Guard CgBI bit depth and color type

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -1431,6 +1431,22 @@ internal sealed class PngDecoderCore : ImageDecoderCore
 
         this.pngColorType = this.header.ColorType;
         this.Dimensions = new Size(this.header.Width, this.header.Height);
+
+        // Apple's pngcrush emits the CgBI chunk before IHDR, so the header
+        // compatibility check is deferred until both chunks have been seen.
+        if (this.isCgbi)
+        {
+            ThrowIfInvalidCgbiContent(this.header);
+        }
+    }
+
+    private static void ThrowIfInvalidCgbiContent(in PngHeader header)
+    {
+        if (header.BitDepth != 8 || (header.ColorType is not PngColorType.Rgb and not PngColorType.RgbWithAlpha))
+        {
+            PngThrowHelper.ThrowInvalidImageContentException(
+                $"CgBI is only supported for 8-bit truecolor images. Was bit depth '{header.BitDepth}', color type '{header.ColorType}'.");
+        }
     }
 
     /// <summary>

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

@@ -767,6 +767,28 @@ public partial class PngDecoderTests
         Assert.Equal(expectedHeight, imageInfo.Height);
     }
 
+    [Theory]
+    [InlineData(TestImages.Png.Cgbi.BitDepth16)]
+    [InlineData(TestImages.Png.Cgbi.Palette)]
+    public void Identify_CgBI_IncompatibleHeader_ThrowsInvalidImageContentException(string imagePath)
+    {
+        TestFile testFile = TestFile.Create(imagePath);
+        using MemoryStream stream = new(testFile.Bytes, false);
+        InvalidImageContentException ex = Assert.Throws<InvalidImageContentException>(() => Image.Identify(stream));
+        Assert.Contains("CgBI is only supported for 8-bit truecolor images", ex.Message);
+    }
+
+    [Theory]
+    [WithFile(TestImages.Png.Cgbi.BitDepth16, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Png.Cgbi.Palette, PixelTypes.Rgba32)]
+    public void Decode_CgBI_IncompatibleHeader_ThrowsInvalidImageContentException<TPixel>(TestImageProvider<TPixel> provider)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        InvalidImageContentException ex = Assert.Throws<InvalidImageContentException>(
+            () => { using Image<TPixel> image = provider.GetImage(PngDecoder.Instance); });
+        Assert.Contains("CgBI is only supported for 8-bit truecolor images", ex.Message);
+    }
+
     [Theory]
     [WithFile(TestImages.Png.Splash, PixelTypes.Rgba32)]
     [WithFile(TestImages.Png.Bike, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -189,6 +189,10 @@ public static class TestImages
 
             // Issue 410: https://github.com/SixLabors/ImageSharp/issues/410
             public const string Issue410 = "Png/issues/Issue_410.png";
+
+            // Synthetic fixtures derived from colors.png to exercise CgBI validation.
+            public const string BitDepth16 = "Png/cgbi/colors-cgbi-bitdepth16.png";
+            public const string Palette = "Png/cgbi/colors-cgbi-palette.png";
         }
 
         public static class Bad

tests/Images/Input/Png/cgbi/colors-cgbi-bitdepth16.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:59610bc03f6ca867e5f71c574b3a0d1942c9e3a230c8a32bf3007cb82f286866
+size 12853

tests/Images/Input/Png/cgbi/colors-cgbi-palette.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:a3a2f20c69ae423523a8f41887e3f37257a338f2220c2ea44d35c87daf8c3aa3
+size 12853