Fix #405

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;
                         }
                     }

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 +134,16 @@ namespace SixLabors.ImageSharp.Tests
                 }
             }
         }
+
+        [Theory]
+        [WithFileCollection(nameof(BadAppExtFiles), PixelTypes)]
+        public void DecodeBadApplicationExtensionLength<TPixel>(TestImageProvider<TPixel> imageProvider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = imageProvider.GetImage())
+            {
+                imageProvider.Utility.SaveTestOutputFile(image, "bmp");
+            }
+        }
     }
 }

tests/ImageSharp.Tests/TestImages.cs

@@ -157,6 +157,12 @@ 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 static readonly string[] All = { Rings, Giphy, Cheers, Trans, Kumin };
         }
     }

tests/Images/Input/Gif/issues/issue405_badappextlength252-2.gif

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:151d394c7c57a1696fedc32116514b667ad70c4a2c7d34db7c489c17d5755eed
+size 24724

tests/Images/Input/Gif/issues/issue405_badappextlength252.gif

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:0549f4500693cdf7eac8351d8cd64d1093da9e9eaaed923e8c6f00476e350f43
+size 41721