Security experts are warning that ready-made code which exploits a recently announced Cisco IOS software vulnerability is circulating and attacks using the exploit are taking place.Cisco\u00a0did not respond to requests for comment, but updated its IOS bulletin to say that it is aware that an exploit had been published on a public mailing list.On Wednesday, Cisco\u00a0warned\u00a0of a widespread and serious flaw in IOS that could make devices using the operating system vulnerable to a denial-of-service (DoS) attack.The flaw affects a wide range of Cisco devices that run IOS and accept data packets using IPv4, including Cisco's popular Catalyst family of switches, 7300 series routers and Aironet family of wireless access points.The exploit was posted to prominent security discussion lists by an unknown individual using the name "Marion Barry" and an e-mail address at the free Hotmail e-mail service.Security provider Internet Security Systems Inc. said that it had tested the code contained in that message and that it works, according to Dan Ingevaldson, engineering director for ISS X-Force.The code contains a small program written in the C programming language that makes it easy to quickly develop an exploit using the IOS vulnerability, Ingevaldson said."It's probably 200 lines. All it does is give you instructions on how to create an exploit. You just point at a target and it will fire an attack," he said.A malicious hacker with a "moderate degree of sophistication" could run the exploit, according to Shawn Hernan, a member of the technical staff at the CERT Coordination Center.ISS received reports Friday of the code being copied widely on the Internet and used in attacks on Internet service providers and major Internet backbone providers.ISS does not know where the exploit came from, but was surprised by the speed with which it appeared, according to Ingevaldson."All I know is that there was an update on the Cisco bulletin and a few hours later there was an exploit," he said, referring to Cisco's modification of its earlier security bulletin regarding the IOS vulnerability.That update bulletin gave more specific information than was first released on what protocols could be used with IPv4 data packets to trigger the IOS vulnerability and create a denial-of-service attack on vulnerable Cisco devices."Maybe it was not as hard as people thought," Ingevaldson said.Providing more information on the vulnerability is a "double-edged sword," Hernan said.The additional information provided by Cisco allowed organizations to create more focused access control lists to thwart attacks, and provided needed information to companies creating intrusion detection signatures to block the new attack traffic, he said.However, it also may have given important clues to those interested in creating code to exploit the vulnerability.CERT was aware of heightened interest in the IOS vulnerability within online communities of individuals interested in computer intrusion, Hernan said. But it is unclear whether the updated bulletin provided the clues needed to create a successful exploit, he said."It might have been a matter of time. No one can really know," he said.The quick appearance of an exploit follows similar releases for vulnerabilities affecting the Apache Web server, and Microsoft's WebDAV HTTP protocol extensions."It kind of unseats the common adage that there's a 30-day window after an exploit is developed," Ingevaldson said.Customers are advised to patch affected Cisco systems at the earliest convenience and also apply workarounds such as access control lists (ACL) to block attack traffic, in keeping with\u00a0guidelines released by Cisco.Ingevaldson noted that even patched routers will pass attack traffic on to other devices, allowing attack traffic to circulate until a vulnerable Cisco device is found, making the use of ACLs more important, he said.But Hernan disputed that, saying that while patched routers will pass traffic along, malicious traffic must target a specific Cisco device.In addition to Cisco, both CERT and the SANS Institute issued warnings about the exploit.The U.S. Department of Homeland Security was also informed of the circulating exploit, according to SANS. A DHS spokeswoman had no comment.Organizations using Cisco products with vulnerable versions of the IOS operating system were strongly encouraged to review the security bulletin and upgrade to a patched version of IOS. In addition, customers should review the updated workaround information provided by Cisco in the bulletin, SANS said.