Middleware and TagHelpers for CSP support in ASP.NET

src/Middleware/CSP/CSP.slnf

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+{
+  "solution": {
+    "path": "..\\..\\..\\AspNetCore.sln",
+    "projects": [
+      "src\\Middleware\\CSP\\src\\Microsoft.AspNetCore.Csp.csproj",
+      "src\\Middleware\\CSP\\test\\UnitTests\\Microsoft.AspNetCore.Csp.Test.csproj",
+      "src\\Middleware\\CSP\\test\\testassets\\CspMiddlewareWebSite.csproj",
+    ]
+  }
+}

src/Middleware/CSP/README.md

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+# CSP
+
+## Description
+
+This directory contains .NET Core middleware for Content Security Policy (CSP). CSP is a very popular security mitigation against XSS and other injection vulnerabilities. CSP comes in many flavours, but we've chosen to add support for the most robust of them: nonce-based, strict-dynamic CSP.
+
+There was a previous discussion about CSP in .NET [here](https://github.com/dotnet/aspnetcore/issues/6001), that we have considered for our design.
+
+## Contributions
+This directory includes the following changes:
+
+* Allow configuration of whether CSP enabled in reporting or enforcement modes.
+* Allows configuration of a report URI, for violation reports sent by the browser.
+* CSP middleware generates a nonce-based, strict-dynamic policy.
+* Middleware adds thepolicy to HTTP responses according to the configuration.
+* Custom <script> TagHelper to set nonce attribute on script blocks automatically.
+* Provides a default implementation of a CSP violation report collection endpoint.
+* Example app that uses our CSP middleware and corresponding basic unit tests.
+
+## Usage:
+
+```
+// CSP configuration. Must come first because other middleware might skip any following middleware.
+
+	app.UseCsp(policyBuilder =>
+policyBuilder.WithCspMode(CspMode.ENFORCING)
+
+.WithReportingUri("/csp"));
+```
+You can find the sample app under `./test/testassets/CspApplication/` directory.
+
+## Authors
+* Co-authored-by: Aaron Shim - [email protected]
+* Co-authored-by: Santiago Diaz - [email protected]

src/Middleware/CSP/src/ContentSecurityPolicy.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Text;
+
+namespace Microsoft.AspNetCore.Csp
+{
+    /// <summary>
+    /// A greedy Content Security Policy generator
+    /// </summary>
+    public class ContentSecurityPolicy
+    {
+        private readonly string _baseAndObject = "base-uri 'none'; object-src 'none'";
+        private readonly Func<INonce, string> policyBuilder;
+
+        private readonly CspMode _cspMode;
+        private readonly bool _strictDynamic;
+        private readonly bool _unsafeEval;
+        private readonly string _reportingUri;
+
+        /// <summary>
+        /// Instantiates a new <see cref="ContentSecurityPolicy"/>.
+        /// </summary>
+        /// <param name="cspMode">Represents whether the current policy is in enforcing or reporting mode.</param>
+        /// <param name="strictDynamic">Whether the policy should enable nonce propagation.</param>
+        /// <param name="unsafeEval">Whether JavaScript's eval should be allowed to run.</param>
+        /// <param name="reportingUri">An absolute or relative URI representing the reporting endpoint</param>
+        public ContentSecurityPolicy(
+            CspMode cspMode,
+            bool strictDynamic,
+            bool unsafeEval,
+            string reportingUri
+        )
+        {
+            _cspMode = cspMode;
+            _strictDynamic = strictDynamic;
+            _unsafeEval = unsafeEval;
+            _reportingUri = reportingUri;
+
+            // compute the static directives of the policy up front to avoid doing so on every request
+            var policyFormat = new StringBuilder()
+                .Append("script-src")
+                .Append(" 'nonce-{0}' ")  // nonce
+                .Append(_strictDynamic ? "'strict-dynamic'" : "")
+                .Append(_unsafeEval ? "'unsafe-eval'" : "")
+                .Append(" https: http:;")  // fall-back allowlist-based CSP for browsers that don't support nonces
+                .Append(_baseAndObject)
+                .Append("; ")               // end of script-src
+                .Append(_reportingUri != null ? "report-uri " + _reportingUri : "")
+                .ToString();
+
+            policyBuilder = nonce => string.Format(policyFormat, nonce.GetValue());
+        }
+
+        public string GetHeaderName()
+        {
+            return _cspMode == CspMode.REPORTING ? CspConstants.CspReportingHeaderName : CspConstants.CspEnforcedHeaderName;
+        }
+        public string GetPolicy(INonce nonce)
+        {
+            return policyBuilder.Invoke(nonce);
+        }
+    }
+
+    public enum CspMode
+    {
+        NONE,
+        REPORTING,
+        ENFORCING
+    }
+}

