Interactsh is an open-source tool for detecting out-of-band interactions. Domains configured with interactsh server prior to version 1.0.0 were vulnerable to subdomain takeover for a specific subdomain, i.e `app.` Interactsh server used to create cname entries for `app` pointing to `projectdiscovery.github.io` as default, which intended to used for hosting interactsh web client using GitHub pages. This is a security issue with a self-hosted interactsh server in which the user may not have configured a web client but still have a CNAME entry pointing to GitHub pages, making them vulnerable to subdomain takeover. This allows a threat actor to host / run arbitrary client side code (cross-site scripting) in a user's browser when browsing the vulnerable subdomain. Version 1.0.0 fixes this issue by making CNAME optional, rather than default.

Subdomain Takeover in Interactsh server