More details in Security Bulletin MS15-058 and KB #3065718.

⚠ This critical on-demand hotfix fixes a memory leak issue for customers running SQL Server 2014 and Azure Storage.

Recommendation: Apply this fix or, better yet, the latest Cumulative Update if you are using 2014 SP1 and currently (or plan to) use Azure Storage for saving database files.

(No word yet on whether SP2 is affected by this issue.) KB #3162589 alone makes this (or the latest) Cumulative Update worth it.

Without the update, you could observe super-latches with virtually exponential growth in worker time (otherwise fixed only by a restart).

One of the more important fixes is one that involves partial (and thus incorrect) results from a clustered columnstore index with a corrupted rowgroup (more gory details in this blog post and KB #3067257).

However, issues were discovered shortly after release: The Analysis Services fixes documented in the KB aren't actually included (in fact SSAS is not touched at all), and slipstreaming RTM SP1 CU1 was not possible.

This is true no matter which KB article you came from.

However, if you are looking for a specific Cumulative Update that doesn't happen to be the most recent one, you can download it from the Microsoft Update Catalog – just don't expect that to always have the latest.

You need to rebuild/recreate the master database as described here.