src/Middleware/CSP/src/ContentSecurityPolicyBuilder.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.Extensions.Logging;
+
+namespace Microsoft.AspNetCore.Csp
+{
+    /// <summary>
+    /// Allows customizing content security policies
+    /// </summary>
+    public class ContentSecurityPolicyBuilder
+    {
+        private CspMode _cspMode;
+        private bool _strictDynamic;
+        private bool _unsafeEval;
+        private string _reportingUri;
+        private LogLevel _logLevel = LogLevel.Information;
+
+        public ContentSecurityPolicyBuilder WithCspMode(CspMode cspMode)
+        {
+            _cspMode = cspMode;
+            return this;
+        }
+
+        public ContentSecurityPolicyBuilder WithStrictDynamic()
+        {
+            _strictDynamic = true;
+            return this;
+        }
+
+        public ContentSecurityPolicyBuilder WithUnsafeEval()
+        {
+            _unsafeEval = true;
+            return this;
+        }
+        public ContentSecurityPolicyBuilder WithReportingUri(string reportingUri)
+        {
+            // TODO: normalize URL
+            _reportingUri = reportingUri;
+            return this;
+        }
+
+        public ContentSecurityPolicyBuilder WithLogLevel(LogLevel logLevel)
+        {
+            _logLevel = logLevel;
+            return this;
+        }
+
+        /// <summary>
+        /// Whether the policy specifies a relative reporting URI.
+        /// </summary>
+        /// <remarks>
+        /// If this method returns true, a handler for the reporting endpoint will be automatically added to this application.
+        /// </remarks>
+        public bool HasLocalReporting()
+        {
+            return _reportingUri != null && _reportingUri.StartsWith("/");
+        }
+
+        public CspReportLogger ReportLogger(ICspReportLoggerFactory loggerFactory)
+        {
+            return loggerFactory.BuildLogger(_logLevel, _reportingUri);
+        }
+
+        public ContentSecurityPolicy Build()
+        {
+            if (_cspMode == CspMode.NONE)
+            {
+                // TODO: Error message
+                throw new InvalidOperationException();
+            }
+
+            if (_cspMode == CspMode.REPORTING && _reportingUri == null)
+            {
+                // TODO: Error message
+                throw new InvalidOperationException();
+            }
+
+            return new ContentSecurityPolicy(
+                _cspMode,
+                _strictDynamic,
+                _unsafeEval,
+                _reportingUri
+            );
+        }
+    }
+}

