Cisco on Wednesday confirmed that routers and other devices running the newest versions of its IOS (Internetwork Operating System) are vulnerable to serious attack.

The San Jose, Calif.-based network hardware maker published a security advisory and recommended that users either upgrade to alternate editions or install fixed versions of IOS.

For its part, security giant Symantec immediately raised its overall Internet threat to "Level 2" from "1" earlier in the day. The last time Symantec had its threat set to "2" was during the Zotob attacks of August.

"Given the recent attention to exploitation of vulnerabilities in Cisco's IOS it is possible that this issue will see attempts at exploit development in the near term," wrote Symantec researchers in an alert to customers of its DeepSight Threat Management System.

The flaw is in the Firewall Authentication Proxy for FTP and/or Telnet Sessions in later versions of IOS -- 12.2 through 12.4 -- and might result in either a denial-of-service (DoS) attack which would likely bring down the device or possibly a more dangerous scenario, where the attacker gains complete control of the device. Or both.

"Successful exploitation of the vulnerability on Cisco IOS may result in a reload of the device or execution of arbitrary code," wrote Cisco in the advisory. "Repeated exploitation could result in a sustained DoS attack or execution of arbitrary code on Cisco IOS devices."

According to Cisco, an attacker could take advantage of this vulnerability by completing a TCP connection to a IOS-running device, and launching the exploit when the device is authenticating the user.

If upgrades or patched versions of IOS can't be deployed, Cisco recommended that customers disable the firewall authentication feature for Telnet and FTP sessions and instead deploy firewall authentication for HTTP and HTTPS sessions. Other workarounds and mitigating strategies were included in the Cisco advisory.

Cisco's operating system has taken a beating this summer, primarily after a research spilled details of a new exploit technique that could knock out much of the Internet's Cisco-based infrastructure. Cisco used a heavy hand, including legal action, to stifle the researcher.

Just days later, Cisco's own Web site was hacked, and all user and customer passwords had to be reset.