Security Update 2018-002 (Sierra and El Capitan)
Apple has released Security Update 2018-002 for macOS 10.12 Sierra and 10.11 El Capitan, patching security vulnerabilities that the company addressed in 10.13.4 High Sierra (see “macOS 10.13.4 High Sierra Adds Business Chat and External GPU Support,” 30 March 2018). Both of the older operating systems also address a logic issue with disk images that could launch an application after mounting, resolve a race condition in iCloud Drive that could grant an application elevated privileges, and fix a memory corruption issue that could allow an application to execute arbitrary code with system privileges. (Free. For 10.12.6 Sierra, 779.6 MB; for 10.11.6 El Capitan, 869.6 MB; security content release notes)
The parallel update for El Capitan, 10.11.6 2018 002 appears to lose track of what was set in the Startup Disk preference and defaulted to starting up from the internal hard drive. I run on an External firewire drive, and was surprised to discover that after the update the Mac started up on the internal hard drive, which was still set to 10.6. To make matters worse, neither the Startup Disk Preferences nor Disk Utility in 10.6 could not find the external drive running 10.11, although it appeared in the Finder. I eventually managed to start up on the External drive, but it took some fiddling.