How do I get a Spring boot to parse thymeleaf-extras-springsecurity tags?

Please try adding something like the following code to your @Configuration(or @SpringBootApplication) class:

@Bean
public SpringTemplateEngine templateEngine(ITemplateResolver templateResolver, SpringSecurityDialect sec) {
    final SpringTemplateEngine templateEngine = new SpringTemplateEngine();
    templateEngine.setTemplateResolver(templateResolver);
    templateEngine.addDialect(sec); // Enable use of "sec"
    return templateEngine;
}

Note that if you are forcing Spring Boot to use Thymeleaf version 3, you have to force also the version 3 of the thymeleaf-extras-springsecurity4 dependency:

<dependency>
    <groupId>org.thymeleaf.extras</groupId>
    <artifactId>thymeleaf-extras-springsecurity4</artifactId>
    <version>3.0.1.RELEASE</version>
</dependency>

See also this related answer.