From maintaining dozens of projects over decades: yes and no.
Your custom library probably won't fall to a library-specific attack unless you were actively aiming for interoperability. However your custom library almost certainly has many vulnerabilities that you haven't heard of yet. Just a few weeks ago I saw a custom library (PHP) with SQL injection vulnerabilities, I couldn't believe it. I suggested to the client that if he still resists having another professional audit it, at least let some frontier LLM have a look. Yes, I recommended this guy to vibe code his security-sensitive code because "professional developers" today still miss the basics.
Your custom library probably won't fall to a library-specific attack unless you were actively aiming for interoperability. However your custom library almost certainly has many vulnerabilities that you haven't heard of yet. Just a few weeks ago I saw a custom library (PHP) with SQL injection vulnerabilities, I couldn't believe it. I suggested to the client that if he still resists having another professional audit it, at least let some frontier LLM have a look. Yes, I recommended this guy to vibe code his security-sensitive code because "professional developers" today still miss the basics.