Add tests and fix for handling invalid input.

src/ImageSharp/Formats/Bmp/BmpImageFormatDetector.cs

@@ -22,9 +22,13 @@ namespace SixLabors.ImageSharp.Formats.Bmp
 
         private bool IsSupportedFileFormat(ReadOnlySpan<byte> header)
         {
-            short fileTypeMarker = BinaryPrimitives.ReadInt16LittleEndian(header);
-            return header.Length >= this.HeaderSize &&
-                   (fileTypeMarker == BmpConstants.TypeMarkers.Bitmap || fileTypeMarker == BmpConstants.TypeMarkers.BitmapArray);
+            if (header.Length >= this.HeaderSize)
+            {
+                short fileTypeMarker = BinaryPrimitives.ReadInt16LittleEndian(header);
+                return fileTypeMarker == BmpConstants.TypeMarkers.Bitmap || fileTypeMarker == BmpConstants.TypeMarkers.BitmapArray;
+            }
+
+            return false;
         }
     }
 }

src/ImageSharp/Formats/Tga/TgaImageFormatDetector.cs

@@ -23,7 +23,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
         {
             if (header.Length >= this.HeaderSize)
             {
-                // There is no magick bytes in a tga file, so at least the image type
+                // There are no magic bytes in a tga file, so at least the image type
                 // and the colormap type in the header will be checked for a valid value.
                 if (header[1] != 0 && header[1] != 1)
                 {
@@ -34,7 +34,7 @@ namespace SixLabors.ImageSharp.Formats.Tga
                 return imageType.IsValid();
             }
 
-            return true;
+            return false;
         }
     }
 }

src/ImageSharp/Image.Decode.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors and contributors.
 // Licensed under the Apache License, Version 2.0.
 
+using System;
 using System.IO;
 using System.Linq;
 using SixLabors.ImageSharp.Formats;
@@ -47,19 +48,26 @@ namespace SixLabors.ImageSharp
         /// <returns>The mime type or null if none found.</returns>
         private static IImageFormat InternalDetectFormat(Stream stream, Configuration config)
         {
-            // This is probably a candidate for making into a public API in the future!
-            int maxHeaderSize = config.MaxHeaderSize;
-            if (maxHeaderSize <= 0)
+            // We take a minimum of the stream length vs the max header size and always check below
+            // to ensure that only formats that headers fit within the given buffer length are tested.
+            int headerSize = (int)Math.Min(config.MaxHeaderSize, stream.Length);
+            if (headerSize <= 0)
             {
                 return null;
             }
 
-            using (IManagedByteBuffer buffer = config.MemoryAllocator.AllocateManagedByteBuffer(maxHeaderSize, AllocationOptions.Clean))
+            using (IManagedByteBuffer buffer = config.MemoryAllocator.AllocateManagedByteBuffer(headerSize, AllocationOptions.Clean))
             {
                 long startPosition = stream.Position;
-                stream.Read(buffer.Array, 0, maxHeaderSize);
+                stream.Read(buffer.Array, 0, headerSize);
                 stream.Position = startPosition;
-                return config.ImageFormatsManager.FormatDetectors.Select(x => x.DetectFormat(buffer.GetSpan())).LastOrDefault(x => x != null);
+
+                // Does the given stream contain enough data to fit in the header for the format
+                // and does that data match the format specification?
+                // Individual formats should still check since they are public.
+                return config.ImageFormatsManager.FormatDetectors
+                    .Where(x => x.HeaderSize <= headerSize)
+                    .Select(x => x.DetectFormat(buffer.GetSpan())).LastOrDefault(x => x != null);
             }
         }
 

tests/ImageSharp.Tests/Formats/GeneralFormatTests.cs

@@ -191,6 +191,20 @@ namespace SixLabors.ImageSharp.Tests
             }
         }
 
+        [Fact]
+        public void IdentifyReturnsNullWithInvalidStream()
+        {
+            byte[] invalid = new byte[10];
+
+            using (var memoryStream = new MemoryStream(invalid))
+            {
+                IImageInfo imageInfo = Image.Identify(memoryStream, out IImageFormat format);
+
+                Assert.Null(imageInfo);
+                Assert.Null(format);
+            }
+        }
+
         private static IImageFormat GetFormat(string format)
         {
             return Configuration.Default.ImageFormats.FirstOrDefault(x => x.FileExtensions.Contains(format));