fix(security): prevent proto pollution in simple assignment

src/index.ts

@@ -208,9 +208,7 @@ function parseComplexParam(queryParams: Object, keys: (string | number)[], value
   let keysLastIndex = keys.length - 1;
   for (let j = 0; j <= keysLastIndex; j++) {
     let key = keys[j] === '' ? (currentParams as any).length : keys[j];
-    if (key === '__proto__') {
-      throw new Error('Prototype pollution detected.');
-    }
+    preventPollution(key);
     if (j < keysLastIndex) {
       // The value has to be an array or a false value
       // It can happen that the value is no array if the key was repeated with traditional style like `list=1&list[]=2`
@@ -267,6 +265,7 @@ export function parseQueryString(queryString: string): Object {
       if (keysLastIndex) {
         parseComplexParam(queryParams, keys, value);
       } else {
+        preventPollution(key);
         queryParams[key] = processScalarParam(queryParams[key], value);
       }
     } else {
@@ -275,3 +274,9 @@ export function parseQueryString(queryString: string): Object {
   }
   return queryParams;
 }
+
+function preventPollution(key: string) {
+  if (key === '__proto__') {
+    throw new Error('Prototype pollution detected.');
+  }
+}

test/path.spec.ts

@@ -337,8 +337,20 @@ describe('query strings', () => {
     });
   });
 
-  it('does not pollute prototype', () => {
-    const path1 = '__proto__[asdf]=asdf';
-    assert.throws(() => parseQueryString(path1), 'Prototype pollution detected');
+  describe('prototype pollution', () => {
+    it('does not allow __proto__ in AccessKey assignment: "__proto__[asdf]=asdf"', () => {
+      const path1 = '__proto__[asdf]=asdf';
+      assert.throws(() => parseQueryString(path1), 'Prototype pollution detected');
+    });
+
+    it('does not attempt on AccessMember like key that has __proto__: "__proto__.asdf=asdf"', () => {
+      const path1 = '__proto__.asdf=asdf';
+      assert.doesNotThrow(() => parseQueryString(path1));
+    });
+
+    it('does not allow __proto__ in simple assignment: "__proto__=x&0[xxx]=xxx"', () => {
+      const path1 = '__proto__=x&0[xxx]=xxx';
+      assert.throws(() => parseQueryString(path1), 'Prototype pollution detected');
+    });
   });
 });