Web application vulnerabilities do not usually come from exotic attacks. Most trace back to a handful of well-documented issues that keep showing up because they are easy to introduce and easy to overlook.
Injection flaws, where untrusted input is passed directly into a database query or command, remain common despite being well understood. The fix is consistent: never trust input, and use parameterized queries or equivalent safeguards everywhere data enters your system.
Broken authentication is another frequent issue, from weak password policies to sessions that never expire properly. Multi-factor authentication and sensible session management close most of this gap.
Sensitive data exposure happens when information that should be encrypted, in transit or at rest, is not. This is often not a deliberate choice but an oversight in a system that grew over time without a security review.
Security misconfiguration covers a wide range of issues: default credentials left in place, unnecessary services left running, overly permissive access controls. It is usually the result of speed prioritized over review, and it is one of the easier categories to fix once found.
Cross-site scripting and insecure deserialization round out the list of usual suspects. Both stem from trusting input or data that should be validated first.
None of these require advanced tools to catch. Regular code review, dependency updates, and periodic security testing catch the large majority of these issues before they become a real problem.