Read and validate the TIFF file header.

src/ImageSharp.Formats.Tiff/TiffConstants.cs

@@ -10,17 +10,17 @@ namespace ImageSharp.Formats
     /// <summary>
     /// Defines constants defined in the TIFF specification.
     /// </summary>
-    internal static class GifConstants
+    internal static class TiffConstants
     {
         /// <summary>
         /// Byte order markers for indicating little endian encoding.
         /// </summary>
-        public const ushort ByteOrderLittleEndian = 0x4949;
+        public const byte ByteOrderLittleEndian = 0x49;
 
         /// <summary>
         /// Byte order markers for indicating big endian encoding.
         /// </summary>
-        public const ushort ByteOrderBigEndian = 0x4D4D;
+        public const byte ByteOrderBigEndian = 0x4D;
 
         /// <summary>
         /// Magic number used within the image file header to identify a TIFF format file.

src/ImageSharp.Formats.Tiff/TiffDecoderCore.cs

@@ -20,6 +20,11 @@ namespace ImageSharp.Formats
         /// </summary>
         private readonly IDecoderOptions options;
 
+        /// <summary>
+        /// A flag indicating if the file is encoded in little-endian or big-endian format.
+        /// </summary>
+        private bool isLittleEndian;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="TiffDecoderCore" /> class.
         /// </summary>
@@ -46,6 +51,8 @@ namespace ImageSharp.Formats
             where TColor : struct, IPixel<TColor>
         {
             this.InputStream = stream;
+
+            uint firstIfdOffset = ReadHeader();
         }
 
         /// <summary>
@@ -54,5 +61,69 @@ namespace ImageSharp.Formats
         public void Dispose()
         {
         }
+
+        private uint ReadHeader()
+        {
+            byte[] headerBytes = new byte[8];
+            ReadBytes(headerBytes, 8);
+
+            if (headerBytes[0] == TiffConstants.ByteOrderLittleEndian && headerBytes[1] == TiffConstants.ByteOrderLittleEndian)
+                isLittleEndian = true;
+            else if (headerBytes[0] != TiffConstants.ByteOrderBigEndian && headerBytes[1] != TiffConstants.ByteOrderBigEndian)
+                throw new ImageFormatException("Invalid TIFF file header.");
+
+            if (ToUInt16(headerBytes, 2) != TiffConstants.HeaderMagicNumber)
+                throw new ImageFormatException("Invalid TIFF file header.");
+
+            uint firstIfdOffset = ToUInt32(headerBytes, 4);
+            if (firstIfdOffset == 0)
+                throw new ImageFormatException("Invalid TIFF file header.");
+
+            return firstIfdOffset;
+        }
+
+        private byte[] ReadBytes(byte[] buffer, int count)
+        {
+            int offset = 0;
+
+            while (count > 0)
+            {
+                int bytesRead = InputStream.Read(buffer, offset, count);
+
+                if (bytesRead == 0)
+                    break;
+
+                offset += bytesRead;
+                count -= bytesRead;
+            }
+
+            return buffer;
+        }
+
+        private Int16 ToInt16(byte[] bytes, int offset)
+        {
+            if (isLittleEndian)
+                return (short)(bytes[offset + 0] | (bytes[offset + 1] << 8));
+            else
+                return (short)((bytes[offset + 0] << 8) | bytes[offset + 1]);
+        }
+
+        private Int32 ToInt32(byte[] bytes, int offset)
+        {
+            if (isLittleEndian)
+                return bytes[offset + 0] | (bytes[offset + 1] << 8) | (bytes[offset + 2] << 16) | (bytes[offset + 3] << 24);
+            else
+                return (bytes[offset + 0] << 24) | (bytes[offset + 1] << 16) | (bytes[offset + 2] << 8) | bytes[offset + 3];
+        }
+
+        private UInt32 ToUInt32(byte[] bytes, int offset)
+        {
+            return (uint)ToInt32(bytes, offset);
+        }
+
+        private UInt16 ToUInt16(byte[] bytes, int offset)
+        {
+            return (ushort)ToInt16(bytes, offset);
+        }
     }
 }

