github · Kwstubbs · Nov 26, 2024 · Nov 26, 2024 · Nov 27, 2024 · Dec 10, 2024
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+---
+category: minorAnalysis
+---
+* C#: Added `cs/web/cors-misconfiguration` queries which looks for CORS misconfigurations with and without credentials.
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+<!DOCTYPE qhelp PUBLIC
+"-//Semmle//qhelp//EN"
+"qhelp.dtd">
+<qhelp>
+
+<overview>
+	<p>
+
+		A server can send the
+		<code>"Access-Control-Allow-Credentials"</code> CORS header to control
+		when a browser may send user credentials in Cross-Origin HTTP
+		requests.
+
+	</p>
+	<p>
+
+		When the <code>Access-Control-Allow-Credentials</code> header
+		is <code>"true"</code>, the <code>Access-Control-Allow-Origin</code>
+		header must have a value different from <code>"*"</code> in order to
+		make browsers accept the header. Therefore, to allow multiple origins
+		for Cross-Origin requests with credentials, the server must
+		dynamically compute the value of the
+		<code>"Access-Control-Allow-Origin"</code> header. Computing this
+		header value from information in the request to the server can
+		therefore potentially allow an attacker to control the origins that
+		the browser sends credentials to.
+
+	</p>
+
+
+
+</overview>
+
+<recommendation>
+	<p>
+
+		When the <code>Access-Control-Allow-Credentials</code> header
+		value is <code>"true"</code>, a dynamic computation of the
+		<code>Access-Control-Allow-Origin</code> header must involve
+		sanitization if it relies on user-controlled input.
+
+
+	</p>
+	<p>
+
+		Since the <code>"null"</code> origin is easy to obtain for an
+		attacker, it is never safe to use <code>"null"</code> as the value of
+		the <code>Access-Control-Allow-Origin</code> header when the
+		<code>Access-Control-Allow-Credentials</code> header value is
+		<code>"true"</code>.
+
+	</p>
+</recommendation>
+
+<example>
+	<p>
+
+		In the example below, the server allows the browser to send
+		user credentials in a Cross-Origin request. The request header
+		<code>origins</code> controls the allowed origins for such a
+		Cross-Origin request.
+
+	</p>
+
+	<sample src="examples/CorsBad.cs"/>
+
+	<p>
+
+		This is not secure, since an attacker can choose the value of
+		the <code>origin</code> request header to make the browser send
+		credentials to their own server. The use of a allowlist containing
+		allowed origins for the Cross-Origin request fixes the issue:
+
+	</p>
+
+	<sample src="examples/CorsGood.cs"/>
+</example>
+
+<references>
+	<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin">CORS, Access-Control-Allow-Origin</a>.</li>
+	<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Credentials">CORS, Access-Control-Allow-Credentials</a>.</li>
+	<li>PortSwigger: <a href="http://blog.portswigger.net/2016/10/exploiting-cors-misconfigurations-for.html">Exploiting CORS Misconfigurations for Bitcoins and Bounties</a></li>
+	<li>W3C: <a href="https://w3c.github.io/webappsec-cors-for-developers/#resources">CORS for developers, Advice for Resource Owners</a></li>
+</references>
+</qhelp>
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+/**
+ * @name Credentialed CORS Misconfiguration
+ * @description Allowing any origin while allowing credentials may result in security issues as third party website may be able to
+ *              access private resources.
+ * @kind problem
+ * @problem.severity error
+ * @security-severity 7.5
+ * @precision high
+ * @id cs/web/cors-misconfiguration
+ * @tags security
+ *       external/cwe/cwe-942
+ */
+
+import csharp
+import CorsMisconfigurationLib
+
+/**
+ * Holds if the application allows an origin using "*" origin.
+ */
+private predicate allowAnyOrigin(MethodCall m) {
+  m.getTarget()
+      .hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsPolicyBuilder",
+        "AllowAnyOrigin")
+}
+
+/**
+ * Holds if the application uses a vulnerable CORS policy.
