vfs: prevent RealFSProvider sandbox escape via symlinks

The containment check in RealFSProvider#resolvePath was thrown inside the
try block wrapping fs.realpathSync(). Because the rejection carried an
ENOENT code, the `if (err?.code !== 'ENOENT') throw err` catch mistook it
for "path does not exist yet" and returned the raw, symlink-containing
path, which the caller then opened -- following the symlink out of the
root. #verifyAncestorInRoot had the same self-swallowing flaw, making it a
no-op. As a result readFile/stat/open/write on a symlink pointing outside
the root read and wrote real files outside the sandbox.

Move the containment checks outside the try/catch so a rejection can no
longer be swallowed by the ENOENT handler, and compare against a
canonicalized root (#realRoot) so the checks are sound even when the root
path itself contains symlinked components (e.g. /tmp -> /private/tmp).

Add regression tests covering read/stat/open/write and new-file creation
through symlinks that resolve outside the provider root.

Author: VinzSpring

lib/internal/vfs/providers/real.js

@@ -238,14 +238,27 @@ class RealFileHandle extends VirtualFileHandle {
  */
 class RealFSProvider extends VirtualProvider {
   #rootPath;
+  #realRoot;
 
   /**
    * @param {string} rootPath The real filesystem path to use as root
    */
   constructor(rootPath) {
     super();
-    // Resolve to absolute path and normalize
+    // Resolve to absolute path and normalize. This is the user-facing root
+    // exposed via `rootPath` and used to translate symlink targets.
     this.#rootPath = path.resolve(getValidatedPath(rootPath, 'rootPath'));
+    // Canonicalize the root (resolving any symlinked path components) for use
+    // in containment checks. `fs.realpathSync()` on a candidate returns a
+    // fully-resolved path, so comparing it against a non-canonical root can
+    // both reject legitimate in-root paths and (together with the symlink
+    // resolution below) fail to reject escapes. Fall back to the resolved
+    // path when the root does not exist on disk yet.
+    try {
+      this.#realRoot = fs.realpathSync(this.#rootPath);
+    } catch {
+      this.#realRoot = this.#rootPath;
+    }
     setOwnProperty(this, 'readonly', false);
     setOwnProperty(this, 'supportsSymlinks', true);
   }
@@ -272,36 +285,45 @@ class RealFSProvider extends VirtualProvider {
       normalized = normalized.slice(1);
     }
 
-    // Join with root and resolve
-    const realPath = path.resolve(this.#rootPath, normalized);
+    // Join with the canonical root and resolve `.`/`..` lexically.
+    const realPath = path.resolve(this.#realRoot, normalized);
 
-    // Security check: ensure the resolved path is within rootPath
-    const rootWithSep = this.#rootPath.endsWith(path.sep) ?
-      this.#rootPath :
-      this.#rootPath + path.sep;
+    // Security check: ensure the lexically-resolved path is within the root.
+    // Catches `..` traversal and sibling-prefix paths before touching disk.
+    const rootWithSep = this.#realRoot.endsWith(path.sep) ?
+      this.#realRoot :
+      this.#realRoot + path.sep;
 
-    if (realPath !== this.#rootPath && !StringPrototypeStartsWith(realPath, rootWithSep)) {
+    if (realPath !== this.#realRoot && !StringPrototypeStartsWith(realPath, rootWithSep)) {
       throw createENOENT('open', vfsPath);
     }
 
-    // Resolve symlinks to prevent escape via symbolic links
+    // Resolve symlinks to prevent escape via symbolic links.
     if (followSymlinks) {
+      let resolved;
       try {
-        const resolved = fs.realpathSync(realPath);
-        if (resolved !== this.#rootPath &&
-            !StringPrototypeStartsWith(resolved, rootWithSep)) {
-          throw createENOENT('open', vfsPath);
-        }
-        return resolved;
+        resolved = fs.realpathSync(realPath);
       } catch (err) {
+        // Only a genuine "does not exist" is handled here. Any other error
+        // (including the containment rejection below) must propagate.
         if (err?.code !== 'ENOENT') throw err;
-        // Path doesn't exist yet - verify deepest existing ancestor
+        // Path doesn't exist yet - verify the deepest existing ancestor is
+        // within the root, then return the in-root path for creation.
         this.#verifyAncestorInRoot(realPath, rootWithSep, vfsPath);
         return realPath;
       }
+      // IMPORTANT: perform the containment check OUTSIDE the try/catch above.
+      // Throwing it inside the try would let the `err?.code !== 'ENOENT'`
+      // catch swallow the rejection (createEACCES/ENOENT share codes with real
+      // fs errors), silently returning a path that escapes the root.
+      if (resolved !== this.#realRoot &&
+          !StringPrototypeStartsWith(resolved, rootWithSep)) {
+        throw createEACCES('open', vfsPath);
+      }
+      return resolved;
     }
 
-    // For lstat/readlink (no final symlink follow), check parent only
+    // For lstat/readlink (no final symlink follow), check ancestors only.
     this.#verifyAncestorInRoot(realPath, rootWithSep, vfsPath);
     return realPath;
   }
@@ -314,18 +336,23 @@ class RealFSProvider extends VirtualProvider {
    */
   #verifyAncestorInRoot(realPath, rootWithSep, vfsPath) {
     let current = path.dirname(realPath);
-    while (current.length >= this.#rootPath.length) {
+    while (current.length >= this.#realRoot.length) {
+      let resolved;
       try {
-        const resolved = fs.realpathSync(current);
-        if (resolved !== this.#rootPath &&
-            !StringPrototypeStartsWith(resolved, rootWithSep)) {
-          throw createENOENT('open', vfsPath);
-        }
-        return;
+        resolved = fs.realpathSync(current);
       } catch (err) {
+        // A non-existent ancestor: keep walking up. Any other error propagates.
         if (err?.code !== 'ENOENT') throw err;
         current = path.dirname(current);
+        continue;
+      }
+      // Deepest existing ancestor found. Enforce containment OUTSIDE the
+      // try/catch so the rejection is not swallowed by the ENOENT handler.
+      if (resolved !== this.#realRoot &&
+          !StringPrototypeStartsWith(resolved, rootWithSep)) {
+        throw createEACCES('open', vfsPath);
       }
+      return;
     }
   }
 
