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/hudson/plugins/rake/RvmUtil.java

@@ -105,7 +105,7 @@ private static String buildGemPath(String currentGem, FilePath global, Collectio
         for (FilePath gem : candidateGems) {
             File gemFile = new File(gem.toURI());
             String canonicalPath = gemFile.getCanonicalPath();
-            if ((canonicalPath.startsWith(currentGem + "@") && gem.child("specifications").exists())) {
+            if ((gemFile.getCanonicalFile().toPath().startsWith(currentGem + "@") && gem.child("specifications").exists())) {
                 paths.add(canonicalPath);
             }
         }