A small security and bug fix update is currently rolling out for most devices in the Chrome OS Stable channel. The update takes devices to build 58.0.3029.112 (Platform version: 9334.69.0). This build from the Chromium team appears to be for a wide range of devices with no exceptions noted in the release notes. This is somewhat different than the last few major releases as there have been specific devices excluded from the updates. Those devices have been for the most part the handful of Chromebooks and Chromeboxes that can run Android apps.
As for what is fixed in this update or patched, that information was not released. That is the SOP (Standard Operating Procedure) for the Chrome OS team as they tend to not release specifics on an update until the majority of devices have been updated.
The release notes also point to “feature enhancements” but again, nothing specific was listed. Having installed this update to my Acer Chromebook 14 this morning, nothing stuck out as different in my cursory overview. If, however, you find something new, do drop a comment and let me know so I can update the post.
The update should hit devices over the course of the next few days. You can, as always, force the issue by typing in Chrome://help in the browser and checking for an update. On my Chromebook 14, the update took less than 5 minutes so it apply.