The stunnel 6.45 documentation for "accept" states,

"If no host specified, defaults to all IPv4 addresses for the local host."

In this case, stunnel actually binds to 127.0.0.1. Indeed, inspection of the relevant code paths reveals that addr_list->passive is never set to 1 in this case. To get stunnel to bind to all addresses, 0.0.0.0 must be explicitly specified.

Furthermore, inspection of the code path for the "local" option reveals that addr_list->passive is set to 1 in this case, which seems to be backwards: The documentation for getaddrinfo states that AI_PASSIVE is ignored in this case.

The logic for these two cases seems to be reversed.