Always read CRC.

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -1141,11 +1141,6 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <param name="chunk">The <see cref="PngChunk"/>.</param>
         private void ValidateChunk(in PngChunk chunk)
         {
-            if (!chunk.IsCritical)
-            {
-                return;
-            }
-
             Span<byte> chunkType = stackalloc byte[4];
 
             BinaryPrimitives.WriteUInt32BigEndian(chunkType, (uint)chunk.Type);
@@ -1155,7 +1150,7 @@ namespace SixLabors.ImageSharp.Formats.Png
             this.crc.Update(chunk.Data.GetSpan());
 
             uint crc = this.ReadChunkCrc();
-            if (this.crc.Value != crc)
+            if (this.crc.Value != crc && chunk.IsCritical)
             {
                 string chunkTypeName = Encoding.ASCII.GetString(chunkType);
                 PngThrowHelper.ThrowInvalidChunkCrc(chunkTypeName);

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -89,6 +89,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             TestImages.Png.Issue1014_5, TestImages.Png.Issue1014_6
         };
 
+        public static readonly string[] TestImagesIssue1177 =
+        {
+            TestImages.Png.Issue1177_1,
+            TestImages.Png.Issue1177_2
+        };
+
         [Theory]
         [WithFileCollection(nameof(CommonTestImages), PixelTypes.Rgba32)]
         public void Decode<TPixel>(TestImageProvider<TPixel> provider)
@@ -243,6 +249,23 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             Assert.Null(ex);
         }
 
+        [Theory]
+        [WithFileCollection(nameof(TestImagesIssue1177), PixelTypes.Rgba32)]
+        public void Issue1177<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            System.Exception ex = Record.Exception(
+                () =>
+                {
+                    using (Image<TPixel> image = provider.GetImage(PngDecoder))
+                    {
+                        image.DebugSave(provider);
+                        image.CompareToOriginal(provider, ImageComparer.Exact);
+                    }
+                });
+            Assert.Null(ex);
+        }
+
         [Theory]
         [WithFile(TestImages.Png.Issue1127, PixelTypes.Rgba32)]
         public void Issue1127<TPixel>(TestImageProvider<TPixel> provider)

tests/ImageSharp.Tests/TestImages.cs

@@ -95,6 +95,10 @@ namespace SixLabors.ImageSharp.Tests
             public const string Issue1014_6 = "Png/issues/Issue_1014_6.png";
             public const string Issue1127 = "Png/issues/Issue_1127.png";
 
+            // Issue 1177: https://github.com/SixLabors/ImageSharp/issues/1177
+            public const string Issue1177_1 = "Png/issues/Issue_1177_1.png";
+            public const string Issue1177_2 = "Png/issues/Issue_1177_2.png";
+
             public static class Bad
             {
                 // Odd chunk lengths