tests/ImageSharp.Formats.Tiff.Tests/Formats/Tiff/TiffDecoderHeaderTests.cs

@@ -0,0 +1,79 @@
+// <copyright file="TiffDecoderHeaderTests.cs" company="James Jackson-South">
+// Copyright (c) James Jackson-South and contributors.
+// Licensed under the Apache License, Version 2.0.
+// </copyright>
+
+namespace ImageSharp.Tests
+{
+    using System.IO;
+    using System.Linq;
+    using Xunit;
+
+    using ImageSharp.Formats;
+
+    public class TiffDecoderHeaderTests
+    {
+        public static object[][] IsLittleEndianValues = new[] { new object[] { false },
+                                                                new object[] { true } };
+
+        [Theory]
+        [MemberData(nameof(IsLittleEndianValues))]
+        public void Decode_ThrowsException_WithInvalidByteOrderMarkers(bool isLittleEndian)
+        {
+            Stream stream = new TiffGenHeader()
+                            {
+                                FirstIfd = new TiffGenIfd(),
+                                ByteOrderMarker = 0x1234
+                            }
+                            .ToStream(isLittleEndian);
+
+            TiffDecoder decoder = new TiffDecoder();
+            
+            ImageFormatException e = Assert.Throws<ImageFormatException>(() => { TestDecode(decoder, stream); });
+            
+            Assert.Equal("Invalid TIFF file header.", e.Message);
+        }
+
+        [Theory]
+        [MemberData(nameof(IsLittleEndianValues))]
+        public void Decode_ThrowsException_WithIncorrectMagicNumber(bool isLittleEndian)
+        {
+            Stream stream = new TiffGenHeader()
+                            {
+                                FirstIfd = new TiffGenIfd(),
+                                MagicNumber = 32
+                            }
+                            .ToStream(isLittleEndian);
+
+            TiffDecoder decoder = new TiffDecoder();
+            
+            ImageFormatException e = Assert.Throws<ImageFormatException>(() => { TestDecode(decoder, stream); });
+            
+            Assert.Equal("Invalid TIFF file header.", e.Message);
+        }
+
+        [Theory]
+        [MemberData(nameof(IsLittleEndianValues))]
+        public void Decode_ThrowsException_WithNoIfdZero(bool isLittleEndian)
+        {
+            Stream stream = new TiffGenHeader()
+                            {
+                                FirstIfd = null
+                            }
+                            .ToStream(isLittleEndian);
+
+            TiffDecoder decoder = new TiffDecoder();
+            
+            ImageFormatException e = Assert.Throws<ImageFormatException>(() => { TestDecode(decoder, stream); });
+            
+            Assert.Equal("Invalid TIFF file header.", e.Message);
+        }
+
+        private void TestDecode(TiffDecoder decoder, Stream stream)
+        {
+            Configuration.Default.AddImageFormat(new TiffFormat());
+            Image image = new Image(1,1);
+            decoder.Decode<Color>(image, stream, null);
+        }
+    }
+}

tests/ImageSharp.Formats.Tiff.Tests/TestUtilities/Tiff/TiffGenHeader.cs

@@ -32,11 +32,17 @@ namespace ImageSharp.Tests
             bytes.AddUInt32(0);
 
             var headerData = new TiffGenDataBlock(bytes.ToArray());
-            var firstIfdData = FirstIfd.GetData(isLittleEndian);
 
-            firstIfdData.First().AddReference(headerData.Bytes, 4);
-
-            return new [] { headerData }.Concat(firstIfdData);
+            if (FirstIfd != null)
+            {
+                var firstIfdData = FirstIfd.GetData(isLittleEndian);
+                firstIfdData.First().AddReference(headerData.Bytes, 4);
+                return new [] { headerData }.Concat(firstIfdData);
+            }
+            else
+            {
+                return new [] { headerData };
+            }
         }
     }
 }