https://github.com/pnggroup/libpng/security/advisories/GHSA-g8hp-mq4h-rqm3 reports:
LIBPNG is a reference library for use in applications
that read, create, and manipulate PNG (Portable Network
Graphics) raster image files. Prior to 1.6.55, an
out-of-bounds read vulnerability exists in the
png_set_quantize() API function. When the function is
called with no histogram and the number of colors in the
palette is more than twice the maximum supported by the
user's display, certain palettes will cause the function to
enter into an infinite loop that reads past the end of an
internal heap-allocated buffer. The images that trigger
this vulnerability are valid per the PNG specification.
This vulnerability is fixed in 1.6.55.