Prevent prototype pollution while parsing query strings (#2494)

* Prevent prototype pollution while parsing query strings

* Update changelog [skip ci]

docs/change-log.md

@@ -17,6 +17,7 @@
 ### Upcoming...
 
 - Ensure vnodes are removed correctly in the face of `onbeforeremove` resolving after new nodes are added ([#2492](https://github.com/MithrilJS/mithril.js/pull/2492) [@isiahmeadows](https://github.com/isiahmeadows))
+- Fix prototype pollution vulnerability in `m.parseQueryString` ([#2494](https://github.com/MithrilJS/mithril.js/pull/2494) [@isiahmeadows](https://github.com/isiahmeadows))
 
 -->
 

querystring/parse.js

@@ -1,7 +1,5 @@
 "use strict"
 
-// The extra `data` parameter is for if you want to append to an existing
-// parameters object.
 module.exports = function(string) {
 	if (string === "" || string == null) return {}
 	if (string.charAt(0) === "?") string = string.slice(1)
@@ -29,9 +27,17 @@ module.exports = function(string) {
 				}
 				level = counters[key]++
 			}
+			// Disallow direct prototype pollution
+			else if (level === "__proto__") break
 			if (isValue) cursor[level] = value
-			else if (cursor[level] == null) cursor[level] = isNumber ? [] : {}
-			cursor = cursor[level]
+			else {
+				// Read own properties exclusively to disallow indirect
+				// prototype pollution
+				value = Object.getOwnPropertyDescriptor(cursor, level)
+				if (value != null) value = value.value
+				if (value == null) value = cursor[level] = isNumber ? [] : {}
+			}
+			cursor = value
 		}
 	}
 	return data

querystring/tests/test-parseQueryString.js

@@ -97,4 +97,16 @@ o.spec("parseQueryString", function() {
 		var data = parseQueryString("a=1&b=2&a=3")
 		o(data).deepEquals({a: "3", b: "2"})
 	})
+	o("doesn't pollute prototype directly, censors `__proto__`", function() {
+		var prev = Object.prototype.toString
+		var data = parseQueryString("a=b&__proto__%5BtoString%5D=123")
+		o(Object.prototype.toString).equals(prev)
+		o(data).deepEquals({a: "b"})
+	})
+	o("doesn't pollute prototype indirectly, retains `constructor`", function() {
+		var prev = Object.prototype.toString
+		var data = parseQueryString("constructor%5Bprototype%5D%5BtoString%5D=123")
+		o(Object.prototype.toString).equals(prev)
+		o(data).deepEquals({a: "b"})
+	})
 })