On July 27th, Microsoft released the Critical Update for UR11.
What this update contains is all COD updates part of UR12, UR13, UR14 and some fixes from the CRM2013 branch and is intended to be installed when you don't want to install UR12 for compatibility reasons (UR12 being more a minor release than an update)
There're some differences in upgrade paths between server and clients:
For Server
If you did not install UR11 beforehand, you just need to install the Critical Update as it contains all changes part of UR11. you need to install UR11 beforehand (which itself needs the UR6 codebase) and then apply the CUR11
You can afterward upgrade to the newer rollups (12+), note that you only need to apply the last rollup, not all intermediates.
For Client
The client needs to be at UR11 level before the Critical Update is applied; so you need to downgrade existing clients that are past UR11; or upgrade existing clients that are pre UR11.
Afterward, the following update to apply would be UR15+, not UR12, UR13 or UR14.
The KB Article can be found here, the direct downlad link here and a very detailled article by the Microsoft Dynamics CRM PFE team can be found here.