@@ -456,9 +483,9 @@ class RealFSProvider extends VirtualProvider {
     }
     const realPath = this.#resolvePath(vfsPath);
     const resolvedTarget = path.resolve(path.dirname(realPath), target);
-    const rootWithSep = this.#rootPath.endsWith(path.sep) ?
-      this.#rootPath : this.#rootPath + path.sep;
-    if (resolvedTarget !== this.#rootPath &&
+    const rootWithSep = this.#realRoot.endsWith(path.sep) ?
+      this.#realRoot : this.#realRoot + path.sep;
+    if (resolvedTarget !== this.#realRoot &&
         !StringPrototypeStartsWith(resolvedTarget, rootWithSep)) {
       throw createEACCES('symlink', vfsPath);
     }
@@ -472,9 +499,9 @@ class RealFSProvider extends VirtualProvider {
     }
     const realPath = this.#resolvePath(vfsPath);
     const resolvedTarget = path.resolve(path.dirname(realPath), target);
-    const rootWithSep = this.#rootPath.endsWith(path.sep) ?
-      this.#rootPath : this.#rootPath + path.sep;
-    if (resolvedTarget !== this.#rootPath &&
+    const rootWithSep = this.#realRoot.endsWith(path.sep) ?
+      this.#realRoot : this.#realRoot + path.sep;
+    if (resolvedTarget !== this.#realRoot &&
         !StringPrototypeStartsWith(resolvedTarget, rootWithSep)) {
       throw createEACCES('symlink', vfsPath);
     }
@@ -485,7 +512,7 @@ class RealFSProvider extends VirtualProvider {
   // because fs.realpathSync (a JS impl) preserves case but fs.promises.realpath
   // (native) canonicalizes the drive letter and other components.
   #resolvedToVfsPath(resolved, vfsPath, syscall) {
-    const rel = path.relative(this.#rootPath, resolved);
+    const rel = path.relative(this.#realRoot, resolved);
     if (rel === '') return '/';
     if (rel === '..' ||
         StringPrototypeStartsWith(rel, '..' + path.sep) ||

test/parallel/test-vfs-real-provider-symlinks.js

@@ -94,6 +94,52 @@ const myVfs = vfs.create(new vfs.RealFSProvider(root));
                          { code: 'EACCES' });
   }
 
+  // Regression test (sandbox escape via symlink): read/stat/open/write through
+  // a symlink whose target is OUTSIDE the root must be rejected, never silently
+  // followed. Previously the containment rejection was thrown inside the
+  // `try { fs.realpathSync() }` block in RealFSProvider.#resolvePath and, since
+  // it carried an ENOENT code, was swallowed by the "path doesn't exist yet"
+  // catch -- so the raw (symlink-containing) path was returned and the caller's
+  // real fs op followed it out of the root.
+  {
+    // esc-link (created above) -> <tmpdir>/outside.txt, which contains
+    // 'forbidden' and lives outside the provider root.
+    assert.throws(() => myVfs.readFileSync('/esc-link'), { code: 'EACCES' });
+    assert.throws(() => myVfs.statSync('/esc-link'), { code: 'EACCES' });
+    assert.throws(() => myVfs.openSync('/esc-link', 'r'), { code: 'EACCES' });
+    await assert.rejects(myVfs.promises.readFile('/esc-link'),
+                         { code: 'EACCES' });
+    await assert.rejects(myVfs.promises.open('/esc-link', 'r'),
+                         { code: 'EACCES' });
+
+    // A write through the symlink must not touch the out-of-root file.
+    assert.throws(() => myVfs.writeFileSync('/esc-link', 'pwned'),
+                  { code: 'EACCES' });
+    assert.strictEqual(
+      fs.readFileSync(path.join(tmpdir.path, 'outside.txt'), 'utf8'),
+      'forbidden');
+  }
+
+  // Regression test: creating a NEW file underneath a directory symlink that
+  // points outside the root must be rejected (the ENOENT "create" branch,
+  // which relied on #verifyAncestorInRoot -- previously a no-op because it
+  // swallowed its own containment rejection).
+  {
+    const outsideDir = path.join(tmpdir.path, 'outside-dir');
+    fs.mkdirSync(outsideDir, { recursive: true });
+    fs.symlinkSync(outsideDir, path.join(root, 'esc-dir'));
+
+    assert.throws(() => myVfs.writeFileSync('/esc-dir/pwned.txt', 'x'),
+                  { code: 'EACCES' });
+    assert.throws(() => myVfs.openSync('/esc-dir/pwned.txt', 'w'),
+                  { code: 'EACCES' });
+    await assert.rejects(myVfs.promises.writeFile('/esc-dir/pwned.txt', 'x'),
+                         { code: 'EACCES' });
+
+    // Nothing must have been created outside the root.
+    assert.strictEqual(fs.existsSync(path.join(outsideDir, 'pwned.txt')), false);
+  }
+
   // Realpath on root and on a subdir
   {
     fs.mkdirSync(path.join(root, 'sub2'), { recursive: true });