src/Middleware/CSP/src/CspConstants.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNetCore.Csp
+{
+    /// <summary>
+    /// CSP-related constants.
+    /// </summary>
+    public static class CspConstants
+    {
+        /// <summary>
+        /// CSP header name in enforcement mode.
+        /// </summary>
+        public static readonly string CspEnforcedHeaderName = "Content-Security-Policy";
+        /// <summary>
+        /// CSP header name in reporting mode.
+        /// </summary>
+        public static readonly string CspReportingHeaderName = "Content-Security-Policy-Report-Only";
+        /// <summary>
+        /// Expected content type for requests containing CSP violation reports.
+        /// </summary>
+        public static readonly string CspReportContentType = "application/csp-report";
+        /// <summary>
+        /// Possible violated directive value used to create textual representations of violation reports.
+        /// </summary>
+        public static readonly string ScriptSrcElem = "script-src-elem";
+        /// <summary>
+        /// Possible blocked URI value used to create textual representations of violation reports.
+        /// </summary>
+        public static readonly string BlockedUriInline = "inline";
+        /// <summary>
+        /// Possible violated directive value used to create textual representations of violation reports.
+        /// </summary>
+        public static readonly string ScriptSrcAttr = "script-src-attr";
+    }
+}

src/Middleware/CSP/src/CspMiddleware.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+
+namespace Microsoft.AspNetCore.Csp
+{
+    /// <summary>
+    /// Middleware for supporting CSP.
+    /// </summary>
+    public class CspMiddleware
+    {
+        private readonly RequestDelegate _next;
+        private readonly ContentSecurityPolicy _csp;
+
+        /// <summary>
+        /// Instantiates a new <see cref="CspMiddleware"/>.
+        /// </summary>
+        /// <param name="next">The next middleware in the pipeline.</param>
+        /// <param name="csp">A content security policy generator.</param>
+        public CspMiddleware(RequestDelegate next, ContentSecurityPolicy csp)
+        {
+            _next = next;
+            _csp = csp;
+        }
+
+        public Task Invoke(HttpContext context, INonce nonce)
+        {
+            context.Response.Headers[_csp.GetHeaderName()] = _csp.GetPolicy(nonce);
+            return _next(context);
+        }
+    }
+}

src/Middleware/CSP/src/CspMiddlewareExtensions.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Microsoft.AspNetCore.Csp
+{
+    /// <summary>
+    /// Extends <see cref="IApplicationBuilder"/> to add CSP middleware support.
+    /// </summary>
+    public static class CspMiddlewareExtensions
+    {
+        /// <summary>
+        /// Adds a CSP middleware to this web application pipeline that will add a custom policy to responses and collect CSP violation reports sent by user agents.
+        /// </summary>
+        /// <param name="app">The IApplicationBuilder passed to the Configure method</param>
+        /// <param name="configurePolicy">A delegate to build a custom content security policy</param>
+        /// <returns>The original app parameter</returns>
+        public static IApplicationBuilder UseCsp(this IApplicationBuilder app, Action<ContentSecurityPolicyBuilder> configurePolicy)
+        {
+            if (app == null)
+            {
+                throw new ArgumentNullException(nameof(app));
+            }
+
+            var policyBuilder = new ContentSecurityPolicyBuilder();
+            configurePolicy(policyBuilder);
+
+            if (policyBuilder.HasLocalReporting())
+            {
+                var loggerFactory = app.ApplicationServices.GetService<ICspReportLoggerFactory>();
+                var reportLogger = policyBuilder.ReportLogger(loggerFactory);
+                app.UseWhen(
+                    context => context.Request.Path.StartsWithSegments(reportLogger.ReportUri),
+                    appBuilder => appBuilder.UseMiddleware<CspReportingMiddleware>(reportLogger));
+            }
+
+            return app.UseMiddleware<CspMiddleware>(policyBuilder.Build());
+        }
+
+        /// <summary>
+        /// Adds the necessary bindings for CSP. Namely, allows adding nonces to script tags automatically and provides a custom logging factory.
+        /// </summary>
+        /// <param name="app">The IApplicationBuilder passed to the Configure method</param>
+        /// <returns>The original services parameter</returns>
+        public static IServiceCollection AddCsp(this IServiceCollection services)
+        {
+            if (services == null)
+            {
+                throw new ArgumentNullException(nameof(services));
+            }
+
+            services.AddScoped<INonce, Nonce>();
+            services.AddSingleton<ICspReportLoggerFactory, CspReportLoggerFactory>();
+
+            return services;
+        }
+    }
+}