About one percent of the Internet web pages are being changed in transit, sometimes in a harmful way, according to researchers at the University of Washington.

In a paper, set to be delivered Wednesday, the researchers document some troubling practices. In July and August they tested data sent to about 50,000 computers and discovered that a small number of Internet service providers (ISPs) were injecting ads into web pages on their networks.

They also found that some web browsing and ad-blocking software was actually making web surfing more dangerous by introducing security vulnerabilities into pages.

"The Web is a lot more wild than we originally expected," said Charles Reis, a PhD student at the University of Washington who co-authored the paper.

The paper, which was co-written by a researcher at the International Computer Science Institute, will be delivered at the Usenix Symposium on Networked Systems Design and Implementation in San Francisco.

To get their data, the team wrote software that would test whether or not someone visiting a test page on the University of Washington's website was viewing HTML that had been altered in transit.

In 16 instances ads were injected into the web page by the visitor's ISP. "We're confirming some rumours that had been in the news last summer, that ISPs had been injecting these ads."

The service providers named by the researchers are generally small ISPs such as RedMoon, Mesa Networks and MetroFi, but the paper also named one of the largest ISPs in the US, XO Communications, as an ad injector.

An XO spokesman said that the company did not engage in this practice and that any ad-injection linked to its network was probably being done by a "downstream" service provider that was purchasing network capacity from XO.

In June 2007 the TechCrunch blog reported RedMoon, a small Texas wireless provider, was using a system built by a Redwood City, California company called NebuAd to insert advertising into the HTML code of web pages.

Critics blasted the ISP for meddling with its customers' traffic and worried that this kind of ad injection undermined the integrity of websites, which had no control over the ads being displayed.

NebuAd has now discontinued its ad-injection product line and now delivers only the standard type of advertising that it buys from web publishers, a company spokesman said Tuesday.

The data also shows that pages were sometimes changed by popup blockers within products such as CheckPoint's ZoneAlarm or CA's Personal Firewall, but also that some products actually inserted security vulnerabilities into the pages they processed.

Even Microsoft's Internet Explorer browser was part of the problem, the researchers claim. IE injected HTML into pages that it saved to the computer's hard drive, making those pages vulnerable to attacks when the page was then reloaded from disk.

The paper's authors characterised their work as a first step and said that more study would be required to get a clearer picture of what exactly is going on within the many networks that make up the Internet.

"One of the next steps for the community is to create better and stronger mechanisms for understanding what is happening," said Tadayoshi Kohno, an assistant professor with the University of Washington. "The Web is still very young and we just don't know what's going to happen next."