Backport GIF LZW fix to 2.1

src/ImageSharp/Formats/Gif/LzwDecoder.cs

@@ -20,6 +20,11 @@ internal sealed class LzwDecoder : IDisposable
         /// </summary>
         private const int MaxStackSize = 4096;
 
+        /// <summary>
+        /// The maximum bits for a lzw code.
+        /// </summary>
+        private const int MaximumLzwBits = 12;
+
         /// <summary>
         /// The null code.
         /// </summary>
@@ -74,12 +79,12 @@ public void DecodePixels(int minCodeSize, Buffer2D<byte> pixels)
             // It is possible to specify a larger LZW minimum code size than the palette length in bits
             // which may leave a gap in the codes where no colors are assigned.
             // http://www.matthewflickinger.com/lab/whatsinagif/lzw_image_data.asp#lzw_compression
-            if (minCodeSize < 2 || clearCode > MaxStackSize)
+            if (minCodeSize < 2 || minCodeSize > MaximumLzwBits || clearCode > MaxStackSize)
             {
                 // Don't attempt to decode the frame indices.
                 // Theoretically we could determine a min code size from the length of the provided
                 // color palette but we won't bother since the image is most likely corrupted.
-                GifThrowHelper.ThrowInvalidImageContentException("Gif Image does not contain a valid LZW minimum code.");
+                return;
             }
 
             // The resulting index table length.

tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs

@@ -279,15 +279,9 @@ public void Issue2012EmptyXmp<TPixel>(TestImageProvider<TPixel> provider)
         public void Issue2012BadMinCode<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel>
         {
-            Exception ex = Record.Exception(
-                () =>
-                {
-                    using Image<TPixel> image = provider.GetImage();
-                    image.DebugSave(provider);
-                });
-
-            Assert.NotNull(ex);
-            Assert.Contains("Gif Image does not contain a valid LZW minimum code.", ex.Message);
+            using Image<TPixel> image = provider.GetImage();
+            image.DebugSave(provider);
+            image.CompareToReferenceOutput(provider);
         }
 
         // https://bugzilla.mozilla.org/show_bug.cgi?id=55918
@@ -301,5 +295,16 @@ public void IssueDeferredClearCode<TPixel>(TestImageProvider<TPixel> provider)
             image.DebugSave(provider);
             image.CompareFirstFrameToReferenceOutput(ImageComparer.Exact, provider);
         }
+
+        // https://github.com/SixLabors/ImageSharp/issues/2743
+        [Theory]
+        [WithFile(TestImages.Gif.Issues.BadMaxLzwBits, PixelTypes.Rgba32)]
+        public void IssueTooLargeLzwBits<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using Image<TPixel> image = provider.GetImage();
+            image.DebugSaveMultiFrame(provider);
+            image.CompareToReferenceOutputMultiFrame(provider, ImageComparer.Exact);
+        }
     }
 }

tests/ImageSharp.Tests/Formats/Gif/GifMetadataTests.cs

@@ -190,5 +190,20 @@ public void Decode_VerifyRepeatCount(string imagePath, uint repeatCount)
                 }
             }
         }
+
+        [Theory]
+        [InlineData(TestImages.Gif.Issues.BadMaxLzwBits, 8)]
+        [InlineData(TestImages.Gif.Issues.Issue2012BadMinCode, 1)]
+        public void Identify_Frames_Bad_Lzw(string imagePath, int framesCount)
+        {
+            TestFile testFile = TestFile.Create(imagePath);
+            using MemoryStream stream = new(testFile.Bytes, false);
+
+            IImageInfo imageInfo = Image.Identify(stream);
+
+            Assert.NotNull(imageInfo);
+            GifMetadata gifMetadata = imageInfo.Metadata.GetGifMetadata();
+            Assert.NotNull(gifMetadata);
+        }
     }
 }

tests/ImageSharp.Tests/TestImages.cs

@@ -455,6 +455,7 @@ public static class Issues
                 public const string BadAppExtLength = "Gif/issues/issue405_badappextlength252.gif";
                 public const string BadAppExtLength_2 = "Gif/issues/issue405_badappextlength252-2.gif";
                 public const string BadDescriptorWidth = "Gif/issues/issue403_baddescriptorwidth.gif";
+                public const string BadMaxLzwBits = "Gif/issues/issue_2743.gif";
                 public const string DeferredClearCode = "Gif/issues/bugzilla-55918.gif";
                 public const string Issue1505 = "Gif/issues/issue1505_argumentoutofrange.png";
                 public const string Issue1530 = "Gif/issues/issue1530.gif";