According to an email sent to the site's affected members, the security breach “may have led to your NT website account (email, password, name and contact information only) being accessed by an unauthorised user.”
The message goes on to say: “Your payment information is safely encrypted, so no financial or booking details have been viewed. The affected system has been secured, however we would ask you to change your password on the NT site and it would be prudent to do so on other sites where you use the same password.”
A spokesperson for the theatre told Whatsonstage.com that the membership system has now been secured and that “significant changes” are being made to guard against future attacks.
According to the website computing.co.uk, the National Theatre upgraded its payment security system last year "to comply with the payment card industry data security standards (PCI DSS)".
If you have any particular concerns or would like further information, contact firstname.lastname@example.org or call +44 207 452 3850.