phpMyFAQ sharing FAQ functionality can easily be abused for phishing purposes

Summary

The 'sharing FAQ' functionality allows any unauthenticated actor to misuse the phpMyFAQ application to send arbitrary emails to a large range of targets.

Details

The phpMyFAQ application has a functionality where anyone can share a FAQ item to others. The front-end of this functionality allows any phpMyFAQ articles to be shared with 5 email addresses. The application will then send these 5 emails. However, there are no controls over what link and content are shared. Furthermore, any unauthenticated actor can perform this action. There is a CAPTCHA in place, however the amount of people you email with a single request is not limited to 5 by the backend. An attacker can thus solve a single CAPTCHA and send thousands of emails at once.

PoC

We send the following form and capture the request.

We now change the body to contain 50 email addresses instead of just 1, and send the request. The attacker can also change the body of the email to any phishing message.

Below are the logs of the email server, proving that all these emails were sent.

An attacker can also change the link that is sent in these emails. Making phishing even more possible.

Impact

An attacker can utilize the target application's email server to send phishing messages. This can get the server on a blacklist, causing all emails to end up in spam. It can also lead to reputational damages.