer bug that first surfaced last October. The flaw was fixed several days later by RealNetworks.

Another surprise, Thompson said, was the speed of the hack's cleanup. Although a Google search still showed thousands of sites infected with the script on Saturday, Thompson claimed that Grisoft's LinkScanner Pro tool indicated that nearly all had actually been scrubbed. "I found that really surprising [that they were cleaned so quickly]," he said in an interview via instant messaging on Sunday. "They're all so disparate. If it was a big server farm, I could understand it being cleaned so quickly, but there doesn't seem to be anything common about them all."

The ISC updated its alert Sunday, saying another round of SQL injection attacks had infected sites with a script referring to a different malicious server. When asked to examine the second domain, Thompson confirmed that it was serving up the same malicious JavaScript as the first. However, many of those sites -- which as of this morning numbered more than 93,000, according to a quick Google search -- had not been cleaned.

"It looks like a bunch of these are still carrying the references to [the malicious domain] but not infectively," said Thompson. "In other words, they're still hacked, but the injection hasn't worked properly."

Microsoft was not immediately available for comment on the SQL Server vulnerability used by the mass hack.