feat: add followSymlinks option to restrict symlink traversal (#297)

index.js

@@ -123,6 +123,10 @@ function SendStream (req, path, options) {
     ? normalizeList(opts.extensions, 'extensions option')
     : []
 
+  this._followSymlinks = opts.followSymlinks !== undefined
+    ? Boolean(opts.followSymlinks)
+    : true
+
   this._immutable = opts.immutable !== undefined
     ? Boolean(opts.immutable)
     : false
@@ -488,6 +492,50 @@ SendStream.prototype.pipe = function pipe (res) {
  */
 
 SendStream.prototype.send = function send (path, stat) {
+  var self = this
+  var root = this._root
+
+  if (this.res.headersSent) {
+    // impossible to send now
+    this.headersAlreadySent()
+    return
+  }
+
+  if (!this._followSymlinks && root !== null) {
+    debug('check symlink "%s"', path)
+    fs.realpath(path, function onrealpath (err, realPath) {
+      if (err) {
+        return self.onStatError(err)
+      }
+
+      fs.realpath(root, function onrealroot (err, realRoot) {
+        if (err) {
+          return self.onStatError(err)
+        }
+
+        if (realPath !== realRoot && !realPath.startsWith(realRoot.endsWith(sep) ? realRoot : realRoot + sep)) {
+          debug('symlink target outside root "%s"', realPath)
+          return self.error(403)
+        }
+
+        self._send(path, stat)
+      })
+    })
+    return
+  }
+
+  this._send(path, stat)
+}
+
+/**
+ * Transfer `path` to response.
+ *
+ * @param {String} path
+ * @param {Object} stat
+ * @api private
+ */
+
+SendStream.prototype._send = function _send (path, stat) {
   var len = stat.size
   var options = this.options
   var opts = {}

test/send.js

@@ -919,6 +919,101 @@ describe('send(file, options)', function () {
     })
   })
 
+  describe('followSymlinks', function () {
+    var petsDir = path.join(fixtures, 'pets')
+    var linkDir = path.join(petsDir, 'outside-dir')
+    var linkFile = path.join(petsDir, 'outside-file.txt')
+    var linkInside = path.join(fixtures, 'inside-symlink.txt')
+    var hasFileSymlink = false
+    var hasDirSymlink = false
+    var hasInsideSymlink = false
+
+    before(function () {
+      try {
+        fs.symlinkSync(path.join(fixtures, 'name.txt'), linkFile)
+        hasFileSymlink = true
+      } catch (e) {
+        // file symlink creation may not be permitted on Windows without privileges
+      }
+
+      try {
+        fs.symlinkSync(path.join(fixtures, 'name.txt'), linkInside)
+        hasInsideSymlink = true
+      } catch (e) {
+        // file symlink creation may not be permitted on Windows without privileges
+      }
+
+      try {
+        var linkType = process.platform === 'win32' ? 'junction' : 'dir'
+        fs.symlinkSync(fixtures, linkDir, linkType)
+        hasDirSymlink = true
+      } catch (e) {
+        // ignore
+      }
+    })
+
+    after(function () {
+      if (hasFileSymlink) {
+        try {
+          fs.unlinkSync(linkFile)
+        } catch (e) {
+          try { fs.rmdirSync(linkFile) } catch (e) {}
+        }
+      }
+      if (hasDirSymlink) {
+        try {
+          fs.unlinkSync(linkDir)
+        } catch (e) {
+          try { fs.rmdirSync(linkDir) } catch (e) {}
+        }
+      }
+      if (hasInsideSymlink) {
+        try {
+          fs.unlinkSync(linkInside)
+        } catch (e) {
+          try { fs.rmdirSync(linkInside) } catch (e) {}
+        }
+      }
+    })
+
+    it('should default to true', function (done) {
+      if (!hasFileSymlink && !hasDirSymlink) return this.skip()
+      var targetUrl = hasFileSymlink ? '/outside-file.txt' : '/outside-dir/name.txt'
+      request(createServer({ root: petsDir }))
+        .get(targetUrl)
+        .expect(200, 'tobi', done)
+    })
+
+    it('should allow symlinks pointing outside root when true', function (done) {
+      if (!hasFileSymlink && !hasDirSymlink) return this.skip()
+      var targetUrl = hasFileSymlink ? '/outside-file.txt' : '/outside-dir/name.txt'
+      request(createServer({ followSymlinks: true, root: petsDir }))
+        .get(targetUrl)
+        .expect(200, 'tobi', done)
+    })
+
+    it('should 403 for symlinks pointing outside root when false', function (done) {
+      if (!hasFileSymlink && !hasDirSymlink) return this.skip()
+      var targetUrl = hasFileSymlink ? '/outside-file.txt' : '/outside-dir/name.txt'
+      request(createServer({ followSymlinks: false, root: petsDir }))
+        .get(targetUrl)
+        .expect(403, done)
+    })
+
+    it('should allow symlinks pointing inside root when false', function (done) {
+      if (!hasInsideSymlink) return this.skip()
+      request(createServer({ followSymlinks: false, root: fixtures }))
+        .get('/inside-symlink.txt')
+        .expect(200, 'tobi', done)
+    })
+
+    it('should allow regular files within root when false', function (done) {
+      request(createServer({ followSymlinks: false, root: petsDir }))
+        .get('/index.html')
+        .expect(200, done)
+    })
+  })
+
   describe('lastModified', function () {
     it('should support disabling last-modified', function (done) {
       request(createServer({ lastModified: false, root: fixtures }))