diff --git a/src/ui/Features/Video/TextToSpeech/VoiceManager/VoicePacks/VoicePack.cs b/src/ui/Features/Video/TextToSpeech/VoiceManager/VoicePacks/VoicePack.cs
index a2046a52567..1119c2fb6dc 100644
--- a/src/ui/Features/Video/TextToSpeech/VoiceManager/VoicePacks/VoicePack.cs
+++ b/src/ui/Features/Video/TextToSpeech/VoiceManager/VoicePacks/VoicePack.cs
@@ -18,7 +18,7 @@ public class VoicePack
     public long SizeBytes { get; }
     public string License { get; }
 
-    /// <summary>Lower-case SHA-256 of the zip; empty skips verification.</summary>
+    /// <summary>Required lower-case SHA-256 of the zip.</summary>
     public string Sha256 { get; }
 
     public VoicePack(string id, string name, string languageName, string languageCode, string description, string url, int voiceCount, long sizeBytes, string license, string sha256)
diff --git a/src/ui/Logic/Download/VoicePackDownloadService.cs b/src/ui/Logic/Download/VoicePackDownloadService.cs
index 87f74e6e7f1..1b8b94d73e8 100644
--- a/src/ui/Logic/Download/VoicePackDownloadService.cs
+++ b/src/ui/Logic/Download/VoicePackDownloadService.cs
@@ -28,16 +28,43 @@ public VoicePackDownloadService(HttpClient httpClient)
 
     public async Task DownloadPack(VoicePack pack, Stream stream, IProgress<float>? progress, CancellationToken cancellationToken)
     {
+        if (string.IsNullOrWhiteSpace(pack.Sha256))
+        {
+            throw new InvalidOperationException($"No SHA-256 is registered for voice pack '{pack.Name}'.");
+        }
+
+        if (!stream.CanRead || !stream.CanSeek)
+        {
+            throw new InvalidOperationException("Voice pack integrity verification requires a readable, seekable stream.");
+        }
+
         await DownloadHelper.DownloadFileAsync(_httpClient, pack.Url, stream, progress, cancellationToken);
+        await VerifyPackAsync(pack, stream, cancellationToken);
+    }
 
-        if (string.IsNullOrEmpty(pack.Sha256) || stream.Length == 0)
+    internal static async Task VerifyPackAsync(VoicePack pack, Stream stream, CancellationToken cancellationToken)
+    {
+        if (string.IsNullOrWhiteSpace(pack.Sha256))
         {
-            return;
+            throw new InvalidOperationException($"No SHA-256 is registered for voice pack '{pack.Name}'.");
         }
 
