Add support for vulnerable CORS middlewares

python/ql/lib/semmle/python/Concepts.qll

@@ -1411,6 +1411,56 @@ module Http {
       override DataFlow::Node getValueArg() { none() }
     }
 
+    /**
+     * A data-flow node that enables or disables CORS
+     * in a global manner.
+     *
+     * Extend this class to refine existing API models. If you want to model new APIs,
+     * extend `CorsMiddleware::Range` instead.
+     */
+    class CorsMiddleware extends DataFlow::Node instanceof CorsMiddleware::Range {
+      /**
+       * Gets the string corresponding to the middleware
+       */
+      string middleware_name() { result = super.middleware_name() }
+
+      /**
+       * Gets the boolean value corresponding to if CORS credentials is enabled
+       * (`true`) or disabled (`false`) by this node.
+       */
+      DataFlow::Node allowed_origins() { result = super.allowed_origins() }
+
+      DataFlow::Node allowed_credentials() { result = super.allowed_credentials() }
+    }
+
+    /** Provides a class for modeling new CORS middleware APIs. */
+    module CorsMiddleware {
+      /**
+       * A data-flow node that enables or disables Cross-site request forgery protection
+       * in a global manner.
+       *
+       * Extend this class to model new APIs. If you want to refine existing API models,
+       * extend `CorsMiddleware` instead.
+       */
+      abstract class Range extends DataFlow::Node {
+        /**
+         * Gets the string corresponding to the middleware
+         */
+        abstract string middleware_name();
+
+        /**
+         * Gets the boolean value corresponding to if CORS credentials is enabled
+         * (`true`) or disabled (`false`) by this node.
+         */
+        abstract DataFlow::Node allowed_credentials();
+
+        /**
+         * Gets the strings corresponding to the origins allowed by the cors policy
+         */
+        abstract DataFlow::Node allowed_origins();
+      }
+    }
+
     /**
      * A data-flow node that enables or disables Cross-site request forgery protection
      * in a global manner.

python/ql/lib/semmle/python/frameworks/FastApi.qll

@@ -30,6 +30,32 @@ module FastApi {
     API::Node instance() { result = cls().getReturn() }
   }
 
+  /**
+   * A call to `app.add_middleware` adding a generic middleware.
+   */
+  private class AddMiddlewareCall extends DataFlow::CallCfgNode {
+    AddMiddlewareCall() { this = App::instance().getMember("add_middleware").getACall() }
+
+    string middleware_name() { result = this.getArg(0).asExpr().(Name).toString() }
+  }
+
+  /**
+   * A call to `app.add_middleware` adding CORSMiddleware.
+   */
+  class AddCorsMiddlewareCall extends Http::Server::CorsMiddleware::Range, AddMiddlewareCall {
+    override string middleware_name() { result = this.getArg(0).asExpr().(Name).toString() }
+
+    override DataFlow::Node allowed_origins() { result = this.getArgByName("allow_origins") }
+
+    override DataFlow::Node allowed_credentials() {
+      result = this.getArgByName("allow_credentials")
+    }
+
+    DataFlow::Node allowed_methods() { result = this.getArgByName("allow_methods") }
+
+    DataFlow::Node allowed_headers() { result = this.getArgByName("allow_headers") }
+  }
+
   /**
    * Provides models for the `fastapi.APIRouter` class
    *

