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>

Author: JLLeitschuh

ttorrent-client/src/main/java/com/turn/ttorrent/client/storage/FileCollectionStorage.java

@@ -76,7 +76,7 @@ public static FileCollectionStorage create(TorrentMetadata metadata, File parent
     for (TorrentFile file : metadata.getFiles()) {
       File actual = new File(parent, file.getRelativePathAsString());
 
-      if (!actual.getCanonicalPath().startsWith(parent.getCanonicalPath())) {
+      if (!actual.getCanonicalFile().toPath().startsWith(parent.getCanonicalFile().toPath())) {
         throw new SecurityException("Torrent file path attempted " +
                 "to break directory jail!");
       }