The Microsoft Security Advisory (961040) for SQL Server was posted on the 22nd of December. Microsoft has done a commendable job and provided a lot of information on this page, with the cross reference of the CVE number (CVE-2008-4270) so you can find more details if you need it. Like any of the store procedures that provide remote code execution, they can be dangerous and are targets for hackers.
You want to patch as soon as Microsoft releases a patch.
Microsoft states that “… MSDE 2000 or SQL Server 2005 Express are at risk of remote attack if they have modified the default installation to accept remote connections, if they allow untrusted users access to MSDE 2000 or SQL Server 2005 Express …”. But I rate the risk higher than what they are saying because of the following:
MSDE 2000 and SQL Server Express 2005 are often bundled/embedded into applications and so their presence is not immediately apparent. There may be copies around that most IT staff are not fully aware of, and/or these applications may be delivered with open permissions because the developer of the application was not concerned with these functions.
Second, replication is an administrative function. sp_replwritetovarbin, along with other stored procedures like sp_resyncexecutesql and sp_resyncexecute functions run as DBO, or Database Owner, so if they are compromised they expose permissions as well as function.
Finally, as MSDE 2000 and SQL Server Express 2005 get used by web developers who run the database on the same machine with the same OS/DBA credentials, you server could be completely compromised with this one.
So follow their advice and run the command:
“use master deny execute on sp_replwritetovarbin to public”
A couple more recommendations, assuming you are a DBA (which is a fair assumption if you are running the suggested workaround) check the master.dbo.sysprotects and master.dbo.sysobjects for public permissions in general. Even if you are patched for this specific vulnerability, or if you are running an unaffected version of the database, you should have this procedure locked down otherwise you remain vulnerable.
Over and above patching the known servers, if you have a scanning and discovery tool, run a scan across your network for the default SQL Server port to see if there are other database engines. That should spotlight the majority of undocumented databases.