#797 throw ImageFormatException when no StartOfFrame marker is found on a jpg image.

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -292,6 +292,11 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                         case JpegConstants.Markers.SOS:
                             if (!metadataOnly)
                             {
+                                if (this.Frame is null)
+                                {
+                                    throw new ImageFormatException("no readable SOF marker found.");
+                                }
+
                                 this.ProcessStartOfScanMarker();
                                 break;
                             }

tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.Baseline.cs

@@ -42,5 +42,13 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             // TODO: We need a public ImageDecoderException class in ImageSharp!
             Assert.ThrowsAny<Exception>(() => provider.GetImage(JpegDecoder));
         }
+
+        [Theory]
+        [WithFile(TestImages.Jpeg.Issues.InvalidJpegThrowsWrongException797, PixelTypes.Rgba32)]
+        public void LoadingImage_InvalidTagLength_ShouldThrow<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            Assert.Throws<ImageFormatException>(() => provider.GetImage());
+        }
     }
 }

tests/ImageSharp.Tests/TestImages.cs

@@ -167,6 +167,7 @@ namespace SixLabors.ImageSharp.Tests
                 public const string OrderedInterleavedProgressive723C = "Jpg/issues/Issue723-Ordered-Interleaved-Progressive-C.jpg";
                 public const string ExifGetString750Transform = "Jpg/issues/issue750-exif-tranform.jpg";
                 public const string ExifGetString750Load = "Jpg/issues/issue750-exif-load.jpg";
+                public const string InvalidJpegThrowsWrongException797 = "Jpg/issues/Issue797-InvalidImage.jpg";
             }
 
             public static readonly string[] All = Baseline.All.Concat(Progressive.All).ToArray();