Security: Prevent Host Header Injection attacks by validating HTTP_HOST
against a whitelist of allowed hostnames before constructing the baseURL.
Changes:
- Add getValidHost() method to validate HTTP_HOST against allowedHostnames
- If allowedHostnames is empty, log warning and fall back to 'localhost'
- If host not in whitelist, log warning and use first allowed hostname
- Update .env.example with allowedHostnames documentation
- Add security configuration section to INSTALL.md
- Add unit tests for host validation
This addresses the security advisory where the application constructed
baseURL from the attacker-controllable HTTP_HOST header, allowing:
- Login form phishing via manipulated form actions
- Cache poisoning via poisoned asset URLs
Fixes GHSA-jchf-7hr6-h4f3
7cb1d95da7