[SECURITY] Fix Partial Path Traversal Vulnerability (#1930)

* vuln-fix: Partial Path Traversal Vulnerability

This fixes a partial path traversal vulnerability.

Replaces `dir.getCanonicalPath().startsWith(parent.getCanonicalPath())`, which is vulnerable to partial path traversal attacks, with the more secure `dir.getCanonicalFile().toPath().startsWith(parent.getCanonicalFile().toPath())`.

To demonstrate this vulnerability, consider `"/usr/outnot".startsWith("/usr/out")`.
The check is bypassed although `/outnot` is not under the `/out` directory.
It's important to understand that the terminating slash may be removed when using various `String` representations of the `File` object.
For example, on Linux, `println(new File("/var"))` will print `/var`, but `println(new File("/var", "/")` will print `/var/`;
however, `println(new File("/var", "/").getCanonicalPath())` will print `/var`.

Weakness: CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
Severity: Medium
CVSSS: 6.1
Detection: CodeQL & OpenRewrite (https://public.moderne.io/recipes/org.openrewrite.java.security.PartialPathTraversalVulnerability)

Reported-by: Jonathan Leitschuh <Jonathan.Leitschuh@gmail.com>
Signed-off-by: Jonathan Leitschuh <Jonathan.Leitschuh@gmail.com>

Bug-tracker: JLLeitschuh/security-research#13

Co-authored-by: Moderne <team@moderne.io>

* Update AbstractFileSource.java

Co-authored-by: Moderne <team@moderne.io>

src/main/java/com/github/tomakehurst/wiremock/common/AbstractFileSource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2012-2021 Thomas Akehurst
+ * Copyright (C) 2012-2022 Thomas Akehurst
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,6 +27,7 @@
 import java.io.IOException;
 import java.net.URI;
 import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.List;
 
 public abstract class AbstractFileSource implements FileSource {
@@ -159,7 +160,7 @@ private void assertFilePathIsUnderRoot(String path) {
               ? new File(path).getCanonicalPath()
               : new File(rootDirectory, path).getCanonicalPath();
 
-      if (!filePath.startsWith(rootPath)) {
+      if (!Paths.get(filePath).normalize().startsWith(rootPath)) {
         throw new NotAuthorisedException("Access to file " + path + " is not permitted");
       }
     } catch (IOException ioe) {