LIBPNG is vulnerable to a heap buffer over-read in `png_write_image_8bit` with grayscale+alpha or RGB/RGBA images

Vulnerability Description

LIBPNG is a reference library for use in applications that read, create, and manipulate PNG (Portable Network Graphics) raster image files. From version 1.6.0 to before 1.6.51, a heap buffer over-read vulnerability exists in libpng's png_write_image_8bit function when processing 8-bit images through the simplified write API with convert_to_8bit enabled. The vulnerability affects 8-bit grayscale+alpha, RGB/RGBA, and images with incomplete row data. A conditional guard incorrectly allows 8-bit input to enter code expecting 16-bit input, causing reads up to 2 bytes beyond allocated buffer boundaries. This issue has been patched in version 1.6.51.

References

• https://github.com/pnggroup/libpng/security/advisories/GHSA-qpr4-xm66-hww6
• https://github.com/pnggroup/libpng/pull/749
• https://github.com/pnggroup/libpng/commit/2bd84c019c300b78e811743fbcddb67c9d9bf821