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

src/org/loklak/LoklakServer.java

@@ -374,7 +374,10 @@ private static void extractContents() {
 
     private static void extract(JarFile jar, JarEntry file) throws IOException {
         Path workingDirectory = Paths.get("").toAbsolutePath();
-        File target = new File(workingDirectory.toString() + File.separator + file.getName());
+        File target = new File(workingDirectory.toString(), file.getName());
+        if (!target.toPath().normalize().startsWith(workingDirectory.toString())) {
+            throw new RuntimeException("Bad zip entry");
+        }
 
         if (target.exists())
             return;