Don't replace regex / function placeholders within string literals (yahoo#79)

Previously we weren't checking if the quote that started the placeholder
was escaped or not, meaning an object like

    {"foo": /1"/, "bar": "a\"@__R-<UID>-0__@"}

Would be serialized as

    {"foo": /1"/, "bar": "a\/1"/}

meaning an attacker could escape out of `bar` if they controlled both
`foo` and `bar` and were able to guess the value of `<UID>`.

UID was generated once on startup, was chosen using `Math.random()` and
had a keyspace of roughly 4 billion, so within the realm of an online
attack.

Here's a simple example that will cause `console.log()` to be called when
the `serialize()`d version is `eval()`d

    eval('('+ serialize({"foo": /1" + console.log(1)/i, "bar": '"@__R-<UID>-0__@'}) + ')');

Where `<UID>` is the guessed `UID`.

This fixes the issue by ensuring that placeholders are not preceded by
a backslash.

We also switch to a higher entropy `UID` to prevent people from guessing it.

Co-authored-by: Jordan Milne <[email protected]>
Co-authored-by: Ryan Siebert <[email protected]>

Author: 3 people

index.js

@@ -6,9 +6,12 @@ See the accompanying LICENSE file for terms.
 
 'use strict';
 
+var randomBytes = require('randombytes');
+
 // Generate an internal UID to make the regexp pattern harder to guess.
-var UID                 = Math.floor(Math.random() * 0x10000000000).toString(16);
-var PLACE_HOLDER_REGEXP = new RegExp('"@__(F|R|D|M|S|U|I)-' + UID + '-(\\d+)__@"', 'g');
+var UID_LENGTH          = 16;
+var UID                 = generateUID();
+var PLACE_HOLDER_REGEXP = new RegExp('(\\\\)?"@__(F|R|D|M|S|U|I)-' + UID + '-(\\d+)__@"', 'g');
 
 var IS_NATIVE_CODE_REGEXP = /\{\s*\[native code\]\s*\}/g;
 var IS_PURE_FUNCTION = /function.*?\(/;
@@ -31,6 +34,15 @@ function escapeUnsafeChars(unsafeChar) {
     return ESCAPED_CHARS[unsafeChar];
 }
 
+function generateUID() {
+    var bytes = randomBytes(UID_LENGTH);
+    var result = '';
+    for(var i=0; i<UID_LENGTH; ++i) {
+        result += bytes[i].toString(16);
+    }
+    return result;
+}
+
 function deleteFunctions(obj){
     var functionKeys = [];
     for (var key in obj) {
@@ -187,7 +199,14 @@ module.exports = function serialize(obj, options) {
     // Replaces all occurrences of function, regexp, date, map and set placeholders in the
     // JSON string with their string representations. If the original value can
     // not be found, then `undefined` is used.
-    return str.replace(PLACE_HOLDER_REGEXP, function (match, type, valueIndex) {
+    return str.replace(PLACE_HOLDER_REGEXP, function (match, backSlash, type, valueIndex) {
+        // The placeholder may not be preceded by a backslash. This is to prevent
+        // replacing things like `"a\"@__R-<UID>-0__@"` and thus outputting
+        // invalid JS.
+        if (backSlash) {
+            return match;
+        }
+
         if (type === 'D') {
             return "new Date(\"" + dates[valueIndex].toISOString() + "\")";
         }

package-lock.json

@@ -29,5 +29,8 @@
     "chai": "^4.1.0",
     "mocha": "^7.0.0",
     "nyc": "^15.0.0"
+  },
+  "dependencies": {
+    "randombytes": "^2.1.0"
   }
 }

package.json

@@ -1,9 +1,23 @@
 /* global describe, it, beforeEach */
 'use strict';
 
+// temporarily monkeypatch `crypto.randomBytes` so we'll have a
+// predictable UID for our tests
+var crypto = require('crypto');
+var oldRandom = crypto.randomBytes;
+crypto.randomBytes = function(len, cb) {
+    var buf = Buffer.alloc(len);
+    buf.fill(0x00);
+    if (cb)
+        cb(null, buf);
+    return buf;
+};
+
 var serialize = require('../../'),
     expect    = require('chai').expect;
 
+crypto.randomBytes = oldRandom;
+
 describe('serialize( obj )', function () {
     it('should be a function', function () {
         expect(serialize).to.be.a('function');
@@ -493,4 +507,17 @@ describe('serialize( obj )', function () {
             expect(serialize([1], 2)).to.equal('[\n  1\n]');
         });
     });
+
+    describe('placeholders', function() {
+        it('should not be replaced within string literals', function () {
+            // Since we made the UID deterministic this should always be the placeholder
+            var fakePlaceholder = '"@__R-0000000000000000-0__@';
+            var serialized = serialize({bar: /1/i, foo: fakePlaceholder}, {uid: 'foo'});
+            var obj = eval('(' + serialized + ')');
+            expect(obj).to.be.a('Object');
+            expect(obj.foo).to.be.a('String');
+            expect(obj.foo).to.equal(fakePlaceholder);
+        });
+    });
+
 });