diff --git a/src/ImageSharp/Formats/Jxl/Processing/Decoder/JxlBoxContentDecoder.cs b/src/ImageSharp/Formats/Jxl/Processing/Decoder/JxlBoxContentDecoder.cs
new file mode 100644
index 000000000..7621102ef
--- /dev/null
+++ b/src/ImageSharp/Formats/Jxl/Processing/Decoder/JxlBoxContentDecoder.cs
@@ -0,0 +1,108 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Buffers;
+using System.IO.Compression;
+using SixLabors.ImageSharp.Formats.Jxl.IO;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Decoder;
+
+/// <summary>
+/// Allows decoding and decompressing box data.
+/// </summary>
+internal sealed class JxlBoxContentDecoder
+{
+    /// <summary>
+    /// Specifies how many bytes to read to fetch box data. This is ignored
+    /// if the box extends till EOF.
+    /// </summary>
+    private ulong boxSize;
+
+    /// <summary>
+    /// When true the box size is ignored and is unbounded - that is, keeps going
+    /// till the end of the file or stream.
+    /// </summary>
+    private bool boxExtendsTillEnd;
+
+    /// <summary>
+    /// This contains flags that determine whether the box is Brotli-compressed or
+    /// not.
+    /// </summary>
+    private JxlBoxCodingMode codingMode;
+
+    /// <summary>
+    /// Prepares parsing the box.
+    /// </summary>
+    /// <param name="codingMode">Specifies box compression.</param>
+    /// <param name="isUnbounded">Specifies whether or not the box size keeps going till the end of stream.</param>
+    /// <param name="size">Specifies the fixed size of the box when it is not unbounded.</param>
+    public void Initialize(JxlBoxCodingMode codingMode, bool isUnbounded, ulong size)
+    {
+        this.boxSize = size;
+        this.codingMode = codingMode;
+        this.boxExtendsTillEnd = isUnbounded;
+    }
+
+    /// <summary>
+    /// Prepares parsing the box.
+    /// </summary>
+    /// <param name="isBrotliCompressed">True if the box is compressed with Brotli. If uncompressed - false.</param>
+    /// <param name="isUnbounded">Specifies whether or not the box size keeps going till the end of stream.</param>
+    /// <param name="size">Specifies the fixed size of the box when it is not unbounded.</param>
+    public void Initialize(bool isBrotliCompressed, bool isUnbounded, ulong size)
+        => this.Initialize(
+            isBrotliCompressed ? JxlBoxCodingMode.Brotli : JxlBoxCodingMode.Uncompressed,
+            isUnbounded,
+            size);
+
+    public void Process(Stream stream, JxlMemoryWriter writer)
+    {
+        byte[] cache = ArrayPool<byte>.Shared.Rent(16384);
+
+        try
+        {
+            if (this.codingMode == JxlBoxCodingMode.Brotli)
+            {
+                using BrotliStream brotli = new(stream, CompressionMode.Decompress, leaveOpen: true);
+
+                int bytesRead;
+                while ((bytesRead = brotli.Read(cache, 0, cache.Length)) > 0)
+                {
+                    writer.Write(cache.AsSpan(0, bytesRead));
+                }
+            }
+            else
+            {
+                if (this.boxExtendsTillEnd)
+                {
+                    int bytesRead;
+                    while ((bytesRead = stream.Read(cache, 0, cache.Length)) > 0)
+                    {
+                        writer.Write(cache.AsSpan(0, bytesRead));
+                    }
+                }
+                else
+                {
+                    ulong bytesLeft = this.boxSize;
+                    while (bytesLeft > 0)
+                    {
+                        int toRead = (int)Math.Min((ulong)cache.Length, bytesLeft);
+                        int bytesRead = stream.Read(cache, 0, toRead);
+
+                        if (bytesRead == 0)
+                        {
+                            throw new EndOfStreamException("Unexpected EOF while reading box content");
+                        }
+
+                        writer.Write(cache.AsSpan(0, bytesRead));
+                        bytesLeft -= (ulong)bytesRead;
+                    }
+                }
+            }
+        }
+        finally
+        {
+            ArrayPool<byte>.Shared.Return(cache);
+        }
+    }
+}
diff --git a/src/ImageSharp/Formats/Jxl/Processing/Decoder/JxlDecoderCore.cs b/src/ImageSharp/Formats/Jxl/Processing/Decoder/JxlDecoderCore.cs
index 016269e4d..7ca3ace0e 100644
--- a/src/ImageSharp/Formats/Jxl/Processing/Decoder/JxlDecoderCore.cs
+++ b/src/ImageSharp/Formats/Jxl/Processing/Decoder/JxlDecoderCore.cs
@@ -1508,7 +1508,7 @@ internal sealed class JxlDecoderCore : ImageDecoderCore, IDisposable
             Span<byte> fileSignature = stackalloc byte[2];
             stream.ReadExactly(fileSignature);
 
-            if (fileSignature[0] != 0xFF || fileSignature[1] != CodestreamMarker)
+            if (fileSignature[0] != 0xFF || fileSignature[1] != JxlShared.CodestreamMarker)
             {
                 throw new InvalidOperationException("The file signature is invalid");
             }
@@ -1553,7 +1553,7 @@ internal sealed class JxlDecoderCore : ImageDecoderCore, IDisposable
         {
             Span<byte> span = this.GetCodeStreamSpan();
 
-            JxlBitReader reader = new(span);
+            JxlBitReader reader = new(this.stream);
             reader.SkipBits64((ulong)this.codestreamBitsAhead);
 
             this.metadata!.CustomTransformData!.NonserializedXybEncoded = this.metadata.ImageMetadata!.XybEncoded;
@@ -2286,7 +2286,7 @@ internal sealed class JxlDecoderCore : ImageDecoderCore, IDisposable
                 {
                     if (this.decoderStage != JxlDecoderStage.CodeStreamFinished || this.JbrdNeedsMoreBoxes())
                     {
-                        return NeedMoreInput;
+                        ThrowNotEnoughData();
                     }
 
                     if (this.inputClosed || (this.eventsWanted & Box) != 0)
@@ -2294,7 +2294,7 @@ internal sealed class JxlDecoderCore : ImageDecoderCore, IDisposable
                         return Success;
                     }
 
-                    return NeedMoreInput;
+                    ThrowNotEnoughData();
                 }
 
                 bool boxedCodestreamDone = ((this.eventsWanted & Box) != 0)
@@ -2556,7 +2556,7 @@ internal sealed class JxlDecoderCore : ImageDecoderCore, IDisposable
 
                 if (!boxDone)
                 {
-                    return NeedMoreInput;
+                    ThrowNotEnoughData();
                 }
 
                 this.boxStage = JxlBoxStage.Header;