Four reasons APIs are more exposed than traditional web apps.
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Auth endpoints are public by design
The login endpoint has to be reachable by everyone — that's its job. Unlike a database or admin panel, you can't firewall it. Every attacker in the world can reach it directly.
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Multiple clients share one auth layer
Mobile app, web app, third-party integrations, partner APIs — all authenticate through the same endpoints. A bug in any one auth path affects all of them.
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No human in the loop
A traditional web login might show a CAPTCHA. An API endpoint is pure automation — exactly what a credential-stuffing script needs. There's no friction, no review, no delay.
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Microservices often skip auth internally
In a microservices architecture, internal services often trust each other without authentication. If an attacker reaches one internal service — via SSRF or a misconfigured endpoint — they may reach all of them without any credential check.