Every month, Google sends out a new security patch for the Nexus devices which are still being supported under the timeline for software security updates, and as of now Google has just started to push the latest software update to Nexus devices which contains the October security patch. The new security software is available for download and installation for a handful of different devices including the Nexus 6P, Nexus 5X, Pixel C tablet, Nexus Player, Nexus 9 Wi-Fi only, Nexus 6, and Nexus 5. It is worth noting that the last two devices - the Nexus 6 and Nexus 5 - while having new security patch software to install it is not Android 7.0 Nougat software which means that users with those devices are still working with Android Marshmallow.
Having said that, all the other devices listed above are working with a latest version of Android 7.0 Nougat for these fixes, with build numbers NBD90X for the Nexus 6P, NBD90W for the Nexus 5X, NRD91D for the Pixel C tablet, NRD91D for the Nexus Player, and NRD91D for the Nexus 9 Wi-Fi only. Google has made both the Factory Images and the OTA Images available for download, so whichever software you're more comfortable with flashing is up for grabs to push over to your device.
As for what bugs or security issues this patch fixes, the security bulletin states that there were quite a few high-level issues that have been taken care of with this patch, related to a number of different vulnerabilities including Elevation of privilege vulnerability in ServiceManager, Elevation of privilege vulnerability in Lock Settings Service, and Elevation of privilege vulnerability in Mediaserver, among others. There were also a few moderate level issues that were fixed in this patch like Elevation of privilege vulnerability in Telephony, and Elevation of privilege vulnerability in Accessibility Services. All of those issues are fixes that are being made using the October 5th patch string that Google is sending out to supported Nexus and Pixel devices starting today. Google also documents an October 1st patch string with a smaller amount of fixes that other OEMs will be issuing to their devices, giving them more time work with the October 5th patch string, likely issuing those fixes in the November 1st patch. If you're looking to flash either the Factory Images or the OTA Images to your compatible Nexus device, and you aren't exactly sure of the process, it's recommended that you wait for Google to push the OTA security patch to your device, although if you do want to flash the software you can find a tutorial to help you the process here, keeping in mind that you are responsible for anything that happens to your device if something goes wrong. To grab the needed files you can find the Factory Images as well as the OTA Images by hitting the buttons down below.
Google has now finally added the Android 7.0 Nougat Factory Images for the Nexus 6. In addition to the Factory Images, the OTA Images for Android 7.0 Nougat on the Nexus 6 are also available.