=== How to fix it in Java SE

:canonicalization_function: java.io.File.getCanonicalPath
include::../../common/fix/code-rationale.adoc[]

[cols="a"]
|===
h| Non-compliant code example
|
[source,java]
----
@RestController
public class ApiController
{
    static private String targetDirectory = "/path/to/target/directory/";

    @GetMapping(value = "/endpoint")
    public void endpoint(@RequestParam("filename") filename) throws IOException {

        File file = new File(targetDirectory + filename);
        file.delete(); // Noncompliant
    }
}
----
h| Compliant solution
|
[source,java]
----
@RestController
public class ApiController
{
    static private String targetDirectory = "/path/to/target/directory/";

    @GetMapping(value = "/endpoint")
    public void endpoint(@RequestParam("filename") filename) throws IOException {

        File file = new File(targetDirectory + filename);
        String canonicalDestinationPath = file.getCanonicalPath();

        if (!canonicalDestinationPath.startsWith(targetDirectory)) {
            throw new IOException("Entry is outside of the target directory");
        }

        file.delete();
    }
}
----
|===

=== How does this work?

include::../../common/fix/how-does-this-work.adoc[]