Python: New query to check for use of jinja2 templates without auto-escaping.

python/ql/src/Security/CWE-079/Jinja2WithoutEscaping.qhelp

@@ -0,0 +1,42 @@
+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+<overview>
+<p>
+
+Cross-site scripting attacks can occur if untrusted input is not escaped. This applies to templates as well as code.
+The <code>jinja2</code> templates may be vulnerable to XSS if the environment has <code>autoescape</code> set to <code>False</code>.
+Unfortunately, <code>jinja2</code> sets <code>autoescape</code> to <code>False</code> by default.
+Explicitly setting <code>autoescape</code> to <code>True</code> when creating an <code>Environment</code> object will prevent this.
+</p>
+</overview>
+
+<recommendation>
+<p>
+Avoid setting jinja2 autoescape to False.
+Jinja2 provides the function <code>select_autoescape</code> to make sure that the correct auto-escaping is chosen.
+For example, it can be used when creating an environment <code>Environment(autoescape=select_autoescape(['html', 'xml'])</code>
+</p>
+</recommendation>
+
+<example>
+<p>
+The following example is a minimal flask app which shows a safe and unsafe way to render the given name back to the page.
+The first view is unsafe as <code>first_name</code> is not escaped, leaving the page vulnerable to cross-site scripting attacks.
+The second view is safe as <code>first_name</code> is escaped, so it is not vulnerable to cross-site scripting attacks.
+</p>
+<sample src="examples/jinja2.py" />
+</example>
+
+<references>
+<li>
+http://jinja.pocoo.org/docs/2.10/api/
+Jinja2: <a href="http://jinja.pocoo.org/docs/2.10/api/">API</a>.
+</li>
+<li>
+Wikipedia: <a href="http://en.wikipedia.org/wiki/Cross-site_scripting">Cross-site scripting</a>.
+</li>
+</references>
+</qhelp>

python/ql/src/Security/CWE-079/Jinja2WithoutEscaping.ql

@@ -0,0 +1,56 @@
+/**
+ * @name Jinja2 templating with autoescape=False
+ * @description Using jinja2 templates with autoescape=False can
+ *              cause a cross-site scripting vulnerability.
+ * @kind problem
+ * @problem.severity error
+ * @precision medium
+ * @id py/jinja2/autoescape-false
+ * @tags security
+ *       external/cwe/cwe-079
+ */
+
+import python
+
+predicate jinja2Environment(Object callable, int autoescape) {
+    exists(ModuleObject jinja2 |
+        jinja2.getName() = "jinja2" |
+        jinja2.getAttribute("Environment") = callable and
+        callable.(ClassObject).getPyClass().getInitMethod().getArg(autoescape+1).asName().getId() = "autoescape"
+        or
+        exists(ModuleObject environment |
+            environment.getAttribute("Template") = callable and
+            callable.(ClassObject).lookupAttribute("__new__").(FunctionObject).getFunction().getArg(autoescape+1).asName().getId() = "autoescape"
+        )
+    )
+}
+
+ControlFlowNode getAutoEscapeParameter(CallNode call) {
+    exists(Object callable |
+        call.getFunction().refersTo(callable) |
+        jinja2Environment(callable, _) and
+        result = call.getArgByName("autoescape")
+        or
+        exists(int arg |
+            jinja2Environment(callable, arg) and
+            result = call.getArg(arg)
+        )
+    )
+}
+
+from CallNode call
+where
+not exists(call.getNode().getStarargs()) and
+not exists(call.getNode().getKwargs()) and
+(
+    not exists(getAutoEscapeParameter(call)) and
+    exists(Object env |
+        call.getFunction().refersTo(env) and
+        jinja2Environment(env, _)
+    )
+    or
+    exists(Object isFalse |
+      getAutoEscapeParameter(call).refersTo(isFalse) and isFalse.booleanValue() = false
+    )
+)
+select call, "Using jinja2 templates with autoescape=False can potentially allow XSS attacks."

python/ql/src/Security/CWE-079/examples/jinja2.py

@@ -0,0 +1,27 @@
+from flask import Flask, request, make_response, escape
+from jinja2 import Environment, select_autoescape, FileSystemLoader
+
+app = Flask(__name__)
+loader = FileSystemLoader( searchpath="templates/" )
+
+unsafe_env = Environment(loader=loader)
+safe1_env = Environment(loader=loader, autoescape=True)
+safe2_env = Environment(loader=loader, autoescape=select_autoescape())
+
+def render_response_from_env(env):
+    name = request.args.get('name', '')
+    template = env.get_template('template.html')
+    return make_response(template.render(name=name))
+
+@app.route('/unsafe')
+def unsafe():
+    return render_response_from_env(unsafe_env)
+
+@app.route('/safe1')
+def safe1():
+    return render_response_from_env(safe1_env)
+
+@app.route('/safe2')
+def safe2():
+    return render_response_from_env(safe2_env)
+