ntopng is a web-based network traffic monitoring application. Prior to 6.7.260721, ndpi_get_custom_category_match writes a four-byte nDPI protocol-category value through a pointer to the two-byte id variable in IpAddress::reloadBlacklist in src/IpAddress.cpp during packet dissection and host initialization. A remote unauthenticated attacker on a monitored network segment can send packets that match custom-category rules, causing stack corruption and denial of service without user interaction. This issue is fixed in version 6.7.260721.