+ */
+private predicate hasDangerousOrigins(MethodCall m) {
+  m.getTarget()
+      .hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsPolicyBuilder",
+        "WithOrigins") and
+  exists(StringLiteral idStr |
+    idStr.getValue().toLowerCase().matches(["null", "*"]) and
+    TaintTracking::localExprTaint(idStr, m.getAnArgument())
+  )
+}
+
+from MethodCall add_policy, MethodCall child
+where
+  (
+    usedPolicy(add_policy) and
+    // Misconfigured origin affects used policy
+    getCallableFromExpr(add_policy.getArgument(1)).calls*(child.getTarget())
+    or
+    add_policy
+        .getTarget()
+        .hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsOptions",
+          "AddDefaultPolicy") and
+    // Misconfigured origin affects added default policy
+    getCallableFromExpr(add_policy.getArgument(0)).calls*(child.getTarget())
+  ) and
+  (hasDangerousOrigins(child) or allowAnyOrigin(child))
+select add_policy, "The following CORS policy may allow requests from 3rd party websites"
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+/**
+ * @name Credentialed CORS Misconfiguration
+ * @description Allowing any origin while allowing credentials may result in security issues as third party website may be able to
+ *              access private resources.
+ * @kind problem
+ * @problem.severity error
+ * @security-severity 7.5
+ * @precision high
+ * @id cs/web/cors-misconfiguration-credentials
+ * @tags security
+ *       external/cwe/cwe-942
+ */
+
+import csharp
+import CorsMisconfigurationLib
+
+/** A call to `CorsPolicyBuilder.AllowCredentials`. */
+class AllowsCredentials extends MethodCall {
+  AllowsCredentials() {
+    this.getTarget()
+        .hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsPolicyBuilder",
+          "AllowCredentials")
+  }
+}
+
+from MethodCall add_policy, MethodCall setIsOriginAllowed, AllowsCredentials allowsCredentials
+where
+  (
+    getCallableFromExpr(add_policy.getArgument(1)).calls*(setIsOriginAllowed.getTarget()) and
+    usedPolicy(add_policy) and
+    getCallableFromExpr(add_policy.getArgument(1)).calls*(allowsCredentials.getTarget())
+    or
+    add_policy
+        .getTarget()
+        .hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsOptions",
+          "AddDefaultPolicy") and
+    getCallableFromExpr(add_policy.getArgument(0)).calls*(setIsOriginAllowed.getTarget()) and
+    getCallableFromExpr(add_policy.getArgument(0)).calls*(allowsCredentials.getTarget())
+  ) and
+  setIsOriginAllowedReturnsTrue(setIsOriginAllowed)
+select add_policy,
+  "The following CORS policy may allow credentialed requests from 3rd party websites"
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+import csharp
+import DataFlow
+
+/**
+ * Gets the actual callable corresponding to the expression `e`.
+ */
+Callable getCallableFromExpr(Expr e) {
+  exists(Expr dcArg | dcArg = e.(DelegateCreation).getArgument() |
+    result = dcArg.(CallableAccess).getTarget() or
+    result = dcArg.(AnonymousFunctionExpr)
+  )
+  or
+  result = e
+}
+
+/**
+ * Holds if the `Callable` c throws any exception other than `ThrowsArgumentNullException`
+ */
+predicate callableMayThrowException(Callable c) {
+  exists(ThrowStmt thre | c = thre.getEnclosingCallable()) and
+  not callableOnlyThrowsArgumentNullException(c)
+}
+
+/**
+ * Holds if any exception being thrown by the callable is of type `System.ArgumentNullException`
+ * It will also hold if no exceptions are thrown by the callable
+ */
+predicate callableOnlyThrowsArgumentNullException(Callable c) {
+  forall(ThrowElement thre | c = thre.getEnclosingCallable() |
+    thre.getThrownExceptionType().hasFullyQualifiedName("System", "ArgumentNullException")
+  )
+}
+
+/**
+ * Hold if the `Expr` e is a `BoolLiteral` with value true,
+ * the expression has a predictable value == `true`,
+ * or if it is a `ConditionalExpr` where the `then` and `else` expressions meet `isExpressionAlwaysTrue` criteria
+ */
+predicate isExpressionAlwaysTrue(Expr e) {
+  e.(BoolLiteral).getBoolValue() = true
+  or
+  e.getValue() = "true"
+  or
+  e instanceof ConditionalExpr and
+  isExpressionAlwaysTrue(e.(ConditionalExpr).getThen()) and
+  isExpressionAlwaysTrue(e.(ConditionalExpr).getElse())
+  or
+  exists(Callable callable |
+    callableHasAReturnStmtAndAlwaysReturnsTrue(callable) and
+    callable.getACall() = e
+  )
+}
+
+/**
+ * Holds if the lambda expression `le` always returns true
+ */
+predicate lambdaExprReturnsOnlyLiteralTrue(AnonymousFunctionExpr le) {
+  isExpressionAlwaysTrue(le.getExpressionBody())
+}
+
+/**
+ * Holds if the callable has a return statement and it always returns true for all such statements
+ */
+predicate callableHasAReturnStmtAndAlwaysReturnsTrue(Callable c) {
+  c.getReturnType() instanceof BoolType and
+  not callableMayThrowException(c) and
+  forex(ReturnStmt rs | rs.getEnclosingCallable() = c |
+    rs.getNumberOfChildren() = 1 and
+    isExpressionAlwaysTrue(rs.getChildExpr(0))
+  )
+}
+
+/**
+ * Holds if `c` always returns `true`.