python/ql/lib/semmle/python/frameworks/Starlette.qll

@@ -25,6 +25,87 @@ private import semmle.python.frameworks.data.ModelsAsData
  * - https://www.starlette.io/
  */
 module Starlette {
+  /**
+   * Provides models for the `starlette.app` class
+   *
+   * See https://www.starlette.io/websockets/.
+   */
+  module App {
+    API::Node cls() { result = API::moduleImport("starlette").getMember("app") }
+
+    /** Gets a reference to a FastAPI application (an instance of `fastapi.FastAPI`). */
+    API::Node instance() { result = cls().getReturn() }
+  }
+
+  /**
+   * A call to any of the execute methods on a `app.add_middleware`.
+   */
+  class AddMiddlewareCall extends DataFlow::CallCfgNode {
+    AddMiddlewareCall() {
+      this = [App::instance().getMember("add_middleware").getACall(), Middleware::instance()]
+    }
+
+    string middleware_name() { result = this.getArg(0).asExpr().(Name).toString() }
+  }
+
+  /**
+   * A call to any of the execute methods on a `app.add_middleware` with CORSMiddleware.
+   */
+  class AddCorsMiddlewareCall extends AddMiddlewareCall, Http::Server::CorsMiddleware::Range {
+    override string middleware_name() { result = this.getArg(0).asExpr().(Name).toString() }
+
+    override DataFlow::Node allowed_origins() { result = this.getArgByName("allow_origins") }
+
+    override DataFlow::Node allowed_credentials() {
+      result = this.getArgByName("allow_credentials")
+    }
+
+    DataFlow::Node allowed_methods() { result = this.getArgByName("allow_methods") }
+
+    DataFlow::Node allowed_headers() { result = this.getArgByName("allow_headers") }
+  }
+
+  /**
+   * Provides models for the `starlette.middleware.Middleware` class
+   *
+   * See https://www.starlette.io/.
+   */
+  module Middleware {
+    /** Gets a reference to the `starlette.middleware.Middleware` class. */
+    API::Node classRef() {
+      result = API::moduleImport("starlette").getMember("middleware").getMember("Middleware")
+      or
+      result = ModelOutput::getATypeNode("starlette.middleware.Middleware~Subclass").getASubclass*()
+    }
+
+    /**
+     * A source of instances of `starlette.middleware.Middleware`, extend this class to model new instances.
+     *
+     * This can include instantiations of the class, return values from function
+     * calls, or a special parameter that will be set when functions are called by an external
+     * library.
+     *
+     * Use the predicate `Middleware::instance()` to get references to instances of `starlette.middleware.middleware`.
+     */
+    abstract class InstanceSource extends DataFlow::LocalSourceNode { }
+
+    /** A direct instantiation of `starlette.middleware.Middleware`. */
+    class ClassInstantiation extends InstanceSource, DataFlow::CallCfgNode {
+      ClassInstantiation() { this = classRef().getACall() }
+    }
+
+    /** Gets a reference to an instance of `starlette.middleware.Middleware`. */
+    private DataFlow::TypeTrackingNode instance(DataFlow::TypeTracker t) {
+      t.start() and
+      result instanceof InstanceSource
+      or
+      exists(DataFlow::TypeTracker t2 | result = instance(t2).track(t2, t))
+    }
+
+    /** Gets a reference to an instance of `starlette.middleware.Middleware`. */
+    DataFlow::Node instance() { instance(DataFlow::TypeTracker::end()).flowsTo(result) }
+  }
+
   /**
    * Provides models for the `starlette.websockets.WebSocket` class
    *

python/ql/src/experimental/Security/CWE-942/CorsMisconfigurationMiddleware.qhelp

@@ -0,0 +1,64 @@
+<!DOCTYPE qhelp PUBLIC
+"-//Semmle//qhelp//EN"
+"qhelp.dtd">
+<qhelp>
+<overview>
+<p>
+	Web browsers, by default, disallow cross-origin resource sharing via direct HTTP requests (i.e. using a JavaScript HTTP client).
+	Still, to satisfy some needs that arose with the growth of the web, an expedient was created to make exceptions possible.
+	CORS (Cross-origin resource sharing) is a mechanism that allows resources of a web endpoint (let's call it "Peer A")
+	to be accessed from another web page belonging to a different domain ("Peer B").
+</p>
+<p>
+	For that to happen, Peer A needs to make available its CORS configuration via special headers on the desired endpoint
+	via the OPTIONS method.
+</p>
+<p>
+	This configuration can also allow the inclusion of cookies on the cross-origin request,
+	(i.e. when the <code>Access-Control-Allow-Credentials</code> header is set to true) 
+	meaning that Peer B can send a request to Peer A that will include the cookies as if the request was executed by the user.
+</p>
+<p>
+	That can have dangerous effects if Peer B origin is not restricted correctly. 
+	An example of a dangerous scenario is when <code>Access-Control-Allow-Origin</code> header is set to a value gotten from the Peer B's request 
+	(and not correctly validated), or is set to special values such as <code>*</code> or <code>null</code>. 
+	The above values can allow any Peer B to send requests to the misconfigured Peer A on behalf of the user.
+</p>
+<p>
+	Example scenario:
+	User is client of a bank that has its API misconfigured to accept CORS requests from any domain.
+	When the user loads an evil page, the evil page sends a request to the bank's API to transfer all funds
+	to evil party's account.
+	Given that the user was already logged in to the bank website, and had its session cookies set,
+	the evil party's request succeeds.
+</p>
+</overview>
+<recommendation>
+<p>
+	When configuring CORS that allow credentials passing,
+	it's best not to use user-provided values for the allowed origins response header,
+	especially if the cookies grant session permissions on the user's account.
+</p>
+<p>
+	It also can be very dangerous to set the allowed origins to <code>null</code> (which can be bypassed).
+</p>
+</recommendation>
+<example>
+<p>
+	The first example shows a few possible CORS misconfiguration cases:
+</p>
+<sample src="CorsMisconfigurationMiddlewareBad.py"/>
+<p>
+	The second example show better configurations:
+</p>
+<sample src="CorsMisconfigurationMiddlewareGood.py"/>
+</example>
+<references>
+<li>
+	Reference 1: <a href="https://portswigger.net/web-security/cors">PortSwigger Web Security Academy on CORS</a>.
+</li>
+<li>
+	Reference 2: <a href="https://www.youtube.com/watch?v=wgkj4ZgxI4c">AppSec EU 2017 Exploiting CORS Misconfigurations For Bitcoins And Bounties by James Kettle</a>.
+</li>
+</references>
+</qhelp>

