fix: use strict mode recommendation

Author: commenthol

README.md

@@ -41,11 +41,39 @@ npm install --save safer-eval
 
 ## Implementation recommendations
 
-Be aware that a `saferEval('function(){while(true){}}()')` may run
+**Use strict mode**
+
+Always use `'use strict'` mode in functions/ files calling `saferEval()`.
+Otherwise a sandbox breakout may be possible.
+
+```js
+
+'use strict'
+const saferEval = require('safer-eval')
+
+function main () {
+  'use strict' //< alternative within function
+  const res = saferEval('new Date()')
+  ...
+}
+
+```
+
+**Run in worker**
+
+Be aware that a
+
+```js
+saferEval('(function () { while (true) {} })()')
+```
+
+may run
 infinitely. Consider using the module from within a worker thread which is terminated
 after timeout.
 
-Avoid passing context props while deserializing data from hostile environments.
+**Avoid context props**
+
+Avoid passing `context` props while deserializing data from hostile environments.
 
 ## Usage
 
@@ -66,19 +94,23 @@ Check the tests under "harmful context"!
 in node:
 
 ```js
-var saferEval = require('safer-eval')
-var code = `{d: new Date('1970-01-01'), b: new Buffer('data')}`
-var res = saferEval(code)
+'use strict' //< NEVER FORGET TO ADD STRICT MODE in file/ function
+             //< running `saferEval`
+const saferEval = require('safer-eval')
+const code = `{d: new Date('1970-01-01'), b: new Buffer('data')}`
+const res = saferEval(code)
 // => toString.call(res.d) = '[object Date]'
 // => toString.call(res.b) = '[object Buffer]'
 ```
 
 in browser:
 
 ```js
-var saferEval = require('safer-eval')
-var code = `{d: new Date('1970-01-01'), b: function () { return navigator.userAgent }`
-var res = saferEval(code, {navigator: window.navigator})
+'use strict' //< NEVER FORGET TO ADD STRICT MODE in file/ function
+             //< running `saferEval`
+const saferEval = require('safer-eval')
+const code = `{d: new Date('1970-01-01'), b: function () { return navigator.userAgent }`
+const res = saferEval(code, {navigator: window.navigator})
 // => toString.call(res.d) = '[object Date]'
 // => toString.call(res.b) = '[object Function]'
 // => res.b() = "Mozilla/5.0 (..."
@@ -87,19 +119,19 @@ var res = saferEval(code, {navigator: window.navigator})
 To minimize any harmful code injection carefully select the methods you allow in `context`
 
 ```js
-var code = `window.btoa('Hello, world')`
+const code = `window.btoa('Hello, world')`
 
 // AVOID passing a GLOBAL context!!!
-var res = saferEval(code, {window: window})
+const res = saferEval(code, {window: window})
 
 // BETTER - code needs only access to window.btoa
 const clones = require('clones')
-var context = {
+const context = {
   window: {
     btoa: clones(window.btoa, window)
   }
 }
-var res = saferEval(code ,context)
+const res = saferEval(code ,context)
 // => res = 'SGVsbG8sIHdvcmxk'
 ```
 
@@ -108,10 +140,12 @@ var res = saferEval(code ,context)
 Use `new SaferEval()` to reuse a once created context.
 
