Werk #20021: Fix rule matching for an empty explicit hosts/services list
| Component | Core & setup | ||||||
| Title | Fix rule matching for an empty explicit hosts/services list | ||||||
| Date | Jul 21, 2026 | ||||||
| Level | Prominent Change | ||||||
| Class | Bug Fix | ||||||
| Compatibility | Incompatible - Manual interaction might be required | ||||||
| Checkmk versions & editions |
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An "Explicit hosts" or "Service" condition with an empty list is a valid condition meaning "matches no host/service." Checkmk's rule matching did not treat this consistently: in some cases it correctly matched no host/service, but in others it incorrectly matched every host/service instead.
This could not normally be triggered through the rule editor, which has always refused to save an empty explicit hosts/services list. It could occur if such a state was introduced through other means, e.g. via the REST API or a hand-edited configuration file.
If you have a rule whose condition is an empty explicit hosts or services list, this fix changes its effective matching behavior: it will now correctly match no host/service, rather than matching every host/service as before. We recommend checking your rules after upgrading if you suspect this situation applies to you.