[SECURITY] Fix Zip Slip Vulnerability (#246)

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 <Jonathan.Leitschuh@gmail.com>
Signed-off-by: Jonathan Leitschuh <Jonathan.Leitschuh@gmail.com>

Bug-tracker: JLLeitschuh/security-research#16

Co-authored-by: Moderne <team@moderne.io>

Author: JLLeitschuh

src/main/java/winstone/HostConfiguration.java

@@ -280,6 +280,12 @@ protected File getWebRoot(File requestedWebroot, File warfile) throws IOExceptio
 
                     // If archive date is newer than unzipped file, overwrite
                     File outFile = new File(unzippedDir, elemName);
+
+                    // Fix Zip Slip vulnerability
+                    if (!outFile.toPath().normalize().startsWith(unzippedDir.toPath().normalize())) {
+                        throw new IOException("Bad zip entry");
+                    }
+
                     if (outFile.exists() && (outFile.lastModified() > warfile.lastModified())) {
                         continue;
                     }