diff --git a/src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs b/src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs
index 011b6100d..e3b0b4bdc 100644
--- a/src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs
+++ b/src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs
@@ -510,7 +510,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// <param name="remaining">The remaining bytes in the segment block.</param>
         private void ProcessApplicationHeaderMarker(int remaining)
         {
-            if (remaining < 5)
+            // We can only decode JFif identifiers.
+            if (remaining < JFifMarker.Length)
             {
                 // Skip the application header length
                 this.InputStream.Skip(remaining);
diff --git a/tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.Images.cs b/tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.Images.cs
index a14c4735f..2f041e3ab 100644
--- a/tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.Images.cs
+++ b/tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.Images.cs
@@ -25,7 +25,8 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 TestImages.Jpeg.Issues.MultiHuffmanBaseline394,
                 TestImages.Jpeg.Issues.ExifDecodeOutOfRange694,
                 TestImages.Jpeg.Issues.InvalidEOI695,
-                TestImages.Jpeg.Issues.ExifResizeOutOfRange696
+                TestImages.Jpeg.Issues.ExifResizeOutOfRange696,
+                TestImages.Jpeg.Issues.InvalidAPP0721
             };
 
         public static string[] ProgressiveTestJpegs =
@@ -54,7 +55,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 TestImages.Jpeg.Issues.MissingFF00ProgressiveBedroom159
             };
 
-        private static readonly Dictionary<string, float> CustomToleranceValues = 
+        private static readonly Dictionary<string, float> CustomToleranceValues =
             new Dictionary<string, float>
             {
                 // Baseline:
diff --git a/tests/ImageSharp.Tests/TestImages.cs b/tests/ImageSharp.Tests/TestImages.cs
index acfad042e..309fb1d4a 100644
--- a/tests/ImageSharp.Tests/TestImages.cs
+++ b/tests/ImageSharp.Tests/TestImages.cs
@@ -155,6 +155,7 @@ namespace SixLabors.ImageSharp.Tests
                 public const string ExifDecodeOutOfRange694 = "Jpg/issues/Issue694-Decode-Exif-OutOfRange.jpg";
                 public const string InvalidEOI695 = "Jpg/issues/Issue695-Invalid-EOI.jpg";
                 public const string ExifResizeOutOfRange696 = "Jpg/issues/Issue696-Resize-Exif-OutOfRange.jpg";
+                public const string InvalidAPP0721 = "Jpg/issues/Issue721-InvalidAPP0.jpg";
             }
 
             public static readonly string[] All = Baseline.All.Concat(Progressive.All).ToArray();
diff --git a/tests/Images/External b/tests/Images/External
index 7f4d2d64c..5f3cbd839 160000
--- a/tests/Images/External
+++ b/tests/Images/External
@@ -1 +1 @@
-Subproject commit 7f4d2d64c6b820ca2b6827e6a8540a1013305ccf
+Subproject commit 5f3cbd839fbbffae615d294d1dabafdcabc64cf9
diff --git a/tests/Images/Input/Jpg/issues/Issue721-InvalidAPP0.jpg b/tests/Images/Input/Jpg/issues/Issue721-InvalidAPP0.jpg
new file mode 100644
index 000000000..adaea47e5
--- /dev/null
+++ b/tests/Images/Input/Jpg/issues/Issue721-InvalidAPP0.jpg
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:9775b7b975422d48a192d54dce140c1df19a181da9889eb99ee7d4331078b460
+size 1225163