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]>

launcher/src/main/java/org/springframework/boot/loader/thin/PathResolver.java

@@ -230,7 +230,7 @@ private void maybeCopyToRoot(String root, File repo, List<Dependency> classPathA
 				if (!path.endsWith(".jar")) {
 					continue;
 				}
-				if (!path.startsWith(parent)) {
+				if (!file.getCanonicalFile().toPath().startsWith(parent)) {
 					throw new IllegalStateException("Not in thin root repository: " + path);
 				}
 				String jar = path.substring(parent.length());

maven-plugin/src/main/java/org/springframework/boot/experimental/maven/ThinJarMojo.java

@@ -171,7 +171,7 @@ protected File downloadThinJar(File outputDirectory) throws MojoFailureException
 		Dependency deployable = decode(thinLauncherArtifact);
 		File result = resolveFile(deployable);
 		try {
-			if (!result.getCanonicalPath().startsWith(outputDirectory.getCanonicalPath())) {
+			if (!result.getCanonicalFile().toPath().startsWith(outputDirectory.getCanonicalFile().toPath())) {
 				String root = new File(this.settings.getLocalRepository()).getCanonicalPath();
 				String path = result.getCanonicalPath().substring(root.length());
 				File target = new File(new File(outputDirectory, "repository"), path);