I think that you have Spring Security on your class path and then spring security is automatically configured with a default user and generated password
Please look into your pom.xml file for:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-security</artifactId>
</dependency>
If you have that in your pom than you should have a log console message like this:
Using default security password: ce6c3d39-8f20-4a41-8e01-803166bb99b6
And in the browser prompt you will import the user user
and the password printed in the console.
Or if you want to configure spring security you can take a look at Spring Boot secured example
It is explained in the Spring Boot Reference documentation in the Security section, it indicates:
The default AuthenticationManager has a single user (‘user’ username and random password, printed at `INFO` level when the application starts up)
Using default security password: 78fa095d-3f4c-48b1-ad50-e24c31d5cf35