This logic failure causes the decompressor to write scanline data significantly past the allocated memory boundaries. Because the write operations lack bounds checking, an attacker can control the amount of memory overwritten by crafting specific fax-compressed tiles. This defect, verified in versions 3.0.0 through 4.1.0, results in a deterministic process crash (Denial of Service) or provides a potential primitive for memory corruption and remote code execution in any application utilizing ImageSharp to process untrusted TIFF files.

Attack Chain

1. Attacker crafts a malicious TIFF file with T4/T6/MH compression enabled.
2. Attacker specifies an unusually high ImageWidth (e.g., 4,000,000 pixels) while defining small TileWidth and TileHeight dimensions (e.g., 16x16) in the TIFF header.
3. The victim application calls Image.Load(stream) on the malicious TIFF file.
4. The library's TiffDecoderCore determines the file is tiled and enters DecodeTilesChunky.
5. The library allocates a tile buffer sized strictly for the TileWidth, which is significantly smaller than the logical width expected by the decompressor.
6. The TiffDecompressorsFactory incorrectly initializes the fax decompressor with the full frame.Width.
7. During decompression, the BitWriterUtils performs linear writes of the pixel scanlines into the undersized buffer, causing a heap memory overflow.
8. The process crashes due to an AccessViolationException or remains in a silent heap-corruption state depending on the pixel data content.

Impact

Successful exploitation results in an immediate Denial of Service (DoS) for the host process. Given the attacker-controllable length and width of the out-of-bounds write, this vulnerability creates a high-risk surface for potential remote code execution (RCE) in any server-side application or desktop utility that processes tiled TIFF imagery, such as image converters, web-based media upload services, or document management systems.

Recommendation

1. Patch immediately by upgrading ImageSharp to version 4.1.1 or later, which addresses the incorrect decompressor width assignment and adds proper bounds checking to the write operations.
2. Implement an immediate block or sanitization layer for any incoming TIFF files that exhibit unusual compression configurations (T4/T6) combined with tiled layouts until patching is completed.
3. Integrate the suggested regression tests - specifically testing Compression modes 2, 3, and 4 against tiled inputs with TileWidth less than the total ImageWidth - into your CI/CD pipeline to detect similar memory safety regressions.