fix: Prevent path traversal in local file manager

The local file manager joined attacker-controlled path segments into the
uploads storage path without ensuring the result stayed under the uploads
root. Any authenticated user could read arbitrary files readable by the
Planka process (e.g. /etc/passwd, /app/.env) via `../` sequences on the
/user-avatars/*, /background-images/* and /favicons/* routes (CWE-22).

Resolve the path and enforce it stays within uploadsBasePath, centrally in
buildPath so every local file operation is covered. Additionally resolve
symlinks in read() and re-check containment, so a symlink inside the root
cannot be used to escape it.

Reported by Alpesh (github.com/Alpastx).

server/api/hooks/file-manager/LocalFileManager.js

@@ -12,7 +12,24 @@ const { rimraf } = require('rimraf');
 
 // const PATH_SEGMENT_TO_URL_REPLACE_REGEX = /(public|private)\//;
 
-const buildPath = (pathSegment) => path.join(sails.config.custom.uploadsBasePath, pathSegment);
+// Reject a path unless it is the uploads root itself or lives strictly
+// beneath it. Callers must pass an already-absolute path.
+const assertWithinRoot = (rootPath, filePath) => {
+  if (filePath !== rootPath && !filePath.startsWith(`${rootPath}${path.sep}`)) {
+    throw new Error('Path is outside of the uploads directory');
+  }
+};
+
+const buildPath = (pathSegment) => {
+  const { uploadsBasePath } = sails.config.custom;
+  const filePath = path.resolve(uploadsBasePath, pathSegment);
+
+  // Ensure the resolved path stays within the uploads root, so that
+  // attacker-controlled path segments (e.g. `../`) cannot escape it.
+  assertWithinRoot(uploadsBasePath, filePath);
+
+  return filePath;
+};
 
 class LocalFileManager {
   // eslint-disable-next-line class-methods-use-this
@@ -41,14 +58,25 @@ class LocalFileManager {
   async read(filePathSegment, { withHeaders = false } = {}) {
     const filePath = buildPath(filePathSegment);
 
+    // Resolve symlinks and re-check containment, so that a symlink placed
+    // inside the uploads root cannot be used to read files outside of it.
+    let realFilePath;
+    try {
+      realFilePath = await fs.promises.realpath(filePath);
+    } catch (error) {
+      throw new Error('File does not exist');
+    }
+    const realBasePath = await fs.promises.realpath(sails.config.custom.uploadsBasePath);
+    assertWithinRoot(realBasePath, realFilePath);
+
     let stat;
     try {
-      stat = await fs.promises.stat(filePath);
+      stat = await fs.promises.stat(realFilePath);
     } catch (error) {
       throw new Error('File does not exist');
     }
 
-    const readStream = fs.createReadStream(filePath);
+    const readStream = fs.createReadStream(realFilePath);
 
     if (withHeaders) {
       return [