vuln-fix: Zip Slip Vulnerability

This fixes a Zip-Slip vulnerability.

This change does one of two things. This change either

1. Inserts a guard to protect against Zip Slip.
OR
2. 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())`.

For number 2, 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: High
CVSSS: 7.4
Detection: CodeQL (https://codeql.github.com/codeql-query-help/java/java-zipslip/) & OpenRewrite (https://public.moderne.io/recipes/org.openrewrite.java.security.ZipSlip)

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

Bug-tracker: JLLeitschuh/security-research#16

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

Author: JLLeitschuh

test/org/jetbrains/java/decompiler/BulkDecompilationTest.java

@@ -76,6 +76,9 @@ private static void unpack(File archive, File targetDir) {
         ZipEntry entry = entries.nextElement();
         if (!entry.isDirectory()) {
           File file = new File(targetDir, entry.getName());
+          if (!file.toPath().normalize().startsWith(targetDir.toPath().normalize())) {
+            throw new RuntimeException("Bad zip entry");
+          }
           assertTrue(file.getParentFile().mkdirs() || file.getParentFile().isDirectory());
           try (InputStream in = zip.getInputStream(entry); OutputStream out = new FileOutputStream(file)) {
             InterpreterUtil.copyStream(in, out);
@@ -87,4 +90,4 @@ private static void unpack(File archive, File targetDir) {
       throw new RuntimeException(e);
     }
   }
-}
+}