The lack of inbuilt security protocols in cheap and resource constraint Internet of Things (IoT) devices 
give privilege to an attacker to exploit these device’s vulnerabilities and break into the target device. 
Attacks like Mirai, Wannacry, Stuxnet, etc. show that a cyber-attack often comprises of a series of exploitation 
of victim device's vulnerabilities. Timely detection and patching of these vulnerabilities can avoid future 
attacks. Penetration testing helps to identify such vulnerabilities.  However, traditional penetration testing 
methods are not End-to-End, which fail to detect multi-hosts and multi-stages attacks. Even if an individual 
system is secure under some threat model, the attacker can use a kill-chain to reach the target system.  
In this paper, we introduced first-of-its-kind, IoT-PEN, a Penetration Testing Framework for IoT. 
The framework follows a client-server architecture with all IoT nodes act as clients and 
``a system with resources'' as a server. IoT-PEN is an End-to-End, scalable, flexible and automatic penetration 
testing framework for discovering all possible ways an attacker can breach the target system using target-graphs. 
Finally, the paper recommends patch prioritization order by identifying critical nodes, critical paths for 
efficient patching.  Our analysis shows that IoT-PEN is easily scalable to large and complex IoT networks.