Microsoft acknowledged the existence of a cross-site scripting (aka XSS) vulnerability in SharePoint Server 2007 and SharePoint Services 3.0.
CVSS(Base): 4.3 (unofficial)
Exploit Availability: public proof-of-concept
Impact: a specifically-crafted URL targeted to the users of an organization will allow arbitrary code to be executed in the context of the user in the security domain of the organization’s SharePoint server.
Patch Availability: patches are currently unavailable and projected to not be available until June 2010.
Workaround: Microsoft has provided workarounds for both the server and the endpoints. Details are available: http://blogs.technet.com/srd/archive/2010/04/29/sharepoint-xss-issue.aspx
Urgency: Although the vulnerability taken by it self is not that severe, it does open up opportunities to leverage other exploits that may not be otherwise exposed by your environment. This issue should not be considered an internal-only problem because your organization’s SharePoint servers are not on the Internet—all users of SharePoint are exposed. It is recommended that most organizations consider the workarounds proposed by Microsoft.