A security patch for ASP.NET v4.0 was released recently.  The details of the patch can be found at http://www.microsoft.com/technet/security/bulletin/ms10-070.mspx.

I really only have one thing to say in this blog post:  If you use an ISAPI Rewrite Filter on IIS 6.0 and you're application pool is using ASP.NET v4.0, then you should disable the ASP.NET extensionless URL feature.  The security patch unfortunately deletes that registry setting, so you will have to reset it after you install the patch.  The security patch deletes all registry values beneath HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ASP.NET\4.0.30319.0, and restores them to their default values.  The rest of this blog post provides more detail. 

The security patch also has an unfortunate side-effect on IIS 6.0 of moving the ASP.NET v4.0 ISAPI Filter (aspnet_filter.dll) to the bottom of the global (effects all web sites) ISAPI Filter list in IIS configuration.  This could break applications if they have an ISAPI Filter that needs to come after aspnet_filter.dll.  If you installed the patch and suddenly started seeing failures with "/eurl.axd/{GUID}" (where {GUID} is a hexadecimal number) in the URL, then this is the most likely cause.  You can try re-ordering your ISAPI Filters list, but if you're not using the ASP.NET extensionless URL feature on IIS 6.0 then you should disable it as described at http://blogs.msdn.com/b/tmarq/archive/2010/06/18/how-to-disable-the-asp-net-v4-0-extensionless-url-feature-on-iis-6-0.aspx.  That blog post also explains how the ASP.NET v4.0 Extensionless URL feature works on IIS 6.0.

To change the order of the ISAPI Filters on IIS 6.0, open IIS Manager, find the "Web Sites" folder in the navigation tree, right click that folder and select "Properties", and finally select the "ISAPI Filters" tab.  From here, you can select an ISAPI Filter in the list and click the "Move up" button to change the order.  The ASP.NET v4.0 filter is the one that appends "/eurl.axd/{GUID}" to URLs, and in the IIS Manager user interface it appears under the name "ASP.NET_4.0.30319.0".  For more details on ISAPI Filters, see http://msdn.microsoft.com/en-us/library/ms524610(VS.90).aspx.  Note that ISAPI Filters can also be installed at the web site level, so you might need to move your custom ISAPI Filter from the web site level (local) to the web sites level (global) in order to re-order it relative to the ASP.NET v4.0 Filter.

The ASP.NET ISAPI Filter (aspnet_filter.dll) appends "/eurl.axd/{GUID}" to extensionless URLs in order to direct URLs to the ASP.NET ISAPI Extension (aspnet_isapi.dll).  The ASP.NET ISAPI Extension then removes "/eurl.axd/{GUID}" from the URL.  The problems with "/eurl.axd/{GUID}" appearing in URLs occur when an ISAPI Rewrite Filter gets between the ASP.NET ISAPI Filter and the ASP.NET ISAPI Extension, and does something (modifies the URL) that makes it impossible for ASP.NET ISAPI Extension to restore the URL, and so "/eurl.axd/{GUID}" remains in the URL. 

The ASP.NET v4.0 Extensionless URL (EURL) feature does its work during the SF_NOTIFY_PREPROC_HEADERS notification on IIS 6.0.  For those of you having problems with the Ionic’s ISAPI Rewrite Filter, I looked at the Ionic's source code and understand why ASP.NET's EURL feature breaks.  Ionic's ISAPI Rewriter Filter used to do rewrite and redirect during SF_NOTIFY_PREPROC_HEADERS, but the latest sources do it during SF_NOTIFY_AUTH_COMPLETE, which happens later.  So an older version of the Ionic’s ISAPI Rewrite Filter would be compatible with the ASP.NET EURL feature as long as it's ISAPI Filter comes first in the ISAPI Filter list (so it can't get between the ASP.NET ISAPI Filter and the ASP.NET ISAPI Extension), and the new version of Ionic’s ISAPI Rewrite Filter is not compatible at all with the ASP.NET EURL feature, no matter how the ISAPI Filters are ordered, because Ionic's does work in SF_NOTIFY_AUTH_COMPLETE which always comes in between the ASP.NET ISAPI Filter and the ASP.NET ISAPI Extension.  Note that the ASP.NET ISAPI Filter (aspnet_filter.dll) does more than implement ASP.NET EURL, and by no means should you uninstall the ASP.NET ISAPI Filter.  All you should do is disable the ASP.NET extensionless URL feature.