+ */
+private predicate alwaysReturnsTrue(Callable c) {
+  callableHasAReturnStmtAndAlwaysReturnsTrue(c)
+  or
+  lambdaExprReturnsOnlyLiteralTrue(c)
+}
+
+/**
+ * Holds if SetIsOriginAllowed always returns true. This sets the Access-Control-Allow-Origin to the requester
+ */
+predicate setIsOriginAllowedReturnsTrue(MethodCall mc) {
+  mc.getTarget()
+      .hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsPolicyBuilder",
+        "SetIsOriginAllowed") and
+  alwaysReturnsTrue(mc.getArgument(0))
+}
+
+/**
+ * Holds if UseCors is called with the relevant cors policy
+ */
+predicate usedPolicy(MethodCall add_policy) {
+  exists(MethodCall uc |
+    uc.getTarget()
+        .hasFullyQualifiedName("Microsoft.AspNetCore.Builder.CorsMiddlewareExtensions", "UseCors") and
+    (
+      // Same hardcoded name
+      uc.getArgument(1).getValue() = add_policy.getArgument(0).getValue() or
+      // Same variable access
+      uc.getArgument(1).(VariableAccess).getTarget() =
+        add_policy.getArgument(0).(VariableAccess).getTarget() or
+      DataFlow::localExprFlow(add_policy.getArgument(0), uc.getArgument(1))
+    )
+  ) and
+  add_policy
+      .getTarget()
+      .hasFullyQualifiedName("Microsoft.AspNetCore.Cors.Infrastructure.CorsOptions", "AddPolicy")
+}
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+using Leaf.Middlewares;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+
+namespace Leaf
+{
+    public class Startup
+    {
+        public Startup(IConfiguration configuration)
+        {
+            Configuration = configuration;
+        }
+
+        public IConfiguration Configuration { get; }
+
+        // This method gets called by the runtime. Use this method to add services to the container.
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services.AddControllers();
+            //services.AddTransient<MySqlConnection>(_ => new MySqlConnection(Configuration["ConnectionStrings:Default"]));
+            services.AddControllersWithViews()
+                    .AddNewtonsoftJson(options =>
+                        options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
+            );
+
+            services.AddCors(options => {
+                options.AddPolicy("AllowPolicy", builder => builder
+                 .WithOrigins("null")
+                 .AllowCredentials()
+                 .AllowAnyMethod()
+                 .AllowAnyHeader());
+            });
+
+        }
+
+        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
+        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
+        {
+            app.UseRouting();
+
+            app.UseCors("AllowPolicy");
+
+            app.UseRequestResponseLogging();
+
+            if (env.IsDevelopment())
+            {
+                app.UseDeveloperExceptionPage();
+            }
+
+            app.UseHttpsRedirection();
+
+            app.UseAuthorization();
+
+            app.UseEndpoints(endpoints =>
+            {
+                endpoints.MapControllers();
+            });
+
+        }
+    }
+}