This recommendation applies to version 10.21 of this plugin, but the most recent version is 10.42. These findings may no longer be correct.
Findings
Doesn’t escape all HTML
Records the IP address of users who submit subscription requests, but the IP address is inserted into the subscription form as a hidden field meaning that the IP address can be changed before submission
This means that when admin users export the subscriptions to CSV they will see the incorrect IP addresses (probably not a security issue)
The plugin attempts to limit the rate at which the form can be submitted from a single IP (when a filter called s2_lockout returns greater than zero, which it doesn’t by default). This flaw would prevent that rate limiting from functioning when faced with a malicious user