The code is too complex, as calculated using a well-defined, quantitative measure.
This issue makes it more difficult to understand and/or maintain the software, which indirectly affects security by making it more difficult or time-consuming to find and/or fix vulnerabilities. It also might make it easier to introduce vulnerabilities.
This issue can make the software perform more slowly. If the relevant code is reachable by an attacker, then this performance problem might introduce a vulnerability.