feat: Optionally omit object key __proto__ and others from parsed output

Add arguments `ignore-proto-key` and `ignore-prototype-keys` and their
corresponding options. If not set, all objects keys will be retained in
the parsed output by default.

BREAKING CHANGE: Object key `__proto__` and other keys from `Object.prototype` are included in the parsed object by default. Earlier, no keys from `Object.prototype` were included. The new behaviour is consistent with `JSON.parse`. If you need the old behaviour, add the argument `ignore-prototype-keys` to the command line, or set the option `ignorePrototypeKeys` to `true`, when calling the `parse` method. If you don't have under control, what will happens with the parsed object, you should consider setting `ignoreProtoKey` to `true`, when calling the `parse` method, to prevent prototype pollution.

README.md

@@ -120,6 +120,8 @@ Usage: `jsonlint [options] [--] [<file, directory, pattern> ...]`
 
     -f, --config <file>          read options from a custom configuration file
     -F, --no-config              disable searching for configuration files
+    --ignore-proto-key           ignore occurrences of "__proto__" object key
+    --ignore-prototype-keys      ignore all keys from "Object.prototype"
     -s, --sort-keys              sort object keys (not when prettifying)
     --sort-keys-ignore-case      sort object keys ignoring the letter case
     --sort-keys-locale <id>      locale identifier to sort object keys with
@@ -199,6 +201,8 @@ The configuration is an object with the following properties, described above, w
 | Parameter | Alias |
 | --------- | ----- |
 | patterns | |
+| ignore-proto-key | ignoreProtoKey |
+| ignore-prototype-keys | ignorePrototypeKeys |
 | sort-keys | sortKeys |
 | sort-keys-ignore-case | sortKeysIgnoreCase |
 | sort-keys-locale | sortKeysLocale |
@@ -274,6 +278,8 @@ The `parse` method offers more detailed [error information](#error-handling), th
 | `ignoreTrailingCommas`     | ignores trailing commas in objects and arrays (boolean)      |
 | `allowSingleQuotedStrings` | accepts strings delimited by single-quotes too (boolean)     |
 | `allowDuplicateObjectKeys` | allows reporting duplicate object keys as an error (boolean) |
+| `ignoreProtoKey` | ignore occurrences of the `__proto__` object key (boolean) |
+| `ignorePrototypeKeys` | ignore all keys from `Object.prototype` (boolean) |
 | `mode`                     | sets multiple options according to the type of input data (string) |
 | `reviver`                  | converts object and array values (function) |
 

lib/cli.js

@@ -17,6 +17,8 @@ Usage: jsonlint [options] [--] [<file, directory, pattern> ...]
 Options:
   -f, --config <file>          read options from a custom configuration file
   -F, --no-config              disable searching for configuration files
+  --ignore-proto-key           ignore occurrences of "__proto__" object key
+  --ignore-prototype-keys      ignore all keys from "Object.prototype"
   -s, --sort-keys              sort object keys (not when prettifying)
   --sort-keys-ignore-case      sort object keys ignoring the letter case
   --sort-keys-locale <id>      locale identifier to sort object keys with
@@ -89,6 +91,12 @@ for (let i = 2, l = argv.length; i < l; ++i) {
         case 'F':
           params.config = false
           return
+        case 'ignore-proto-key':
+          params.ignoreProtoKey = flag
+          return
+        case 'ignore-prototype-keys':
+          params.ignorePrototypeKeys = flag
+          return
         case 's': case 'sort-keys':
           params.sortKeys = flag
           return
@@ -238,6 +246,8 @@ for (let i = 2, l = argv.length; i < l; ++i) {
 }
 
 const paramNames = {
+  'ignore-proto-key': 'ignoreProtoKey',
+  'ignore-prototype-keys': 'ignorePrototypeKeys',
   'trailing-commas': 'trailingCommas',
   'single-quoted-strings': 'singleQuotedStrings',
   'duplicate-keys': 'duplicateKeys',
@@ -323,7 +333,9 @@ function processContents (source, file) {
       ignoreComments: params.comments,
       ignoreTrailingCommas: params.trailingCommas || params.trimTrailingCommas,
       allowSingleQuotedStrings: params.singleQuotedStrings,
-      allowDuplicateObjectKeys: params.duplicateKeys
+      allowDuplicateObjectKeys: params.duplicateKeys,
+      ignoreProtoKey: params.ignoreProtoKey,
+      ignorePrototypeKeys: params.ignorePrototypeKeys
     }
     if (params.validate.length) {
       const schemas = params.validate.map((file, index) => {

lib/validator.js

@@ -167,7 +167,9 @@
       ignoreComments: options.ignoreComments,
       ignoreTrailingCommas: options.ignoreTrailingCommas,
       allowSingleQuotedStrings: options.allowSingleQuotedStrings,
-      allowDuplicateObjectKeys: options.allowDuplicateObjectKeys
+      allowDuplicateObjectKeys: options.allowDuplicateObjectKeys,
+      ignoreProtoKey: options.ignoreProtoKey,
+      ignorePrototypeKeys: options.ignorePrototypeKeys
     }
     const validate = compileSchema(ajv, schema, parseOptions)
     return function (data, input, options) {

