Cpanel::JSON::XS versions from 4.20 before 4.80 for Perl write out of bounds when a tied or shared hash grows during canonical encoding in encode_hv. With canonical, the keys of a tied or threads::shared hash are counted before a second pass collects them for sorting. The encoder uses a fixed 64-entry stack array or, for larger counts, a heap array sized from the first pass. If the second pass exceeds that capacity, entries are written out of bounds before the counts are compared. This corrupts stack or heap memory and can crash the process.