Customers of U.K. mobile network operator O2 may believe that photos taken with their mobile phones and sent via MMS to friends are private. But if the recipient's phone isn't capable of receiving MMS data, as is the case with Apple's iPhone 3G, those pictures may be publicly accessible.

O2's security for this scenario is security through obscurity: It makes pictures sent via MMS viewable on non-MMS devices by posting them online with a URL that's difficult to guess.

But thanks to Google, there's no need to guess the URL. It can be found using the inurl: search query operator with mms2legacy as the argument.

The reason for this, explained Ken Simpson, CEO of anti-spam company MailChannels, is that one's Google Toolbar may be configured to pass URLs that one visits to Google for indexing. "If you run Google Toolbar, it knows pages you visit," he said.

As Google explains in its Google Toolbar privacy policy, "Certain optional Toolbar features operate by sending Google the addresses or other information about sites when you visit them. Web History, PageRank, and Safe Browsing in Enhanced Mode all work this way."

The ostensibly private O2 picture-share URLs could probably be guessed even without Google. The code that protects them is a 16-digit hexadecimal number and many people are capable of writing a script to try every code combination.

A spokesperson for O2 wasn't immediately available for comment because it was late Friday evening in the United Kingdom.

Simpson, however, believes that someone at the company is aware of the problem and has been trying to cover it up. He said that someone had posted information about the issue in the online forum for O2 customers and that the post had subsequently vanished.

Another O2 customer, however, has chimed in. "I have reported the case to several agencies in the U.K. -- including the ICO, Ofcom, O2, and my excellent local MP," said someone using the name "duguk." "Hopefully something will be done about this."

Simpson said that while he hadn't come across any overly embarrassing pictures, many of the photos that are exposed show people's children.

In a post on the MailChannels blog, senior software engineer David Cawley notes that, ironically, O2 maintains a Protect Our Children Web site. "Well, a good first step would be to avoid leaking customer photos," he said.