Don't let path.normalize get above the root.

Any path.join or path.normalize that starts with a / will not go "above" that after normalization.  This is important because /../foo is almost *always* some sort of error, and doesn't match the corollary in sh: `cd $p; pwd`

At the worse, this can be a vector for exploits, since a static file server might do path.join(docroot, path.normalize("/"+req)) to get the file.  If the normalized request path could be something like "/../../../etc/passwd" then bad things could happen.

lib/path.js

@@ -14,6 +14,10 @@ exports.normalizeArray = function (parts, keepBlanks) {
     // if it's a dot, and there was some previous dir already, then skip it.
     if (directory === "." && prev !== undefined) continue;
 
+    // if it starts with "", and is a . or .., then skip it.
+    if (directories.length === 1 && directories[0] === "" && (
+        directory === "." || directory === "..")) continue;
+
     if (
       directory === ".."
       && directories.length

test/simple/test-path.js

@@ -38,9 +38,11 @@ assert.equal(path.extname("file.ext.ext"), ".ext");
 assert.equal(path.extname("file."), ".");
 
 assert.equal(path.join(".", "fixtures/b", "..", "/b/c.js"), "fixtures/b/c.js");
+assert.equal(path.join("/foo", "../../../bar"), "/bar");
 
 assert.equal(path.normalize("./fixtures///b/../b/c.js"), "fixtures/b/c.js");
 assert.equal(path.normalize("./fixtures///b/../b/c.js",true), "fixtures///b/c.js");
+assert.equal(path.normalize("/foo/../../../bar"), "/bar");
 
 assert.deepEqual(path.normalizeArray(["fixtures","b","","..","b","c.js"]), ["fixtures","b","c.js"]);
 assert.deepEqual(path.normalizeArray(["fixtures","","b","..","b","c.js"], true), ["fixtures","","b","c.js"]);