Harden image URL validation against followed redirects

Endpoints/EndpointHelpers.cs

@@ -122,6 +122,8 @@ internal static class EndpointHelpers
         {
             using var head = new HttpRequestMessage(HttpMethod.Head, uri);
             var headResp = await client.SendAsync(head, HttpCompletionOption.ResponseHeadersRead, cts.Token);
+            if (WasRedirected(uri, headResp))
+                return false;
             if (headResp is { IsSuccessStatusCode: true, StatusCode: not System.Net.HttpStatusCode.Redirect })
             {
                 if (headResp.Content.Headers.ContentLength is > MaxImageBytes)
@@ -142,6 +144,8 @@ internal static class EndpointHelpers
             using var get = new HttpRequestMessage(HttpMethod.Get, uri);
             get.Headers.Range = new System.Net.Http.Headers.RangeHeaderValue(0, 1023);
             var resp = await client.SendAsync(get, HttpCompletionOption.ResponseHeadersRead, cts.Token);
+            if (WasRedirected(uri, resp))
+                return false;
             if (!resp.IsSuccessStatusCode)
                 return false;
             if (resp.StatusCode is System.Net.HttpStatusCode.Redirect)
@@ -179,6 +183,15 @@ internal static class EndpointHelpers
 
     private const long MaxImageBytes = 5 * 1024 * 1024; // 5 MB guard
 
+    private static bool WasRedirected(Uri requestedUri, HttpResponseMessage response)
+    {
+        var finalUri = response.RequestMessage?.RequestUri;
+        if (finalUri is null)
+            return false;
+
+        return !requestedUri.Equals(finalUri);
+    }
+
     private static async Task<bool> IsSafePublicHostAsync(Uri uri, CancellationToken ct)
     {
         try

GameList.Tests/HelperTests.cs

@@ -113,6 +113,14 @@ public class HelperTests
         Assert.False(tooLarge);
     }
 
+    [Fact]
+    public async Task IsReachableImageAsync_rejects_followed_redirect_chain()
+    {
+        var handler = new RedirectFollowingHandler();
+        var reachable = await EndpointHelpers.IsReachableImageAsync("http://example.com/img.png", new StubHttpClientFactory(new StubHttpMessageHandler()), handler);
+        Assert.False(reachable);
+    }
+
     [Fact]
     public async Task IsReachableImageAsync_rejects_non_image_content()
     {
@@ -195,4 +203,29 @@ public class HelperTests
         public string ContentRootPath { get; set; } = "";
         public IFileProvider ContentRootFileProvider { get; set; } = null!;
     }
+
+    private sealed class RedirectFollowingHandler : HttpMessageHandler
+    {
+        private static readonly Uri Source = new("http://example.com/img.png");
+        private static readonly Uri Destination = new("http://example.com/final.png");
+
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            if (request.RequestUri != Source)
+            {
+                return Task.FromResult(new HttpResponseMessage(HttpStatusCode.NotFound) { RequestMessage = request });
+            }
+
+            var redirectedRequest = new HttpRequestMessage(request.Method, Destination);
+            var response = new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                RequestMessage = redirectedRequest,
+                Content = new ByteArrayContent("PNG"u8.ToArray())
+            };
+            response.Content.Headers.ContentType = new MediaTypeHeaderValue("image/png");
+            response.Content.Headers.ContentLength = 3;
+
+            return Task.FromResult(response);
+        }
+    }
 }

Program.cs

@@ -36,7 +36,7 @@ builder.Services.AddDbContext<AppDbContext>(options => options.UseSqlite(connect
 
 builder.Services.ConfigureHttpJsonOptions(options => { options.SerializerOptions.Converters.Add(new JsonStringEnumConverter()); });
 
-builder.Services.AddHttpClient();
+builder.Services.AddHttpClient("imageValidation").ConfigurePrimaryHttpMessageHandler(() => new HttpClientHandler { AllowAutoRedirect = false });
 builder.Services.AddDataProtection().PersistKeysToFileSystem(new DirectoryInfo(dataProtectionDirectory));
 
 builder.Services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme).AddCookie(options =>