The app did not require a password to attendees’ accounts – making it possible to access the email addresses and mobile phone numbers of Chancellor Philip Hammond and former foreign secretary Boris Johnson.
Sky News has found that it was also possible to change the photos of cabinet members, MPs, journalists and local councillors.
The security flaw has now been fixed.
The mobile app is created by events technology company CrowdComms, and the terms and conditions have contact details of the firm’s offices in Australia.
Sky News has contacted the Conservatives for comment.