JS: Add o[o.length] = y taint step

Author: asgerf

javascript/ql/src/semmle/javascript/dataflow/TaintTracking.qll

@@ -584,22 +584,26 @@ module TaintTracking {
 
   /**
    * A taint propagating data flow edge for assignments of the form `o[k] = v`, where
-   * `k` is not a constant and `o` refers to some object literal; in this case, we consider
-   * taint to flow from `v` to that object literal.
+   * one of the following holds:
    *
-   * The rationale for this heuristic is that if properties of `o` are accessed by
-   * computed (that is, non-constant) names, then `o` is most likely being treated as
-   * a map, not as a real object. In this case, it makes sense to consider the entire
-   * map to be tainted as soon as one of its entries is.
+   * - `k` is not a constant and `o` refers to some object literal. The rationale
+   *   here is that `o` is most likely being used like a dictionary object.
+   *
+   * - `k` refers to `o.length`, that is, the assignment is of form `o[o.length] = v`.
+   *   In this case, the assignment behaves like `o.push(v)`.
    */
-  private class DictionaryTaintStep extends SharedTaintStep {
+  private class ComputedPropWriteTaintStep extends SharedTaintStep {
     override predicate heapStep(DataFlow::Node pred, DataFlow::Node succ) {
-      exists(AssignExpr assgn, IndexExpr idx, DataFlow::ObjectLiteralNode obj |
+      exists(AssignExpr assgn, IndexExpr idx, DataFlow::SourceNode obj |
         assgn.getTarget() = idx and
         obj.flowsToExpr(idx.getBase()) and
         not exists(idx.getPropertyName()) and
         pred = DataFlow::valueNode(assgn.getRhs()) and
         succ = obj
+      |
+        obj instanceof DataFlow::ObjectLiteralNode
+        or
+        obj.getAPropertyRead("length").flowsToExpr(idx.getPropertyNameExpr())
       )
     }
   }

javascript/ql/test/library-tests/TaintTracking/BasicTaintTracking.expected

@@ -11,6 +11,7 @@ typeInferenceMismatch
 | array-mutation.js:27:16:27:23 | source() | array-mutation.js:28:8:28:8 | g |
 | array-mutation.js:31:33:31:40 | source() | array-mutation.js:32:8:32:8 | h |
 | array-mutation.js:35:36:35:43 | source() | array-mutation.js:36:8:36:8 | i |
+| array-mutation.js:39:17:39:24 | source() | array-mutation.js:40:8:40:8 | j |
 | booleanOps.js:2:11:2:18 | source() | booleanOps.js:4:8:4:8 | x |
 | booleanOps.js:2:11:2:18 | source() | booleanOps.js:13:10:13:10 | x |
 | booleanOps.js:2:11:2:18 | source() | booleanOps.js:19:10:19:10 | x |

javascript/ql/test/library-tests/TaintTracking/array-mutation.js

@@ -34,4 +34,8 @@ function test(x, y) {
   let i = [];
   Array.prototype.unshift.apply(i, source());
   sink(i); // NOT OK
+
+  let j = [];
+  j[j.length] = source();
+  sink(j); // NOT OK
 }