src/configurable-parser.js

@@ -6,7 +6,8 @@ const oldNode = typeof process !== 'undefined' && process.version.startsWith('v4
 function needsCustomParser (options) {
   return options.ignoreBOM || options.ignoreComments || options.ignoreTrailingCommas ||
   options.allowSingleQuotedStrings || options.allowDuplicateObjectKeys === false ||
-  options.mode === 'cjson' || options.mode === 'json5' || isSafari || oldNode
+  options.ignoreProtoKey || options.ignorePrototypeKeys || options.mode === 'cjson' ||
+  options.mode === 'json5' || isSafari || oldNode
 }
 
 function getReviver (options) {

src/custom-parser.js

@@ -37,6 +37,8 @@ const unescapeMap = {
 
 const ownsProperty = Object.prototype.hasOwnProperty
 
+const emptyObject = {}
+
 function parseInternal (input, options) {
   if (typeof input !== 'string' || !(input instanceof String)) {
     input = String(input)
@@ -46,6 +48,8 @@ function parseInternal (input, options) {
   const ignoreBOM = options.ignoreBOM
   const ignoreComments = options.ignoreComments || options.mode === 'cjson' || json5
   const ignoreTrailingCommas = options.ignoreTrailingCommas || json5
+  const ignoreProtoKey = options.ignoreProtoKey
+  const ignorePrototypeKeys = options.ignorePrototypeKeys
   const allowSingleQuotedStrings = options.allowSingleQuotedStrings || json5
   const allowDuplicateObjectKeys = options.allowDuplicateObjectKeys
   const reviver = options.reviver
@@ -313,8 +317,7 @@ function parseInternal (input, options) {
   }
 
   function parseObject () {
-    const result = {}
-    const emptyObject = {}
+    let result = {}
     let isNotEmpty = false
 
     while (position < inputLength) {
@@ -346,15 +349,20 @@ function parseInternal (input, options) {
           }
         }
 
-        if (key in emptyObject || emptyObject[key] != null) {
+        if ((ignorePrototypeKeys && (key in emptyObject || emptyObject[key] != null)) ||
+            (ignoreProtoKey && key === '__proto__')) {
           // silently ignore it
         } else {
           if (reviver) {
             value = reviver(key, value)
           }
           if (value !== undefined) {
             isNotEmpty = true
-            result[key] = value
+            if (key === '__proto__') {
+              result = Object.assign(JSON.parse(`{"__proto__":${JSON.stringify(value)}}`), result)
+            } else {
+              result[key] = value
+            }
           }
         }
 

test/parse2.js

@@ -168,6 +168,18 @@ test('no prototype pollution', function () {
   assert.notDeepEqual(parsed, { polluted: true })
 })
 
+test('forbid __proto__ key', function () {
+  const parsed = parse('{ "constructor": true, "__proto__": { "polluted": true } }', { ignoreProtoKey: true })
+  assert.notDeepEqual(parsed, JSON.parse('{ "constructor": true, "__proto__": { "polluted": true } }'))
+  assert.strictEqual(parsed.constructor, true)
+})
+
+test('forbid prototype keys', function () {
+  const parsed = parse('{ "constructor": true, "__proto__": { "polluted": true } }', { ignorePrototypeKeys: true })
+  assert.notDeepEqual(parsed, JSON.parse('{ "constructor": true, "__proto__": { "polluted": true } }'))
+  assert.strictEqual(typeof parsed.constructor, 'function')
+})
+
 test('random numbers', function () {
   for (let i = 0; i < 100; ++i) {
     const str = '-01.e'.split('')

web/jsonlint.html

@@ -119,6 +119,14 @@ <h1>JSON Lint</h1>
               <input type="checkbox" checked id="duplicate-object-keys">
               <label for="duplicate-object-keys">Allow duplicate object keys</label>
             </div>
+            <div>
+              <input type="checkbox" checked id="ignore-proto-key">
+              <label for="ignore-proto-key">Ignore __proto__ key</label>
+            </div>
+            <div>
+              <input type="checkbox" checked id="ignore-prototype-keys">
+              <label for="ignore-prototype-keys">Ignore Object.prototype keys</label>
+            </div>
           </div>
           <div>
             <span>Formatting:</span>
@@ -203,7 +211,7 @@ <h2>Result</h2>
   </main>
   <hr>
   <footer>
-    <small>Copyright &copy; 2012-2023 Zachary Carter, Ferdinand Prantl. See the <a href="https://github.com/prantlf/jsonlint#json-lint">project pages</a> to learn about command-line validation and programmatic usage.</small>
+    <small>Copyright &copy; 2012-2024 Zachary Carter, Ferdinand Prantl. See the <a href="https://github.com/prantlf/jsonlint#json-lint">project pages</a> to learn about command-line validation and programmatic usage.</small>
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   </footer>
@@ -367,6 +375,8 @@ <h2>Result</h2>
             allowSingleQuotedStrings: document.getElementById('single-quoted-strings').checked ||
                                       mode === 'json5',
             allowDuplicateObjectKeys: document.getElementById('duplicate-object-keys').checked,
+            ignoreProtoKey: document.getElementById('ignore-proto-key').checked,
+            ignorePrototypeKeys: document.getElementById('ignore-prototype-keys').checked
           }
         }
       }