I got the below error while starting the weblogic admin server.

<Sep 6, 2011 3:22:38 AM CDT> <Critical> <Security> <BEA-090403> <Authentication for user  denied>
<Sep 6, 2011 3:22:38 AM CDT> <Critical> <WebLogicServer> <BEA-000386> <Server subsystem failed. Reason: weblogic.security.SecurityInitializationException: Authentication for user  denied
weblogic.security.SecurityInitializationException: Authentication for user  denied
        at weblogic.security.service.CommonSecurityServiceManagerDelegateImpl.doBootAuthorization(CommonSecurityServiceManagerDelegateImpl.java:965)
        at weblogic.security.service.CommonSecurityServiceManagerDelegateImpl.initialize(CommonSecurityServiceManagerDelegateImpl.java:1050)
        at weblogic.security.service.SecurityServiceManager.initialize(SecurityServiceManager.java:875)
        at weblogic.security.SecurityService.start(SecurityService.java:141)
        at weblogic.t3.srvr.SubsystemRequest.run(SubsystemRequest.java:64)
        Truncated. see log file for complete stacktrace

Caused By: javax.security.auth.login.FailedLoginException: [Security:090304]Authentication Failed: User  javax.security.auth.login.LoginException: [Security:090301]Password Not Supplied
        at weblogic.security.providers.authentication.LDAPAtnLoginModuleImpl.login(LDAPAtnLoginModuleImpl.java:250)
        at com.bea.common.security.internal.service.LoginModuleWrapper$1.run(LoginModuleWrapper.java:110)
        at com.bea.common.security.internal.service.LoginModuleWrapper.login(LoginModuleWrapper.java:106)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        Truncated. see log file for complete stacktrace
>

This error is because of the corrupt or missing boot.properties.

Googling this error message would give you multiple solutions, but I followed the below one.
1. Create a new boot.properties. The location would be <bea_home>/user_projects/domains/<domain>/servers/<admin server name>/security/boot.properties
2. Add the following in the boot.properties
username=<weblogic admin user name>
password=<weblogic admin password>
3. Restart the server.