Fix Relative Path Traversal Attack on note creation

Impact
---

An attacker can read arbitrary `.md` files from the server's filesystem due to an [improper input validation](https://cwe.mitre.org/data/definitions/20.html), which results in the ability to perform a [relative path traversal](https://cwe.mitre.org/data/definitions/23.html).

CVSSv3 string: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

PoC / Quicktest
---

To verify if you are affected, you can try to open the following URL: `http://localhost:3000/..%2F..%2FREADME#` (replace `http://localhost:3000` with your instance's base-URL e.g. `https://demo.hedgedoc.org/..%2F..%2FREADME#`).
- If you see a README page being rendered, you run an affected version.

Analysis
---

The attack works due the fact that [the internal router, passes the url-encoded alias](https://github.com/hedgedoc/hedgedoc/blob/master/lib/web/note/router.js#L26) to the `noteController.showNote`-function. This function passes the input directly to [`findNote()`](78a732abe6/lib/web/note/util.js (L10)) utility function, that will pass it on the the [`parseNoteId()`](78a732abe6/lib/models/note.js (L188-L258))-function, that tries to make sense out of the noteId/alias and check if a note already exists and if so, if a corresponding file on disk was updated.

If no note exists the [note creation-function is called](78a732abe6/lib/models/note.js (L240-L245)), which pass this unvalidated alias, with a `.md` appended, into a [`path.join()`-function](78a732abe6/lib/models/note.js (L99)) which is read from the filesystem in the follow up routine and provides the pre-filled content of the new note.

This allows an attacker to not only read arbitrary `.md` files from the filesystem, but also  observes changes to them.

The usefulness of this attack can be considered limited, since mainly markdown files are use the file-ending `.md` and all markdown files contained in the hedgedoc project, like the README, are public anyway. If other protections such as a chroot or container or proper file permissions are in place, this attack's usefulness is rather limited.

Workarounds
---
On a reverse-proxy level one can force a URL-decode, which will prevent this attack because the router will not accept such a path.

For more information
---

If you have any questions or comments about this advisory:
* Open an topic on [our community forum](https://community.hedgedoc.org)
* Join our [matrix room](https://chat.hedgedoc.org)

Advisory link
---

https://github.com/hedgedoc/hedgedoc/security/advisories/GHSA-p528-555r-pf87

Signed-off-by: Christoph (Sheogorath) Kern <sheogorath@shivering-isles.com>

lib/models/note.js

@@ -94,7 +94,7 @@ module.exports = function (sequelize, DataTypes) {
             let body = null
             let filePath = null
             if (note.alias) {
-              filePath = path.join(config.docsPath, note.alias + '.md')
+              filePath = path.join(config.docsPath, path.basename(note.alias) + '.md')
             }
             if (!filePath || !Note.checkFileExist(filePath)) {
               filePath = config.defaultNotePath
@@ -196,7 +196,7 @@ module.exports = function (sequelize, DataTypes) {
           }
         }).then(function (note) {
           if (note) {
-            const filePath = path.join(config.docsPath, noteId + '.md')
+            const filePath = path.join(config.docsPath, path.basename(noteId) + '.md')
             if (Note.checkFileExist(filePath)) {
               // if doc in filesystem have newer modified time than last change time
               // then will update the doc in db
@@ -238,7 +238,7 @@ module.exports = function (sequelize, DataTypes) {
               return callback(null, note.id)
             }
           } else {
-            const filePath = path.join(config.docsPath, noteId + '.md')
+            const filePath = path.join(config.docsPath, path.basename(noteId) + '.md')
             if (Note.checkFileExist(filePath)) {
               Note.create({
                 alias: noteId,