Experts are skeptical of the threat posed by a reported design flaw in Microsoft Active Directory, which is used by many enterprises to control employee access to the corporate network.Israeli security firm Aorato reported Monday that it created a proof-of-concept attack in which it was able to change a person's network password, thereby making it possible to access other sensitive systems.[Critical design flaw in Active Directory could allow for a password change]Aorato claims the problem stems from Active Directory's backward compatibility with an authentication protocol called NTLM that was the default in versions of Windows older than Windows XP SP3.Newer versions of Windows use a more secure protocol called Kerberos, which Microsoft has been encouraging customers to upgrade to for years.NTLM is vulnerable to a so-called "pass-the-hash" attack in which an attacker steals the login credentials for a computer and then uses the hash for those credentials in changing a user's password. The new password can then be used to access other services, such as remote desktop protocol (RDP) or the Outlook web application.Microsoft says Aorato has not shown anything new."This is a well-known industry limitation in the Kerberos Network Authentication Service standard," the company said in a statement sent to CSOonline Tuesday. "Information on how to manage this limitation when using Windows can be found on the Microsoft TechNet site."Security experts agreed that the problem is not a major risk for businesses."It does not seem to be as serious as pictured since the conditions where an actual attack can happen are very complex," Ehsan Foroughi, director of research at Security Compass, said.Zak Dehlawi, a senior security engineer for Security Innovation agreed, saying pass-the-hash attacks are "most commonly a post-exploitation technique.""An attacker would already need to have gained access to a victim machine through other vulnerabilities before attempting this new procedure," Dehlawi said.Foroughi was more troubled by Aorato's contention that Windows' event logging system would not show any indication of a pass-the-hash attack."The logging part is the most troubling issue for forensics sake," Foroughi said. "But there is not enough there to warrant enterprises to stop using Active Directory tomorrow."Monitoring Active Directory for password resets would be one way to prevent the Aorato attack, Blake Hutchinson, security engineer at Casaba Security, said.Another option would be establishing a baseline of activities for each employee and then monitoring for behavior that falls outside the norm, Feris Rifai, chief executive of Bay Dynamics, said."That will enable corporate IT teams to efficiently and effectively investigate potential threats, and minimize the impact of a breach," Rifai said.[Microsoft's picture-authentication welcomed given password fatigue]While others argued the vulnerability was not critical, Trey Ford, global security strategist at Rapid7, said any security weakness that enables an attacker to impersonate a legitimate user is a "real concern."In addition, the research highlighted how "significant vulnerabilities are often created thanks to backwards compatibility functionality," he said.