PNG: Throw exception, if palette chunk is missing

src/ImageSharp/Formats/Png/PngScanlineProcessor.cs

@@ -240,6 +240,11 @@ public static void ProcessPaletteScanline<TPixel>(
             byte[] paletteAlpha)
             where TPixel : unmanaged, IPixel<TPixel>
         {
+            if (palette.IsEmpty)
+            {
+                PngThrowHelper.ThrowMissingPalette();
+            }
+
             TPixel pixel = default;
             ref byte scanlineSpanRef = ref MemoryMarshal.GetReference(scanlineSpan);
             ref TPixel rowSpanRef = ref MemoryMarshal.GetReference(rowSpan);

src/ImageSharp/Formats/Png/PngThrowHelper.cs

@@ -21,6 +21,9 @@ public static void ThrowInvalidImageContentException(string errorMessage, Except
         [MethodImpl(InliningOptions.ColdPath)]
         public static void ThrowNoData() => throw new InvalidImageContentException("PNG Image does not contain a data chunk");
 
+        [MethodImpl(InliningOptions.ColdPath)]
+        public static void ThrowMissingPalette() => throw new InvalidImageContentException("PNG Image does not contain a palette chunk");
+
         [MethodImpl(InliningOptions.ColdPath)]
         public static void ThrowInvalidChunkType() => throw new InvalidImageContentException("Invalid PNG data.");
 

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

@@ -265,6 +265,22 @@ public void Decode_MissingDataChunk_ThrowsException<TPixel>(TestImageProvider<TP
             Assert.Contains("PNG Image does not contain a data chunk", ex.Message);
         }
 
+        [Theory]
+        [WithFile(TestImages.Png.Bad.MissingPaletteChunk1, PixelTypes.Rgba32)]
+        [WithFile(TestImages.Png.Bad.MissingPaletteChunk2, PixelTypes.Rgba32)]
+        public void Decode_MissingPaletteChunk_ThrowsException<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            Exception ex = Record.Exception(
+                () =>
+                {
+                    using Image<TPixel> image = provider.GetImage(PngDecoder);
+                    image.DebugSave(provider);
+                });
+            Assert.NotNull(ex);
+            Assert.Contains("PNG Image does not contain a palette chunk", ex.Message);
+        }
+
         [Theory]
         [WithFile(TestImages.Png.Bad.BitDepthZero, PixelTypes.Rgba32)]
         [WithFile(TestImages.Png.Bad.BitDepthThree, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -127,6 +127,8 @@ public static class Bad
                 public const string MissingDataChunk = "Png/xdtn0g01.png";
                 public const string WrongCrcDataChunk = "Png/xcsn0g01.png";
                 public const string CorruptedChunk = "Png/big-corrupted-chunk.png";
+                public const string MissingPaletteChunk1 = "Png/missing_plte.png";
+                public const string MissingPaletteChunk2 = "Png/missing_plte_2.png";
 
                 // Zlib errors.
                 public const string ZlibOverflow = "Png/zlib-overflow.png";