When you are using the agent bakery for creating and distributing your monitoring agents it is likely that this change is relevant for you.
In some parts of the GUI, after entering the passphrase of the agent signing keys, it could happen that the signing key passphrase you enter is written to the apache access log of your Check_MK server. As a result it may be visible in clear text to local system users (e.g. users with access to the command line) which scan the logs for it.
This affects the access log of the system apache (normally located at /var/log/apache2) and the access log (/omd/sites/[site]/var/log/apache/*) of the sites apache (master site in distributed setups).
You may want to scan the log files for the string "key_p_passphrase" to check whether or not you are affected. It can be done e.g. with:
zgrep key_p_passphrase /var/log/apache2/access* /omd/sites/*/var/log/apache/access*
In case you find something, you should clean it up. Delete the file or remove the secrets from that file.
Even when it seems unlikely that your key has been compromised, it is recommended to stop using this signing key. Create a new key and proceed with this one.