From 4387db97f5ce1ceaa799565e9544a3012d0085f2 Mon Sep 17 00:00:00 2001
From: Brian Popow <popow@gmx.de>
Date: Fri, 31 May 2024 20:11:51 +0200
Subject: [PATCH] Limit lzw bits to a maximum of 12 bits, fixes issue #2743

---
 src/ImageSharp/Formats/Gif/LzwDecoder.cs        |  9 +++++++--
 .../Formats/Gif/GifDecoderTests.cs              | 17 +++++++++++++++++
 tests/ImageSharp.Tests/TestImages.cs            |  1 +
 tests/Images/Input/Gif/issues/issue_2743.gif    |  3 +++
 4 files changed, 28 insertions(+), 2 deletions(-)
 create mode 100644 tests/Images/Input/Gif/issues/issue_2743.gif

diff --git a/src/ImageSharp/Formats/Gif/LzwDecoder.cs b/src/ImageSharp/Formats/Gif/LzwDecoder.cs
index 4d282f26c6..1a05178fdb 100644
--- a/src/ImageSharp/Formats/Gif/LzwDecoder.cs
+++ b/src/ImageSharp/Formats/Gif/LzwDecoder.cs
@@ -19,6 +19,11 @@ 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>
@@ -73,7 +78,7 @@ internal sealed class LzwDecoder : IDisposable
         // 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
@@ -245,7 +250,7 @@ internal sealed class LzwDecoder : IDisposable
         // 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
diff --git a/tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs b/tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs
index 6399e1193e..7aabe03481 100644
--- a/tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs
@@ -318,4 +318,21 @@ public class GifDecoderTests
         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>
+    {
+        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);
+    }
 }
diff --git a/tests/ImageSharp.Tests/TestImages.cs b/tests/ImageSharp.Tests/TestImages.cs
index a45931e29e..7aaaac6f81 100644
--- a/tests/ImageSharp.Tests/TestImages.cs
+++ b/tests/ImageSharp.Tests/TestImages.cs
@@ -516,6 +516,7 @@ public static class TestImages
             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";
diff --git a/tests/Images/Input/Gif/issues/issue_2743.gif b/tests/Images/Input/Gif/issues/issue_2743.gif
new file mode 100644
index 0000000000..4ce61340d9
--- /dev/null
+++ b/tests/Images/Input/Gif/issues/issue_2743.gif
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:4be51cb9c258a6518d791ad2810fa0d71449805a5d5a8f95dcc7da2dc558ed73
+size 166413