ImageSharp is a 2D graphics library. From 2.0.0 until 4.1.2, the TIFF CCITT Group 3 encoder allocates an undersized compressed-data buffer for narrow 1-bit images. TiffCcittCompressor.Initialize does not reserve enough space for the row data and T4 end-of-line codes, and T4BitCompressor.CompressStrip reaches unchecked writes when TiffCompression.CcittGroup3Fax is selected directly or inherited from decoded TIFF metadata. An attacker-controlled encode or decode-and-re-encode flow can write beyond the logical output span, corrupt process memory, and terminate the process. This issue is fixed in version 4.1.2.
Affected: NuGet: SixLabors.ImageSharp
### 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, <= 4.1.1` - Commit `0815358f9202a78bc7f3b83e19282dc3654b500f` corresponds to release **v4.1.1**.
The 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. ### Preconditions and impact
The 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.
`TiffCompressorFactory` creates `T4BitCompressor` for `CcittGroup3Fax`. `TiffCcittCompressor.Initialize` allocates `Width * rowsPerStrip` bytes, but non-modified T4 writes a 12-bit EOL befor…