C# .NET The most common flaw type in .NET applications was information leakage

Information leakage was most common flaw type for .NET. To review, Information Leakage is a type of security flaw that can allow malicious user s to reveal confidential information from within an application.

Here's a primer - Information Leakage Tutorial (veracode.com).

Source of image, full PDF - state-of-software-security-volume-11-flaw-frequency-by-language.pdf (veracode.com)