== How to fix it in Java I/O API

=== Code examples

:code_impact: delete

include::../../common/fix/code-rationale.adoc[]

==== Noncompliant code example

[source,java,diff-id=1,diff-type=noncompliant]
----
@Controller
public class ExampleController
{
    static private String targetDirectory = "/path/to/target/directory/";

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

        File file = new File(targetDirectory + filename);
        file.delete();
    }
}
----

==== Compliant solution

[source,java,diff-id=1,diff-type=compliant]
----
@Controller
public class ExampleController
{
    private static String targetDirectory = "/path/to/target/directory/";
    private static Path targetPath        = new File(targetDirectory).toPath().normalize();

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

        File file = new File(targetPath + filename);

        if (!file.toPath().normalize().startsWith(targetPath)) {
            throw new IOException("Entry is outside of the target directory");
        }

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

=== How does this work?

:canonicalization_function: java.io.File.getCanonicalPath

include::../../common/fix/self-validation.adoc[]

=== Pitfalls

include::../../common/pitfalls/partial-path-traversal.adoc[]

For example, the following code is vulnerable to partial path injection. Note
that the string `targetDirectory` does not end with a path separator:


[source, java]
----
static private String targetDirectory = "/Users/John";

@GetMapping(value = "/endpoint")
public void endpoint(@RequestParam("folder") File fileName) throws IOException {

    String canonicalizedFileName = fileName.getCanonicalPath();

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

This check can be bypassed because `"/Users/Johnny".startsWith("/Users/John")`
returns `true`. Thus, for validation, `"/Users/John"` should actually be
`"/Users/John/"`.

**Warning**: Some functions, such as `.getCanonicalPath`, remove the
terminating path separator in their return value. +
The validation code should be tested to ensure that it cannot be impacted by this
issue.

https://github.com/aws/aws-sdk-java/security/advisories/GHSA-c28r-hw5m-5gv3[Here is a real-life example of this vulnerability.]


:joining_docs: https://docs.oracle.com/javase/8/docs/api/java/nio/file/Path.html
:joining_func: java.nio.file.Path.resolve

include::../../common/pitfalls/oob-specific-path-joining.adoc[]