A flaw in the implementation of SMB Signing in Windows 2000 & Windows XP could enable an attacker to silently downgrade the SMB Signing settings on an affected system. To do this, the attacker would need access to the session negotiation data as it was exchanged between a client and server, & would need to modify the data in a way that exploits the flaw. This would cause either or both systems to send unsigned data regardless of the signing policy the administrator had set. After having downgraded the signing setting, the attacker could continue to monitor the session & change data within it; the lack of signing would prevent the communicants from detecting the changes.
Microsoft Windows 2000
Microsoft Windows XP
Note - This update is already included in XP Service Pack 1.