diff --git a/src/ImageSharp/Formats/Gif/GifDecoderCore.cs b/src/ImageSharp/Formats/Gif/GifDecoderCore.cs
index 80ee3d130..ae20be7d5 100644
--- a/src/ImageSharp/Formats/Gif/GifDecoderCore.cs
+++ b/src/ImageSharp/Formats/Gif/GifDecoderCore.cs
@@ -155,10 +155,15 @@ namespace SixLabors.ImageSharp.Formats.Gif
                                 this.ReadComments();
                                 break;
                             case GifConstants.ApplicationExtensionLabel:
-                                this.Skip(12); // No need to read.
+
+                                // The application extension length should be 11 but we've got test images that incorrectly
+                                // set this to 252.
+                                int appLength = stream.ReadByte();
+                                this.Skip(appLength); // No need to read.
                                 break;
                             case GifConstants.PlainTextLabel:
-                                this.Skip(13); // Not supported by any known decoder.
+                                int plainLength = stream.ReadByte();
+                                this.Skip(plainLength); // Not supported by any known decoder.
                                 break;
                         }
                     }
@@ -438,7 +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];
 
diff --git a/tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs b/tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs
index a4a27bd83..76a31c186 100644
--- a/tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs
@@ -18,6 +18,7 @@ namespace SixLabors.ImageSharp.Tests
 
         public static readonly string[] TestFiles = { TestImages.Gif.Giphy, TestImages.Gif.Rings, TestImages.Gif.Trans };
 
+        public static readonly string[] BadAppExtFiles = { TestImages.Gif.Issues.BadAppExtLength, TestImages.Gif.Issues.BadAppExtLength_2 };
 
         [Theory]
         [WithFileCollection(nameof(TestFiles), PixelTypes)]
@@ -30,6 +31,7 @@ namespace SixLabors.ImageSharp.Tests
                 imageProvider.Utility.SaveTestOutputFile(image, "gif");
             }
         }
+
         [Theory]
         [WithFileCollection(nameof(TestFiles), PixelTypes)]
         public void DecodeResizeAndSave<TPixel>(TestImageProvider<TPixel> imageProvider)
@@ -132,5 +134,27 @@ namespace SixLabors.ImageSharp.Tests
                 }
             }
         }
+
+        [Theory]
+        [WithFileCollection(nameof(BadAppExtFiles), PixelTypes.Rgba32)]
+        public void DecodeBadApplicationExtensionLength<TPixel>(TestImageProvider<TPixel> imageProvider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = imageProvider.GetImage())
+            {
+                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");
+            }
+        }
     }
 }
\ No newline at end of file
diff --git a/tests/ImageSharp.Tests/TestImages.cs b/tests/ImageSharp.Tests/TestImages.cs
index 7e98e50ad..365aea04a 100644
--- a/tests/ImageSharp.Tests/TestImages.cs
+++ b/tests/ImageSharp.Tests/TestImages.cs
@@ -157,6 +157,13 @@ namespace SixLabors.ImageSharp.Tests
             public const string Trans = "Gif/trans.gif";
             public const string Kumin = "Gif/kumin.gif";
 
+            public class Issues
+            {
+                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 };
         }
     }
diff --git a/tests/Images/Input/Gif/issues/issue403_baddescriptorwidth.gif b/tests/Images/Input/Gif/issues/issue403_baddescriptorwidth.gif
new file mode 100644
index 000000000..fd13fbfb7
Binary files /dev/null and b/tests/Images/Input/Gif/issues/issue403_baddescriptorwidth.gif differ
diff --git a/tests/Images/Input/Gif/issues/issue405_badappextlength252-2.gif b/tests/Images/Input/Gif/issues/issue405_badappextlength252-2.gif
new file mode 100644
index 000000000..6b7caa31d
Binary files /dev/null and b/tests/Images/Input/Gif/issues/issue405_badappextlength252-2.gif differ
diff --git a/tests/Images/Input/Gif/issues/issue405_badappextlength252.gif b/tests/Images/Input/Gif/issues/issue405_badappextlength252.gif
new file mode 100644
index 000000000..f3e36c941
Binary files /dev/null and b/tests/Images/Input/Gif/issues/issue405_badappextlength252.gif differ