Fix #403

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -443,7 +443,8 @@ namespace SixLabors.ImageSharp.Formats.Gif
 
                 var rgba = new Rgba32(0, 0, 0, 255);
 
-                for (int x = descriptor.Left; x < descriptor.Left + descriptor.Width; x++)
+                // #403 The left + width value can be larger than the image width
+                for (int x = descriptor.Left; x < descriptor.Left + descriptor.Width && x < rowSpan.Length; x++)
                 {
                     int index = indices[i];
 

tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs

@@ -136,7 +136,7 @@ namespace SixLabors.ImageSharp.Tests
         }
 
         [Theory]
-        [WithFileCollection(nameof(BadAppExtFiles), PixelTypes)]
+        [WithFileCollection(nameof(BadAppExtFiles), PixelTypes.Rgba32)]
         public void DecodeBadApplicationExtensionLength<TPixel>(TestImageProvider<TPixel> imageProvider)
             where TPixel : struct, IPixel<TPixel>
         {
@@ -145,5 +145,16 @@ namespace SixLabors.ImageSharp.Tests
                 imageProvider.Utility.SaveTestOutputFile(image, "bmp");
             }
         }
+
+        [Theory]
+        [WithFile(TestImages.Gif.Issues.BadDescriptorWidth, PixelTypes.Rgba32)]
+        public void DecodeBadDescriptorDimensionsLength<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage())
+            {
+                provider.Utility.SaveTestOutputFile(image, "bmp");
+            }
+        }
     }
 }

tests/ImageSharp.Tests/TestImages.cs

@@ -161,6 +161,7 @@ namespace SixLabors.ImageSharp.Tests
             {
                 public const string BadAppExtLength = "Gif/issues/issue405_badappextlength252.gif";
                 public const string BadAppExtLength_2 = "Gif/issues/issue405_badappextlength252-2.gif";
+                public const string BadDescriptorWidth = "Gif/issues/issue403_baddescriptorwidth.gif";
             }
 
             public static readonly string[] All = { Rings, Giphy, Cheers, Trans, Kumin };