From ea2986708f6be8e49bdd6fee75b94024cb34e02b Mon Sep 17 00:00:00 2001
From: Wleeaf <179125857+wleeaf@users.noreply.github.com>
Date: Mon, 26 Oct 2026 16:48:33 +0000
Subject: [PATCH] Set the UTF-8 flag for zip entry names encoded as UTF-8
 without a BOM

---
 .../Writers/Zip/ZipCentralDirectoryEntry.cs   |  7 +++--
 src/SharpCompress/Writers/Zip/ZipWriter.cs    |  9 ++++--
 .../SharpCompress.Test/Zip/ZipWriterTests.cs  | 28 +++++++++++++++++++
 3 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/src/SharpCompress/Writers/Zip/ZipCentralDirectoryEntry.cs b/src/SharpCompress/Writers/Zip/ZipCentralDirectoryEntry.cs
index 3a498073..3f084d76 100644
--- a/src/SharpCompress/Writers/Zip/ZipCentralDirectoryEntry.cs
+++ b/src/SharpCompress/Writers/Zip/ZipCentralDirectoryEntry.cs
@@ -104,9 +104,10 @@ internal class ZipCentralDirectoryEntry
             version = 20;
         }
 
-        var flags = Equals(archiveEncoding.GetEncoding(), Encoding.UTF8)
-            ? HeaderFlags.Efs
-            : HeaderFlags.None;
+        var flags =
+            archiveEncoding.GetEncoding().CodePage == Encoding.UTF8.CodePage
+                ? HeaderFlags.Efs
+                : HeaderFlags.None;
         if (UsesDataDescriptor)
         {
             flags |= HeaderFlags.UsePostDataDescriptor;
diff --git a/src/SharpCompress/Writers/Zip/ZipWriter.cs b/src/SharpCompress/Writers/Zip/ZipWriter.cs
index 84debe9d..e1a1e5b6 100644
--- a/src/SharpCompress/Writers/Zip/ZipWriter.cs
+++ b/src/SharpCompress/Writers/Zip/ZipWriter.cs
@@ -207,9 +207,12 @@ public partial class ZipWriter : AbstractWriter
         {
             stream.Write(stackalloc byte[] { 63, 0 }); //version says we used PPMd or LZMA
         }
-        var flags = Equals(WriterOptions.ArchiveEncoding.GetEncoding(), Encoding.UTF8)
-            ? HeaderFlags.Efs
-            : 0;
+        // Compare code pages: Encoding.Default on .NET Core is a UTF-8 encoding without a BOM,
+        // which isn't Equal to Encoding.UTF8.
+        var flags =
+            WriterOptions.ArchiveEncoding.GetEncoding().CodePage == Encoding.UTF8.CodePage
+                ? HeaderFlags.Efs
+                : 0;
         if (usesDataDescriptor)
         {
             flags |= HeaderFlags.UsePostDataDescriptor;
diff --git a/tests/SharpCompress.Test/Zip/ZipWriterTests.cs b/tests/SharpCompress.Test/Zip/ZipWriterTests.cs
index 70c2632d..d22a857c 100644
--- a/tests/SharpCompress.Test/Zip/ZipWriterTests.cs
+++ b/tests/SharpCompress.Test/Zip/ZipWriterTests.cs
@@ -1,3 +1,5 @@
+using System;
+using System.Buffers.Binary;
 using System.IO;
 using System.Text;
 using SharpCompress.Common;
@@ -47,6 +49,32 @@ public class ZipWriterTests : WriterTests
         Assert.True(memoryStream.Length > 0);
     }
 
+    [Fact]
+    public void Zip_Write_Utf8_Name_Sets_Efs_Flag()
+    {
+        // Encoding.Default is a UTF-8 encoding without a BOM on .NET Core
+        using var memoryStream = new MemoryStream();
+        var options = new WriterOptions(CompressionType.Deflate)
+        {
+            ArchiveEncoding = new ArchiveEncoding { Default = new UTF8Encoding(false) },
+        };
+
+        using (var writer = WriterFactory.OpenWriter(memoryStream, ArchiveType.Zip, options))
+        {
+            writer.Write("d\u00e9j\u00e0-vu.txt", new MemoryStream(new byte[] { 1, 2, 3 }));
+        }
+
+        var zip = memoryStream.ToArray();
+        var central = BinaryPrimitives.ReadInt32LittleEndian(zip.AsSpan(zip.Length - 6));
+
+        // Bit 11 of the general purpose flags marks the name as UTF-8 for other readers
+        Assert.Equal(0x800, BinaryPrimitives.ReadUInt16LittleEndian(zip.AsSpan(6)) & 0x800);
+        Assert.Equal(
+            0x800,
+            BinaryPrimitives.ReadUInt16LittleEndian(zip.AsSpan(central + 8)) & 0x800
+        );
+    }
+
     [Fact]
     public void Zip_Deflate_Write() =>
         Write(