For the fourth consecutive day, technical problems hindered access to Microsoft's vast network of Web sites and services.

Microsoft acknowledged late Friday that another round of attacks had briefly blocked access to the software giant's Web sites. The outage followed a similar attack Thursday and a technical glitch that made its sites inaccessible for a nearly 24-hour period on Tuesday and Wednesday. Friday's attack "was similar to Thursday's attack, in which someone attempted to block legitimate access to our Web properties by flooding our network routers with large volumes of bogus requests," the company said in a statement. "Unfortunately, as we have learned over the last few days, we did not apply sufficient self-defense techniques to our use of some third-party products at the front-end of parts of our core network infrastructure," the statement continued, without naming the products. Friday's problems came less than 24 hours after the company said it stopped a denial-of-service attack on its systems that slowed traffic to a crawl for more than two hours on Thursday. That attack followed an outage that began Tuesday night and lasted nearly a day, which Microsoft blamed on a mistake by its own technicians. The outages came as Microsoft is trying to bolster its reputation among corporate customers.