Web servers running a long-outdated version of the Linux kernel were attacked with dramatic speed over two days last week, Cisco Systems said on Thursday. All the affected servers were running the 2.6 version, first released in December 2003, of the Linux kernel, which is the core of the operating system. Most were running a 2.6 Linux kernel version released in 2007 or earlier, wrote Martin Lee, technical lead of Threat Intelligence for Cisco.