CHRISTOS K. DIMITRIADIS
"When comparing the risks of insider threats vs. outsider threats..."
The answer to this question depends on the threat model of each enterprise, but usually the response is both.
There are a number of insider threats related to data leakage (accidental or on purpose), fraudulent action through an integrity breach, loss of availability, or business continuity as a result of insider mistakes or deliberate action. In general, insider threats can cause significant business impact, not only because of the privileges that employees have over information technology but also because the company theoretically has its employees under its control, being responsible for their actions, in contradiction to external threats.
Source: 47 Security Experts weigh in the issue: https://digitalguardian.com/blog/insider-outsider-data-security-threats
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