Python : Arbitrary code execution due to Js2Py

python/ql/src/experimental/Security/CWE-094/Js2Py.qhelp

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+<!DOCTYPE qhelp SYSTEM "qhelp.dtd">
+<qhelp>
+    <overview>
+        <p>
+            Passing untrusted inputs to a JavaScript interpreter like `Js2Py` can lead to arbitrary
+            code execution.
+        </p>
+    </overview>
+    <recommendation>
+        <p> This vulnerability can be prevented either by preventing an untrusted user input to flow
+            to an <code>eval_js</code> call. Or, the impact of this vulnerability can be
+            significantly reduced by disabling imports from the interepreted code (note that in a <a
+                href="https://github.com/PiotrDabkowski/Js2Py/issues/45#issuecomment-258724406">
+            comment</a> the author of the library highlights that Js2Py is still insecure with this
+            option).</p>
+    </recommendation>
+    <example>
+        <p>In the example below, the Javascript code being evaluated is controlled by the user and
+            hence leads to arbitrary code execution.</p>
+        <sample src="Js2pyBad.py" />
+        <p>This can be fixed by disabling imports before evaluating the user passed buffer.</p>
+        <sample src="Js2pyGood.py" />
+    </example>
+</qhelp>

python/ql/src/experimental/Security/CWE-094/Js2Py.ql

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+/**
+ * @name JavaScript code execution.
+ * @description Passing user supplied arguments to a Javascript to Python translation engine such as Js2Py can lead to remote code execution.
+ * @problem.severity error
+ * @security-severity 9.3
+ * @precision high
+ * @kind path-problem
+ * @id py/js2py-rce
+ * @tags security
+ *       experimental
+ *       external/cwe/cwe-94
+ */
+
+import python
+import semmle.python.ApiGraphs
+import semmle.python.dataflow.new.TaintTracking
+import semmle.python.dataflow.new.RemoteFlowSources
+import semmle.python.Concepts
+
+module Js2PyFlowConfig implements DataFlow::ConfigSig {
+  predicate isSource(DataFlow::Node node) { node instanceof RemoteFlowSource }
+
+  predicate isSink(DataFlow::Node node) {
+    API::moduleImport("js2py").getMember(["eval_js", "eval_js6", "EvalJs"]).getACall().getArg(_) =
+      node
+  }
+}
+
+module Js2PyFlow = TaintTracking::Global<Js2PyFlowConfig>;
+
+import Js2PyFlow::PathGraph
+
+from Js2PyFlow::PathNode source, Js2PyFlow::PathNode sink
+where
+  Js2PyFlow::flowPath(source, sink) and
+  not exists(API::moduleImport("js2py").getMember("disable_pyimport").getACall())
+select sink, source, sink, "This input to Js2Py depends on a $@.", source.getNode(),
+  "user-provided value"

python/ql/src/experimental/Security/CWE-094/Js2pyBad.py

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+@bp.route("/bad")
+def bad():
+    jk = flask.request.form["jk"]
+    jk = eval_js(f"{jk} f()")

python/ql/src/experimental/Security/CWE-094/Js2pyGood.py

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+@bp.route("/good")
+def good():
+    # disable python imports to prevent execution of malicious code 
+    js2py.disable_pyimport()
+    jk = flask.request.form["jk"]
+    jk = eval_js(f"{jk} f()")

python/ql/test/experimental/query-tests/Security/CWE-094/Js2Py.expected

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+edges
+| Js2PyTest.py:9:5:9:6 | ControlFlowNode for jk | Js2PyTest.py:10:18:10:28 | ControlFlowNode for Fstring | provenance |  |
+| Js2PyTest.py:9:10:9:22 | ControlFlowNode for Attribute | Js2PyTest.py:9:5:9:6 | ControlFlowNode for jk | provenance | AdditionalTaintStep |
+nodes
+| Js2PyTest.py:9:5:9:6 | ControlFlowNode for jk | semmle.label | ControlFlowNode for jk |
+| Js2PyTest.py:9:10:9:22 | ControlFlowNode for Attribute | semmle.label | ControlFlowNode for Attribute |
+| Js2PyTest.py:10:18:10:28 | ControlFlowNode for Fstring | semmle.label | ControlFlowNode for Fstring |
+subpaths
+#select
+| Js2PyTest.py:10:18:10:28 | ControlFlowNode for Fstring | Js2PyTest.py:9:10:9:22 | ControlFlowNode for Attribute | Js2PyTest.py:10:18:10:28 | ControlFlowNode for Fstring | This input to Js2Py depends on a $@. | Js2PyTest.py:9:10:9:22 | ControlFlowNode for Attribute | user-provided value |

python/ql/test/experimental/query-tests/Security/CWE-094/Js2Py.qlref

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+experimental/Security/CWE-094/Js2Py.ql

python/ql/test/experimental/query-tests/Security/CWE-094/Js2PyTest.py

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+
+import flask
+from js2py import eval_js, disable_pyimport
+
+bp = flask.Blueprint("app", __name__, url_prefix="/")
+
+@bp.route("/bad")
+def bad():
+    jk = flask.request.form["jk"]
+    jk = eval_js(f"{jk} f()")