Add security changelog for 2.13.1

Author: N-o-X

CHANGELOG.md

@@ -9,7 +9,18 @@ Released closed milestones can be found on [GitHub](https://github.com/Icinga/ic
 
 ## 2.13.1 (2021-08-19)
 
-Version 2.13.1 fixes two issues indroduced with the 2.13.0 release.
+The main focus of this version is a security vulnerability in the TLS certificate verification of our metrics writers ElasticsearchWriter, GelfWriter, InfluxdbWriter and Influxdb2Writer.
+
+Version 2.13.1 also fixes two issues indroduced with the 2.13.0 release.
+
+### Security
+
+* Add TLS server certificate validation to ElasticsearchWriter, GelfWriter, InfluxdbWriter and Influxdb2Writer ([GHSA-cxfm-8j5v-5qr2](https://github.com/Icinga/icinga2/security/advisories/GHSA-cxfm-8j5v-5qr2))
+
+Depending on your setup, manual intervention beyond installing the new versions
+may be required, so please read the more detailed information in the
+[release blog post](https://icinga.com/blog/2021/08/19/icinga-2-13-1-security-release//)
+carefully
 
 ### Bugfixes