Shadow IT refers to the use of information technology systems, devices, software, applications, and services without explicit IT department approval. This is the most likely cause of introducing vulnerabilities on a corporate network by deploying unapproved software, as such software may not have been vetted for security compliance, increasing the risk of vulnerabilities.
References =
CompTIA Security+ SY0-701 Course Content: The concept of Shadow IT is discussed as a significant risk due to the introduction of unapproved and potentially vulnerable software into the corporate network.