@Override
    protected AuthenticationHandlerExecutionResult authenticateUsernamePasswordInternal(final UsernamePasswordCredential transformedCredential,
                                                                                        final String originalPassword) throws Throwable {
        try {
            if (this.fileName == null) {
                throw new FileNotFoundException("Filename does not exist");
            }
            val username = transformedCredential.getUsername();
            val passwordOnRecord = getPasswordOnRecord(username);
            if (StringUtils.isBlank(passwordOnRecord)) {
                throw new AccountNotFoundException(username + " not found in backing file.");
            }
            if (matches(originalPassword, passwordOnRecord)) {
                val principal = this.principalFactory.createPrincipal(username);
                return createHandlerResult(transformedCredential, principal, new ArrayList<>());
            }
        } catch (final IOException e) {
            throw new PreventedException(e);
        }
        throw new FailedLoginException();
    }
The filename to read the list of usernames from.
support/cas-server-support-generic/src/main/java/org/apereo/cas/adaptors/generic/FileAuthenticationHandler.java