python/ql/src/experimental/Security/CWE-942/CorsMisconfigurationMiddleware.ql

@@ -0,0 +1,36 @@
+/**
+ * @name SOP protection weak with credentials
+ * @description Disabling or weakening SOP protection may make the application
+ *              vulnerable to a CORS attack.
+ * @kind problem
+ * @problem.severity warning
+ * @security-severity 8.8
+ * @precision high
+ * @id py/insecure-cors-setting
+ * @tags security
+ *       external/cwe/cwe-352
+ */
+
+ import python
+ import semmle.python.Concepts
+ private import semmle.python.dataflow.new.DataFlow
+ predicate containsStar(DataFlow::Node array){
+     (array.asExpr() instanceof List and 
+     array.asExpr().getASubExpression().(StringLiteral).getText().matches("*")) or
+     (array.asExpr().(StringLiteral).getText().matches(["*", "null"]))
+
+ }
+ 
+ predicate isCorsMiddleware(Http::Server::CorsMiddleware middleware){
+     middleware.middleware_name().matches("CORSMiddleware")
+ }
+ 
+ predicate credentialsAllowed(Http::Server::CorsMiddleware  middleware){
+     middleware.allowed_credentials().asExpr() instanceof True
+ }
+ 
+ from Http::Server::CorsMiddleware a
+ where credentialsAllowed(a) and 
+ containsStar(a.allowed_origins().getALocalSource()) and
+ isCorsMiddleware(a)
+ select a, "This CORS middleware uses a vulnerable configuration that leaves it open to attacks from arbitrary websites"

python/ql/src/experimental/Security/CWE-942/CorsMisconfigurationMiddlewareBad.py

@@ -0,0 +1,21 @@
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+app = FastAPI()
+
+origins = [
+    "*"
+]
+
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=origins,
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+
+@app.get("/")
+async def main():
+    return {"message": "Hello World"}

python/ql/src/experimental/Security/CWE-942/CorsMisconfigurationMiddlewareGood.py

@@ -0,0 +1,24 @@
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+app = FastAPI()
+
+origins = [
+    "http://localhost.tiangolo.com",
+    "https://localhost.tiangolo.com",
+    "http://localhost",
+    "http://localhost:8080",
+]
+
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=origins,
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+
+@app.get("/")
+async def main():
+    return {"message": "Hello World"}

python/ql/test/query-tests/Security/CWE-942-CorsMisconfigurationMiddleware/CorsMisconfigurationMiddleware.expected

@@ -0,0 +1,2 @@
+| fastapi.py:10:1:16:1 | ControlFlowNode for Attribute() | This CORS middleware uses a vulnerable configuration that leaves it open to attacks from arbitrary websites |
+| starlette.py:8:5:8:75 | ControlFlowNode for Middleware() | This CORS middleware uses a vulnerable configuration that leaves it open to attacks from arbitrary websites |

python/ql/test/query-tests/Security/CWE-942-CorsMisconfigurationMiddleware/CorsMisconfigurationMiddleware.qlref

@@ -0,0 +1 @@
+experimental/Security/CWE-942/CorsMisconfigurationMiddleware.ql

python/ql/test/query-tests/Security/CWE-942-CorsMisconfigurationMiddleware/fastapi.py

@@ -0,0 +1,21 @@
+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+app = FastAPI()
+
+origins = [
+    "*"
+]
+
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=origins,
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+
+@app.get("/")
+async def main():
+    return {"message": "Hello World"}

python/ql/test/query-tests/Security/CWE-942-CorsMisconfigurationMiddleware/starlette.py

@@ -0,0 +1,11 @@
+from starlette.applications import Starlette
+from starlette.middleware import Middleware
+from starlette.middleware.cors import CORSMiddleware
+
+routes = ...
+
+middleware = [
+    Middleware(CORSMiddleware, allow_origins=['*'], allow_credentials=True)
+]
+
+app = Starlette(routes=routes, middleware=middleware)