A hacker successfully attacked a Web page within Microsoft's U.K. domain on Wednesday, resulting in the display of a photograph of a child waving the flag of Saudi Arabia.

It was "unfortunate" that the site was vulnerable, said Roger Halbheer, chief security advisor for Microsoft in Europe, the Middle East and Africa, on Friday.

The problem has since been fixed. However, the hack highlights how large software companies with technical expertise can still prove vulnerable to hackers.

The hacker, who posted his name as "rEmOtEr," exploited a programming mistake in the site by using a technique known as SQL (Structured Query Language) injection to get unauthorized access to a database, Halbheer said. The site took SQL queries of a particular form, embedded in URLs (uniform resource locators), and passed them to a database. By embedding a query with an unexpected form in the requested URL, the hacker prompted the server to return error messages, Halbheer said.

From those error messages, a hacker can get an idea of how the database is structured and refine a SQL query that the database will process as an instruction to insert, rather than retrieve, data. Eventually, the hacker found the right combination and inserted a link to an external Web site into the database.

That meant when the normal Web page was called into a browser, the database would download data from an external link. In this case, it was two photos and a graphic, a screen shot of which is available on Zone-H.org , which tracks hacked Web sites.

There are two ways to avoid this style of attack. First, the database should not be allowed to return error messages, Halbheer said. Secondly, the Web application should have validated the URL the hacker entered and rejected ones that should not be processed, he said.

If a programmer makes a mistake, "the bad guy can leverage it," Halbheer said.

SQL injection attacks are on the rise, overall, since valuable data is held within databases, said Paul Davie, founder and chief operating officer of Secerno, a security vendor that develops technology to protect databases from SQL attacks.

"I don't think Microsoft are unique in this respect and shouldn't be held up as particularly slipshod," Davie said. "This could have happened to practically anybody."