Validate screenshot URLs server- and client-side

Endpoints/EndpointHelpers.cs

@@ -34,6 +34,16 @@ internal static class EndpointHelpers
                 ? t[..Math.Min(t.Length, max)]
                 : null;
 
+    public static bool IsValidImageUrl(string? url)
+    {
+        if (string.IsNullOrWhiteSpace(url)) return true; // empty is acceptable
+        if (!Uri.TryCreate(url, UriKind.Absolute, out var uri)) return false;
+        if (uri.Scheme is not ("http" or "https")) return false;
+        var path = uri.AbsolutePath.ToLowerInvariant();
+        return path.EndsWith(".png") || path.EndsWith(".jpg") || path.EndsWith(".jpeg")
+               || path.EndsWith(".gif") || path.EndsWith(".webp") || path.EndsWith(".avif");
+    }
+
     public static async Task<bool> IsAdmin(HttpContext ctx, AppDbContext db, IConfiguration config)
     {
         var player = await GetAuthenticatedPlayer(ctx, db);

Endpoints/SuggestEndpoints.cs

@@ -51,6 +51,11 @@ public static class SuggestEndpoints
                 return Results.BadRequest(new { error = "Name is required and must be <= 100 characters." });
             }
 
+            if (!EndpointHelpers.IsValidImageUrl(request.ScreenshotUrl))
+            {
+                return Results.BadRequest(new { error = "Screenshot URL must be http(s) and end with an image file extension." });
+            }
+
             var player = await EndpointHelpers.GetAuthenticatedPlayer(ctx, db);
             if (player is null) return Results.Unauthorized();
 
@@ -118,6 +123,11 @@ public static class SuggestEndpoints
                 return Results.BadRequest(new { error = "Name is required and must be <= 100 characters." });
             }
 
+            if (!EndpointHelpers.IsValidImageUrl(request.ScreenshotUrl))
+            {
+                return Results.BadRequest(new { error = "Screenshot URL must be http(s) and end with an image file extension." });
+            }
+
             var suggestion = await db.Suggestions.FirstOrDefaultAsync(s => s.Id == id);
             if (suggestion == null) return Results.NotFound(new { error = "Suggestion not found." });
 

wwwroot/app.js

@@ -296,6 +296,9 @@ function setupHandlers() {
     const form = e.target;
     const data = Object.fromEntries(new FormData(form).entries());
     if (!data.name) return toast("Name required", true);
+    if (data.screenshotUrl && !isValidImageUrl(data.screenshotUrl)) {
+      return toast("Screenshot URL must be http(s) and end with an image file.", true);
+    }
     try {
       await api.createSuggestion(data);
       form.reset();
@@ -468,6 +471,9 @@ function openEditModal(s) {
   form?.addEventListener("submit", async (e) => {
     e.preventDefault();
     const data = Object.fromEntries(new FormData(form).entries());
+    if (data.screenshotUrl && !isValidImageUrl(data.screenshotUrl)) {
+      return toast("Screenshot URL must be http(s) and end with an image file.", true);
+    }
     if (!data.name?.trim()) return toast("Name required", true);
     try {
       await api.updateSuggestion(s.id, data);
@@ -516,3 +522,16 @@ async function main() {
 }
 
 main();
+
+function isValidImageUrl(url) {
+  if (!url) return true;
+  try {
+    const u = new URL(url);
+    const allowed = ["http:", "https:"];
+    if (!allowed.includes(u.protocol)) return false;
+    const path = u.pathname.toLowerCase();
+    return [".png", ".jpg", ".jpeg", ".gif", ".webp", ".avif"].some(ext => path.endsWith(ext));
+  } catch {
+    return false;
+  }
+}