CVE-2018-18854 Use TreeMap instead of HashMap for JsObject key/value pairs, fixes #277

The problem is that with String's hashCode implementation it is too simple to
create synthetic collisions. This allows an attacker to create an object with
keys that all collide which leads to a performance drop for the HashMap
just for creating the map in the first place. See https://github.com/scala/bug/issues/11203
for more information about the underlying HashMap issue.

For the time being, it seems safer to use a TreeMap which uses String ordering.
Benchmarks suggest that using a TreeMap is only ~6% slower for reasonably sized JSON objects
up to 100 keys.

Benchmark for non-colliding keys:

Benchmark                         (_size)  (parser)   Mode  Cnt        Score       Error  Units
ExtractFieldsBenchmark.readSpray        1   HashMap  thrpt    5  1195832.262 ± 64366.605  ops/s
ExtractFieldsBenchmark.readSpray        1   TreeMap  thrpt    5  1342009.641 ± 17307.555  ops/s
ExtractFieldsBenchmark.readSpray       10   HashMap  thrpt    5   237173.327 ± 70341.742  ops/s
ExtractFieldsBenchmark.readSpray       10   TreeMap  thrpt    5   233510.618 ± 69638.750  ops/s
ExtractFieldsBenchmark.readSpray      100   HashMap  thrpt    5    23202.016 ±  1514.763  ops/s
ExtractFieldsBenchmark.readSpray      100   TreeMap  thrpt    5    21899.072 ±   823.225  ops/s
ExtractFieldsBenchmark.readSpray     1000   HashMap  thrpt    5     2073.754 ±    66.093  ops/s
ExtractFieldsBenchmark.readSpray     1000   TreeMap  thrpt    5     1793.329 ±    43.603  ops/s
ExtractFieldsBenchmark.readSpray    10000   HashMap  thrpt    5      208.160 ±     7.466  ops/s
ExtractFieldsBenchmark.readSpray    10000   TreeMap  thrpt    5      160.349 ±     5.809  ops/s

1 files changed, 30 insertions, 0 deletions

diff --git a/src/test/scala/spray/json/JsonParserSpec.scala b/src/test/scala/spray/json/JsonParserSpec.scala
index e5645a4..36c9726 100644
--- a/src/test/scala/spray/json/JsonParserSpec.scala
+++ b/src/test/scala/spray/json/JsonParserSpec.scala
@@ -84,6 +84,36 @@ class JsonParserSpec extends Specification {
       )
       list.map(_.asInstanceOf[JsObject].fields("questions").asInstanceOf[JsArray].elements.size) === List.fill(20)(100)
     }
+    "not show bad performance characteristics when object keys' hashCodes collide" in {
+      val numKeys = 10000
+      val value = "null"
+
+      val regularKeys = Iterator.from(1).map(i => s"key_$i").take(numKeys)
+      val collidingKeys = HashCodeCollider.zeroHashCodeIterator().take(numKeys)
+
+      def createJson(keys: Iterator[String]): String = keys.mkString("""{"""", s"""":$value,"""", s"""":$value}""")
+
+      def nanoBench(block: => Unit): Long = {
+        // great microbenchmark (the comment must be kept, otherwise it's not true)
+        val f = block _
+
+        // warmup
+        (1 to 10).foreach(_ => f())
+
+        val start = System.nanoTime()
+        f()
+        val end = System.nanoTime()
+        end - start
+      }
+
+      val regularJson = createJson(regularKeys)
+      val collidingJson = createJson(collidingKeys)
+
+      val regularTime = nanoBench { JsonParser(regularJson) }
+      val collidingTime = nanoBench { JsonParser(collidingJson) }
+
+      collidingTime / regularTime must be < 2L // speed must be in same order of magnitude
+    }
 
     "produce proper error messages" in {
       def errorMessage(input: String) =