GeoServer - change Master Password - masterpw.info missing

If you can log into the Geoserver UI with the admin account, you should be able to access the Passwords link under the Security tab on the left nav. One of the options here is to pipe the master password out to a file, as well as reset it.

If you're unable to login, you should be able to reset the admin's password from encrypted to a plaintext one, temporarily: In /data/security/usergroup/default/users.xml, change

<user enabled="true" name="admin" password="digest1:D9miJH/hVgfxZJscMafEtbt...JOSV4POi55PQI4tw"/>

to

<user enabled="true" name="admin" password="plain:new-password-here"/>

Then, once you are able to login, you can go into your account, update your password and save it, which will save it back in an encrypted digest form.

The documentation is outdated/wrong.

It's possible to dump the master password by navigating to Security > Passwords and clicking the Master password forgotten ? link. Turns out that the default password is the same as for the admin account: geoserver