ean_leading_zeroes in backend/upcean.c in Zint Barcode Generator 2.9.1 has a stack-based buffer overflow that is reachable from the C API through an application that includes the Zint Barcode Generator library code.
A NULL Pointer Dereference exists in libzint in Zint 2.7.1 because multiple + characters are mishandled in add_on in upcean.c, when called from eanx in upcean.c during EAN barcode generation.