 ```js
-const {SaferEval} = require('safer-eval')
+'use strict' //< NEVER FORGET TO ADD STRICT MODE in file/ function
+             //< running `saferEval`
+const { SaferEval } = require('safer-eval')
 const safer = new SaferEval()
-var code = `{d: new Date('1970-01-01'), b: new Buffer('data')}`
-var res = safer.runInContext(code)
+const code = `{d: new Date('1970-01-01'), b: new Buffer('data')}`
+const res = safer.runInContext(code)
 ```
 
 ## License

package.json

@@ -23,13 +23,17 @@
     "test": "test"
   },
   "scripts": {
-    "all": "npm run clean && npm run lint && npm run transpile && npm test",
+    "preall": "npm run clean",
+    "all": "npm test",
     "clean": "rimraf lib",
     "coverage": "nyc -r html -r text npm test",
+    "prekarma": "npm run transpile",
     "karma": "karma start",
-    "lint": "eslint --fix \"**/*.js\"",
+    "lint": "eslint --fix src test *.js",
     "prepublishOnly": "npm run all",
+    "pretest": "npm run transpile",
     "test": "mocha",
+    "posttest": "npm run lint",
     "transpile": "babel -d lib src",
     "zuul": "zuul --no-coverage --local 3000 -- test/*.js"
   },

src/common.js

@@ -8,6 +8,8 @@ exports.hasWindow = hasWindow
 const hasGlobal = (typeof global !== 'undefined')
 exports.hasGlobal = hasGlobal
 
+const FN_NOOP = 'function () {}'
+
 const NON_IDENTIFIER = /^\d|-|^(break|case|catch|continue|debugger|default|delete|do|else|finally|for|function|if|in|instanceof|new|return|switch|this|throw|try|typeof|var|void|while|with|class|const|enum|export|extends|import|super|implements|interface|let|package|private|protected|public|static|yield|null|true|false)$/
 
 const isIdentifier = key => !NON_IDENTIFIER.test(key)
@@ -47,15 +49,15 @@ exports.createContext = function () {
     cloneFunctions(context)
     context.Buffer = _protect('Buffer')
     context.console = clones(console, console) // console needs special treatment
-    context.console.constructor.constructor = 'function () {}'
+    context.console.constructor.constructor = FN_NOOP
   }
   if (hasWindow) {
     fillContext(window, true)
     cloneFunctions(context)
     protectBuiltInObjects(context)
     context.console = clones(console, console) // console needs special treatment
     try {
-      context.Object.constructor.constructor = 'function () {}'
+      context.Object.constructor.constructor = FN_NOOP
     } catch (e) {
     }
   }
@@ -123,7 +125,6 @@ function cloneFunctions (context) {
 function protectBuiltInObjects (context) {
   ;[
     'Object',
-    // 'Object.constructor.constructor',
     'Boolean',
     'Symbol',
     'Error',

src/index.js

@@ -40,14 +40,12 @@ class SaferEval {
     if (typeof code !== 'string') {
       throw new TypeError('not a string')
     }
-    let src = 'Object.constructor = function () {};\n'
+    let src = '(function () {"use strict";\n'
+    src += 'Object.constructor = function () {};\n'
     src += 'return ' + code + ';\n'
+    src += '})()'
 
-    return vm.runInContext(
-      '(function () {"use strict"; ' + src + '})()',
-      this._context,
-      this._options
-    )
+    return vm.runInContext(src, this._context, this._options)
   }
 }
 
@@ -72,6 +70,7 @@ class SaferEval {
 * // => toString.call(res.b) = '[object Buffer]'
 */
 function saferEval (code, context) {
+  'use strict'
   return new SaferEval(context).runInContext(code)
 }
 

test/saferEval.spec.js

@@ -1,5 +1,7 @@
 /* eslint no-new-func:0 */
 
+'use strict'
+
 var assert = require('assert')
 var clones = require('clones')
 var saferEval = require('..')
@@ -295,6 +297,23 @@ describe('#saferEval', function () {
         }
         assert.strictEqual(res, undefined)
       })
+      it('should prevent a breakout using function.caller - NEEDS "use strict" being set', function () {
+        'use strict'
+
+        let res
+        try {
+          const stmt = `(() => {
+            function f() {
+              return f.caller.constructor('return global')();
+            }
+            return f.constructor('return ' + f)();
+          })();
+          `
+          res = saferEval(stmt)()
+        } catch (e) {
+        }
+        assert.strictEqual(res, undefined)
+      })
     })
 
     describeBrowser('in browser', function () {
@@ -331,6 +350,22 @@ describe('#saferEval', function () {
         }
         assert.strictEqual(res, undefined)
       })
+      it('should prevent a breakout using function.caller - NEEDS "use strict" being set', function () {
+        'use strict'
+
+        let res
+        try {
+          const stmt = `(() => {
+            function f() {
+              return f.caller.constructor('return window')();
+            }
+            return f.constructor('return ' + f)();
+            })()`
+          res = saferEval(stmt)()
+        } catch (e) {
+        }
+        assert.strictEqual(res, undefined)
+      })
     })
   })