Today’s update to bring macOS Catalina to version 10.15.7 came as something of a surprise, particularly as all the signs are that Big Sur isn’t far from release either.

Apple claims this relatively small update fixes problems connecting automatically to Wi-Fi networks, failure to sync files through iCloud Drive, and problems with Radeon Pro 5700 XT graphics support on the latest iMac Retina 5K, 27-inch, 2020. There are no new firmware updates provided.

Security fixes are detailed for all three here. There are only four in total, of which two apply to Catalina. The most serious of them, which affects all three versions of macOS, is a sandbox vulnerability which gives access to restricted files (CVE-2020-9968).

Among the bundled apps, only Mail has changed, by a small build increment from 3608.120.23.2.1 to 3608.120.23.2.4. Accompanying that are similarly small increments in many of Mail’s support files, including the MailMigrator plugin, private Email frameworks, and Mail and IMAP private frameworks. These are strong evidence that this update attempts to fix the remaining bug(s) in Mail, although Apple hasn’t mentioned that in its release notes. Let’s hope it succeeds.

Looking in more detail at other /System/Library updates, there are the following:

• a minor build increment to Software Update.app, and to its frameworks;
• many build increments across AMD Radeon kernel extensions;
• build increments to the FileProvider framework for Cloud docs, and the CloudDocs and CloudDocsDaemon frameworks;
• a build increment to the ImageIO framework, probably reflecting a security fix;
• build increments to several Message private frameworks, which also appear to fix existing bugs (I hope).

Changes in JavaScriptCore and WebKit frameworks, and the Safari private framework, are from the recent Safari update.

Oddly, the Software Update version of this update installs an old version 1.62 of Apple’s Malware Removal Tool, MRT. Once the installation has completed, you should be able to download and install an update to bring that back up to 1.66, the same version that should have been installed prior to this update. Whether this is an installer error or intentional isn’t clear. This doesn’t appear to be the case with the standalone updater, though.

Standalone updaters are available as follows:

• for 10.15.7, the ‘delta’ update, and Combo update;
• for 10.14, Mojave Security Update 2020-005;
• for 10.13, High Sierra Security Update 2020-005.

That should, finally, be the last update to Catalina, and will almost certainly be the last Security Update for High Sierra, which now passes into the unsupported list.

Postscript: Many users are reporting problems following installation of the initial release of Mojave Security Update 2020-005. These range from minor, such as warnings when running softwareupdate, to major problems with memory usage and impaired performance. These are detailed here by Mr Macintosh, and extensively across other Mac sites.

Minor problems after any of these updates can be resolved by restarting in Safe mode, then after a couple of minutes restarting normally. More severe issues can be resolved by rolling back to a pre-update snapshot (if you’re running Time Machine), by re-installing Mojave, or by upgrading to Catalina (which is being suggested by Apple). However 10.15.7 isn’t without its own glitches. I’ll be taking a look at these updates in more detail on Monday 28. If you haven’t updated yet, you might like to hold off for now, as Apple may well replaced one or more of them.

(Updated 0740 UTC 27 September 2020.)