@PathVariable containing backslash quote returns 400 Bad Request

If you run this simple RestController with Spring Boot (2.5.3):

@RestController
public class SampleRestController {

    @GetMapping("/search/{criteria}")
    public String hello(@PathVariable(name = "criteria") String criteria) {
        return "Hello: " + criteria;
    }

}

And try to open this link in your browser:

 http://localhost:8080/search/%22%5C%22bug%5C%22%22 

Then you will get "400 Bad Request", returned by the embedded Tomcat.

I don't understand, is this a bug in Tomcat ? Is this not a valid URL ?

EDIT: As per some of the replies: I went step-by-step through the Tomcat 9.0.50 source-code and saw the line about ALLOW_BACKSLASH. And neither of the values true or false is good for me, because with true it replaced \ with / and with false it returns 400 Bad Request. What I needed was to allow backslash without replacing it with slash.

My question is really whether this is a bug in Tomcat, since for me the URL seems to be valid. I am not technically putting a \ into the URL, I am putting a %-encoded backslash. What is the purpose of the %-encoding if not to allow the user to send any character in the URL ?

While samabcde's answer provides a solution for your problem, let me answer to your more general questions:

• your URL is perfectly valid although RFC 3986, section 7.3 allows some restriction to be imposed for security reasons,
• these restrictions are not a Tomcat bug, as they were introduced as an answer to CVE-2007-0450. Before Tomcat 6.0.10 the sequences /, %2F and %5C were used to split the request URI into components. If Tomcat was behind a proxy that forwarded only an application (let's say /app), you could use /app/%5C../manager/html to access Tomcat Manager.

If you set org.apache.catalina.connector.CoyoteAdapter.ALLOW_BACKSLASH=true you expose your system to the aforementioned vulnerability and all %5C sequences will appear as / in the servletPath and pathInfo ServletRequest properties. Since the requestURI property contains the undecoded URI path (cf. this question) your example will work.