+        if (!stream.CanRead || !stream.CanSeek)
+        {
+            throw new InvalidOperationException("Voice pack integrity verification requires a readable, seekable stream.");
+        }
+
+        string actual;
         stream.Position = 0;
-        var actual = await Sha256Util.ComputeSha256Async(stream, cancellationToken);
-        stream.Position = 0;
+        try
+        {
+            actual = await Sha256Util.ComputeSha256Async(stream, cancellationToken);
+        }
+        finally
+        {
+            stream.Position = 0;
+        }
+
         if (!string.Equals(pack.Sha256, actual, StringComparison.OrdinalIgnoreCase))
         {
             throw new IOException($"Voice pack '{pack.Name}' failed integrity check (expected SHA-256 {pack.Sha256}, got {actual}).");
diff --git a/tests/UI/Logic/Download/VoicePackDownloadServiceTests.cs b/tests/UI/Logic/Download/VoicePackDownloadServiceTests.cs
new file mode 100644
index 00000000000..72c2d7e9719
--- /dev/null
+++ b/tests/UI/Logic/Download/VoicePackDownloadServiceTests.cs
@@ -0,0 +1,143 @@
+using System.Linq;
+using System.Net;
+using System.Text;
+using Nikse.SubtitleEdit.Features.Video.TextToSpeech.VoiceManager.VoicePacks;
+using Nikse.SubtitleEdit.Logic.Download;
+
+namespace UITests.Logic.Download;
+
+public class VoicePackDownloadServiceTests
+{
+    [Theory]
+    [InlineData("en-standard", "8935dcb18c71fe261e95c0e7c8e4f6cbca153c76109bb946ba4dfe1f115fcada")]
+    [InlineData("de", "c541c76ab960ab31165d753385016012622bd36e43f0fdb3e53e96b31741b88d")]
+    [InlineData("es", "dcd84cf4286152c2dd23436ce36d42427a587af5f9213162f4f61adc6510ec84")]
+    [InlineData("fr", "f0a81f8558d9c3c42de55915b5721a98ef0d1db4f4da52d7e14d848349d7e71f")]
+    [InlineData("it", "ad082e544bc515a54d52736d882b90f00e7a88f07ce5f69e66eb18ec823640f2")]
+    [InlineData("nl", "260a8f4900242b9fd655d4bde7a9d5b2d9e4b5ddfd24f159e78ff1af08541d2a")]
+    [InlineData("pl", "d2f43bdf5d9d955f1523d65dfe91de502892fca1708322510a9bab7d3efae55a")]
+    [InlineData("pt", "7ef61545300c4cfbd0c837acb80625835f3f4e673461ac4eae3b444eb33a8685")]
+    public void CatalogHash_MatchesPublishedReleaseDigest(string id, string expected)
+    {
+        var pack = VoicePackCatalog.All.Single(p => p.Id == id);
+        Assert.Equal(expected, pack.Sha256);
+    }
+
+    [Fact]
+    public async Task DownloadPack_MissingDigest_FailsBeforeHttp()
+    {
+        var handler = new StaticResponseHandler(Encoding.ASCII.GetBytes("unused"));
+        using var httpClient = new HttpClient(handler);
+        var service = new VoicePackDownloadService(httpClient);
+        var pack = CreatePack(string.Empty);
+        await using var stream = new MemoryStream();
+
+        await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            service.DownloadPack(pack, stream, progress: null, TestContext.Current.CancellationToken));
+
+        Assert.Equal(0, handler.RequestCount);
+    }
+
+    [Fact]
+    public async Task DownloadPack_TamperedPayload_IsRejectedAndRewound()
+    {
+        var handler = new StaticResponseHandler(Encoding.ASCII.GetBytes("tampered"));
+        using var httpClient = new HttpClient(handler);
+        var service = new VoicePackDownloadService(httpClient);
+        var pack = CreatePack("cea23dd4b87e8b8634567c2f6c9a3a31ce51ec44c2c45a70ac999aa00516a04b");
+        await using var stream = new MemoryStream();
+
+        await Assert.ThrowsAsync<IOException>(() =>
+            service.DownloadPack(pack, stream, progress: null, TestContext.Current.CancellationToken));
+
+        Assert.Equal(0, stream.Position);
+        Assert.True(handler.RequestCount > 0);
+    }
+
+    [Fact]
+    public async Task VerifyPackAsync_ValidPayload_RewindsStream()
+    {
+        var pack = CreatePack("ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad");
+        await using var stream = new MemoryStream(Encoding.ASCII.GetBytes("abc"));
+        stream.Position = stream.Length;
+
+        await VoicePackDownloadService.VerifyPackAsync(
+            pack,
+            stream,
+            TestContext.Current.CancellationToken);
+
+        Assert.Equal(0, stream.Position);
+    }
+
+    [Fact]
+    public async Task VerifyPackAsync_EmptyStream_IsRejectedAndRewound()
+    {
+        var pack = VoicePackCatalog.All[0];
+        await using var stream = new MemoryStream();
+
+        await Assert.ThrowsAsync<IOException>(() =>
+            VoicePackDownloadService.VerifyPackAsync(
+                pack,
+                stream,
+                TestContext.Current.CancellationToken));
+
+        Assert.Equal(0, stream.Position);
+    }
+
+    [Fact]
+    public async Task DownloadPack_NonSeekableStream_FailsBeforeHttp()
+    {
+        var handler = new StaticResponseHandler(Encoding.ASCII.GetBytes("unused"));
+        using var httpClient = new HttpClient(handler);
+        var service = new VoicePackDownloadService(httpClient);
+        var pack = VoicePackCatalog.All[0];
+        await using var stream = new NonSeekableReadStream(Encoding.ASCII.GetBytes("data"));
+
+        await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            service.DownloadPack(pack, stream, progress: null, TestContext.Current.CancellationToken));
+
+        Assert.Equal(0, handler.RequestCount);
+    }
+
+    private static VoicePack CreatePack(string sha256)
+    {
+        return new VoicePack(
+            "test",
+            "Test pack",
+            "Test",
+            "xx",
+            "Test voice pack",
+            "https://example.test/voices.zip",
+            1,
+            123,
+            "Test license",
+            sha256);
+    }
+
+    private sealed class StaticResponseHandler(byte[] payload) : HttpMessageHandler
+    {
+        public int RequestCount { get; private set; }
+
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            RequestCount++;
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+            {
+                Content = new ByteArrayContent(payload),
+            });
+        }
+    }
+
+    private sealed class NonSeekableReadStream(byte[] data) : MemoryStream(data)
+    {
+        public override bool CanSeek => false;
+
+        public override long Position
+        {
+            get => base.Position;
+            set => throw new NotSupportedException();
+        }
+
+        public override long Seek(long offset, SeekOrigin loc) => throw new NotSupportedException();
+    }
+}