The virus-like attack, which began about 12:30 a.m. ET, sought out vulnerable computers on the Internet to infect using a known flaw in popular database software from Microsoft Corp., called “SQL Server 2000.” But the attacking software code was scanning for victim computers so randomly and so aggressively — sending out thousands of probes each second — that it overwhelmed many Internet data pipelines.

“This is like Code Red all over again,” said Marc Maiffret, an executive with eEye Digital Security, whose engineers were among the earliest to study samples of the attack software. “The sheer number of attacks is eating up so much bandwidth that normal operations can’t take place.”