Microsoft has explained why it took seven years to patch a known vulnerability. Fixing the bug earlier would have taken out network applications and potential exploits alike, it explained.
Security bulletin MS08-068 fixed a flaw in the SMB (Server Message Block) component of Windows, first demonstrated by Sir Dystic of Cult of the Dead Cow fame at a hacking conference in 2001, if not before. The flaw opened the door to SMB replay or reflection attacks that would have allowed the operator of a malicious SMB server to run exploits on vulnerable PCs.
The flaw was rated as important by Microsoft but critical by some independent security watchers, such as the SANS Institute's Internet Storm Centre.
Microsoft explained the delay on issuing a patch on the effect a fix would have had on network-based applications. In a post on Microsoft's Security Response Blog, Christopher Budd explains that the SMBRElay attack worked in much the same way as its legacy NTLM protocol.
"When this issue was first raised back in 2001, we said that we could not make changes to address this issue without negatively impacting network-based applications. And to be clear, the impact would have been to render many (or nearly all) customers’ network-based applications then inoperable," Budd explained.
Interaction between programs, for example Outlook 2000 and Exchange 2000, would have been affected by a patch at the time, according to Budd. Redmond advised customers to apply a workaround - involving SMB signing - but always knew this approach had its limitations.
"We did say that customers who were concerned about this issue could use SMB signing as an effective mitigation, but, the reality was that there were similar constraints that made it infeasible for customers to implement SMB signing," Budd added.
Gradual changes Microsoft has introduced with successive versions of its software have reduced the scope of the flaw and allowing Redmond to (finally) issue a patch last Tuesday.
Security vendors criticised Microsoft for its delay in developing a patch, a failure which left customers open to attack. "Microsoft has known of this problem since 2001 and was not able to (or chose not to) fix it until now. This also means that working exploit code has been available for all Operating Systems including Windows NT 4, Windows 2000, XP, Windows Server 2003, Vista and Windows 2008. However, as Microsoft correctly states, exploitation is severely mitigated on Vista and Windows 2008," Eric Schultze, CTO at security tool firm Shavlik notes.
Tests for the vulnerability have been included with the Metasploit security tool since July 2007. Its developers are critical of Microsoft's long-waited update. "The MS08-068 patch addresses this attack only in the case where the attacker connects back to the victim ... The patch does NOT address the case where the attacker relays the connection to a third-party host that the victim has access to," Metasploit said. ®