harden(gateway): prefix-boundary + path-traversal checks; simplify ticket validation

Tighten the stream-ticket authorization in the gateway middleware:
- Enforce a path-segment boundary on the prefix check instead of a bare
  StartsWith, so a ticket for 'users/12' no longer grants 'users/1234/...'
  (exact-match still allowed for single-object grants)
- Reject object keys containing '..'/'.' segments before proxying, closing a
  path-traversal bypass that SeaweedFS would otherwise resolve outside the
  authorized prefix

Also simplify TicketValidator.ValidateTicketAsync from ErrorOr<string> to
string? (null = invalid) since it has a single failure mode; drop the now
unused ErrorOr package reference from the gateway.

Verified: build clean; the authorization predicate passes a table of cases
covering both the escalation and traversal vulnerabilities.

Build: 0 warnings, 0 errors.

backend/Musify.StreamingGateway/Authentication/TicketValidator.cs

@@ -1,5 +1,4 @@
 using System.Security.Cryptography;
-using ErrorOr;
 using Microsoft.Extensions.Options;
 using Microsoft.IdentityModel.JsonWebTokens;
 using Microsoft.IdentityModel.Tokens;
@@ -35,25 +34,24 @@ public sealed class TicketValidator : IDisposable
         };
     }
 
-    public async Task<ErrorOr<string>> ValidateTicketAsync(string? token)
+    /// <summary>
+    /// Validates the stream ticket and returns the authorized object-key prefix,
+    /// or <see langword="null"/> if the ticket is missing, invalid, or carries no prefix.
+    /// </summary>
+    public async Task<string?> ValidateTicketAsync(string? token)
     {
         if (string.IsNullOrWhiteSpace(token))
-            return Error.Unauthorized();
+            return null;
 
         var result = await _handler.ValidateTokenAsync(token, _validationParameters);
         if (!result.IsValid)
-            return Error.Unauthorized();
+            return null;
 
-        if (!result.Claims.TryGetValue("prefix", out var claim))
-            return Error.Unauthorized();
-
-        if (claim is not string prefix)
-            return Error.Unauthorized();
-
-        if (string.IsNullOrEmpty(prefix))
-            return Error.Unauthorized();
-
-        return prefix;
+        return result.Claims.TryGetValue("prefix", out var claim)
+               && claim is string prefix
+               && !string.IsNullOrEmpty(prefix)
+            ? prefix
+            : null;
     }
 
     public void Dispose() => _rsa.Dispose();

backend/Musify.StreamingGateway/Middleware/TicketValidationMiddleware.cs

@@ -28,19 +28,27 @@ public sealed class TicketValidationMiddleware(
             token = context.Request.Headers[options.HeaderName].ToString();
         }
 
-        var validation = await ticketValidator.ValidateTicketAsync(token);
+        var prefix = await ticketValidator.ValidateTicketAsync(token);
 
-        if (validation.IsError)
+        if (prefix is null)
         {
             logger.LogWarning("Rejected media request {Path}: missing or invalid ticket", path);
             context.Response.StatusCode = StatusCodes.Status401Unauthorized;
             return;
         }
 
-        var prefix = validation.Value;
-
         var objectKey = remaining.Value?.TrimStart('/') ?? string.Empty;
-        if (!objectKey.StartsWith(prefix, StringComparison.Ordinal))
+
+        if (HasTraversalSegment(objectKey))
+        {
+            logger.LogWarning("Rejected media request {ObjectKey}: contains a path traversal segment", objectKey);
+            context.Response.StatusCode = StatusCodes.Status400BadRequest;
+            return;
+        }
+
+        var prefixBoundary = prefix.EndsWith('/') ? prefix : prefix + "/";
+        if (!string.Equals(objectKey, prefix, StringComparison.Ordinal)
+            && !objectKey.StartsWith(prefixBoundary, StringComparison.Ordinal))
         {
             logger.LogWarning("Rejected media request {ObjectKey}: outside authorized prefix {Prefix}", objectKey, prefix);
             context.Response.StatusCode = StatusCodes.Status403Forbidden;
@@ -51,6 +59,9 @@ public sealed class TicketValidationMiddleware(
         await next(context);
     }
 
+    private static bool HasTraversalSegment(string objectKey) =>
+        objectKey.Split('/').Any(segment => segment is ".." or ".");
+
     private static void StripQueryParameter(HttpContext context, string name)
     {
         var remaining = context.Request.Query

backend/Musify.StreamingGateway/Musify.StreamingGateway.csproj

@@ -5,7 +5,6 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="ErrorOr" />
     <PackageReference Include="Microsoft.IdentityModel.JsonWebTokens" />
     <PackageReference Include="Yarp.ReverseProxy" />
   </ItemGroup>