nopCommerce v4.70 and prior, and version 4.80.3, does not invalidate session cookies after logout or session termination, allowing an attacker who has a a valid session cookie access to privileged endpoints (such as /admin) even after the legitimate user has logged out, enabling session hijacking. Any version above 4.70 that is not 4.80.3 fixes the vulnerability.
According to WASC, “Insufficient Session Expiration is when a web site permits an attacker to reuse old session credentials or session IDs for authorization.”