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/org/apache/commons/io/FileUtils.java

@@ -1042,7 +1042,7 @@ public static void copyDirectory(File srcDir, File destDir,
 
         // Cater for destination being directory within the source directory (see IO-141)
         List<String> exclusionList = null;
-        if (destDir.getCanonicalPath().startsWith(srcDir.getCanonicalPath())) {
+        if (destDir.getCanonicalFile().toPath().startsWith(srcDir.getCanonicalFile().toPath())) {
             File[] srcFiles = filter == null ? srcDir.listFiles() : srcDir.listFiles(filter);
             if (srcFiles != null && srcFiles.length > 0) {
                 exclusionList = new ArrayList<String>(srcFiles.length);