The initial releases of macOS Sierra and High Sierra Security Updates 2019-002 on Monday 25 March were not complete. Apple has now released updates to both of these: if you applied either security update before 29 March, you should install this revised update when you can.
Apple states that current updated build numbers you should see are:
- for Sierra, 16G1918;
- for High Sierra, 17G6030.
If your system shows an earlier build number, you need to install the updated security update.
Standalone installers are available from the same links:
Sierra Security Update 2019-002 is here,
High Sierra Security Update 2019-002 is here.
Those who updated Mojave to 10.14.4 aren’t affected by this.
(Thanks to Al for alerting me to this.)