libvips is a demand-driven, horizontally threaded image processing library. The heifsave operation could incorrectly determine the presence of an alpha channel in an input when it was not possible to determine the colour interpretation, known internally within libvips as "multiband". There aren't many ways to create a "multiband" input, but it is possible with a well-crafted TIFF image. If a "multiband" TIFF input image had 4 channels and HEIF-based output was requested, this led to libvips creating a 3 channel HEIF image without an alpha channel but then attempting to write 4 channels of data. This caused a heap buffer overflow, which could crash the process. This vulnerability is fixed in 8.16.1.
A heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc().