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 <[email protected]>
Signed-off-by: Jonathan Leitschuh <[email protected]>

Bug-tracker: JLLeitschuh/security-research#13

Co-authored-by: Moderne <[email protected]>

src/main/java/cloudsync/connector/LocalFilesystemConnector.java

@@ -510,7 +510,7 @@ public Item getItem(File file, final FollowLinkType followlinks, List<String> fo
 				}
 				target = Paths.get(target).toFile().getCanonicalPath();
 
-                if (!followlinks.equals(FollowLinkType.NONE) && followlinks.equals(FollowLinkType.EXTERNAL) && !target.startsWith(localPath + Item.SEPARATOR) )
+                if (!followlinks.equals(FollowLinkType.NONE) && followlinks.equals(FollowLinkType.EXTERNAL) && !Paths.get(target).toFile().getCanonicalFile().toPath().startsWith(localPath + Item.SEPARATOR) )
 				{
                     boolean foundLink = false;
                     for( String followedLinkPath: followedLinkPaths )