1. Vulnerabilities in the code: Writing secure code is one of the most important aspects of building a secure web application. Developers must be aware of common vulnerabilities such as SQL injection, cross-site scripting, and malicious file uploads, and must ensure that their code is not vulnerable to these attacks.

2. Weak authentication: Weak authentication can allow attackers to gain access to sensitive resources without the user’s knowledge. Strong authentication methods such as multi-factor authentication must be implemented to ensure that only authorized users have access.

3. Data encryption: Data encryption is essential to protect data from being intercepted while being transmitted across the network. SSL/TLS should be used to encrypt data in transit and data at rest should be encrypted using a strong encryption algorithm.

4. Access control: Access control must be implemented to ensure that users are only allowed to access the resources that they are authorized to access. This can be done through role-based access control or other access control mechanisms.

5. Application security: Application security is essential to protect applications from malicious attacks and unauthorized access. Developers must be aware of common web application vulnerabilities such as cross-site scripting, SQL injection, and malicious file uploads, and must ensure that their applications are protected against these attacks.

6. Security testing: Security testing is essential to ensure that applications are secure and free from vulnerabilities. Security testing must be done regularly to ensure that any newly introduced vulnerabilities are detected and fixed.

7. Security monitoring: Security monitoring is essential to detect any malicious activity on the system. Security logs should be monitored regularly to detect any suspicious activity and take appropriate action.