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]>

byte-buddy-dep/src/main/java/net/bytebuddy/build/Plugin.java

@@ -3397,7 +3397,7 @@ public void retain(Source.Element element) throws IOException {
                     String name = element.getName();
                     if (!name.endsWith("/")) {
                         File target = new File(folder, name), resolved = element.resolveAs(File.class);
-                        if (!target.getCanonicalPath().startsWith(folder.getCanonicalPath())) {
+                        if (!target.getCanonicalFile().toPath().startsWith(folder.getCanonicalFile().toPath())) {
                             throw new IllegalArgumentException(target + " is not a subdirectory of " + folder);
                         } else if (!target.getParentFile().isDirectory() && !target.getParentFile().mkdirs()) {
                             throw new IOException("Could not create directory: " + target.getParent());