Fix open redirect vulnerability

src/Aspire.Dashboard/Components/Pages/Login.razor.cs

@@ -108,7 +108,7 @@ private async Task SubmitAsync()
 
     private string GetRedirectUrl()
     {
-        return ReturnUrl ?? DashboardUrls.ResourcesUrl();
+        return UrlValidationHelper.IsSafeRedirectUrl(ReturnUrl) ? ReturnUrl : DashboardUrls.ResourcesUrl();
     }
 
     public async ValueTask DisposeAsync()

src/Aspire.Dashboard/Model/ValidateTokenMiddleware.cs

@@ -68,7 +68,10 @@ public async Task InvokeAsync(HttpContext context)
 
     private static void RedirectAfterValidation(HttpContext context)
     {
-        if (context.Request.Query.TryGetValue("returnUrl", out var returnUrl))
+        // An optional returnurl value must be a valid URL and it must be local.
+        // We don't want an open redirect (the potential to create a URL that redirects to a third-party site).
+        if (context.Request.Query.TryGetValue("returnUrl", out var returnUrl)
+            && UrlValidationHelper.IsSafeRedirectUrl(returnUrl.ToString()))
         {
             context.Response.Redirect(returnUrl.ToString());
         }

src/Aspire.Dashboard/Utils/UrlValidationHelper.cs

@@ -0,0 +1,83 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics.CodeAnalysis;
+
+namespace Aspire.Dashboard.Utils;
+
+/// <summary>
+/// Helpers for validating redirect URLs.
+/// </summary>
+internal static class UrlValidationHelper
+{
+    /// <summary>
+    /// Checks whether a URL is safe to use as a redirect target.
+    /// The URL must be a valid URI and a local path (not absolute or protocol-relative).
+    /// </summary>
+    internal static bool IsSafeRedirectUrl([NotNullWhen(true)] string? url)
+    {
+        return Uri.TryCreate(url, UriKind.Relative, out _) && IsLocalUrl(url);
+    }
+
+    // Copied from ASP.NET Core's IsLocalUrl implementation:
+    // https://github.com/dotnet/aspnetcore/blob/7cbda0e023075490b4365a0754ca410ce6eff59a/src/Shared/ResultsHelpers/SharedUrlHelper.cs#L33
+    internal static bool IsLocalUrl([NotNullWhen(true)] string? url)
+    {
+        if (string.IsNullOrEmpty(url))
+        {
+            return false;
+        }
+
+        // Allows "/" or "/foo" but not "//" or "/\".
+        if (url[0] == '/')
+        {
+            // url is exactly "/"
+            if (url.Length == 1)
+            {
+                return true;
+            }
+
+            // url doesn't start with "//" or "/\"
+            if (url[1] != '/' && url[1] != '\\')
+            {
+                return !HasControlCharacter(url.AsSpan(1));
+            }
+
+            return false;
+        }
+
+        // Allows "~/" or "~/foo" but not "~//" or "~/\".
+        if (url[0] == '~' && url.Length > 1 && url[1] == '/')
+        {
+            // url is exactly "~/"
+            if (url.Length == 2)
+            {
+                return true;
+            }
+
+            // url doesn't start with "~//" or "~/\"
+            if (url[2] != '/' && url[2] != '\\')
+            {
+                return !HasControlCharacter(url.AsSpan(2));
+            }
+
+            return false;
+        }
+
+        return false;
+
+        static bool HasControlCharacter(ReadOnlySpan<char> readOnlySpan)
+        {
+            // URLs may not contain ASCII control characters.
+            for (var i = 0; i < readOnlySpan.Length; i++)
+            {
+                if (char.IsControl(readOnlySpan[i]))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+    }
+}

tests/Aspire.Dashboard.Tests/Middleware/ValidateTokenMiddlewareTests.cs

@@ -63,6 +63,32 @@ public async Task ValidateToken_BrowserTokenAuth_RightToken_RedirectsToReturnUrl
         Assert.Equal("/test", response.Headers.Location?.OriginalString);
     }
 
+    [Theory]
+    [InlineData("https://evil.com")]
+    [InlineData("http://evil.com")]
+    [InlineData("//evil.com")]
+    [InlineData("//evil.com/path")]
+    [InlineData("/\\evil.com")]
+    public async Task ValidateToken_NotBrowserTokenAuth_AbsoluteReturnUrl_RedirectsToHomepage(string returnUrl)
+    {
+        using var host = await SetUpHostAsync(FrontendAuthMode.Unsecured, string.Empty).DefaultTimeout();
+        var response = await host.GetTestClient().GetAsync($"/login?t=test&returnUrl={Uri.EscapeDataString(returnUrl)}").DefaultTimeout();
+        Assert.Equal("/", response.Headers.Location?.OriginalString);
+    }
+
+    [Theory]
+    [InlineData("https://evil.com")]
+    [InlineData("http://evil.com")]
+    [InlineData("//evil.com")]
+    [InlineData("//evil.com/path")]
+    [InlineData("/\\evil.com")]
+    public async Task ValidateToken_BrowserTokenAuth_RightToken_AbsoluteReturnUrl_RedirectsToHomepage(string returnUrl)
+    {
+        using var host = await SetUpHostAsync(FrontendAuthMode.BrowserToken, "token").DefaultTimeout();
+        var response = await host.GetTestClient().GetAsync($"/login?t=token&returnUrl={Uri.EscapeDataString(returnUrl)}").DefaultTimeout();
+        Assert.Equal("/", response.Headers.Location?.OriginalString);
+    }
+
     private static async Task<IHost> SetUpHostAsync(FrontendAuthMode authMode, string expectedToken)
     {
         return await new HostBuilder()