CORS Header Access-Control-Allow-Origin Contains Multiple Values, Invalid Error
When a browser reports Access-Control-Allow-Origin contains multiple values, more than one layer in your request's path is independently adding this header —...
Category
Cybersecurity / Networking
Practical fixes for authentication errors, API security issues, and network debugging basics.
CORS Missing Access-Control-Allow-Credentials Header on Authenticated Requests
A CORS error specifically on requests that include credentials (cookies, HTTP authentication) but not on simpler, unauthenticated requests to the same endpoint...
JWT Algorithm Confusion Vulnerability (HS256 vs RS256), Basic Explanation
The JWT algorithm confusion vulnerability exploits a library or application that trusts the algorithm specified inside the token itself, rather than enforcing...
JWT "nbf" (Not Before) Claim Error Causing Validation Failures Across Timezones
An nbf (Not Before) validation failure is virtually never actually a timezone problem, even though it often gets diagnosed that way — JWT timestamp claims are...
OAuth 2.0 PKCE Code Verifier Mismatch on Mobile App Authentication Flow
A PKCE code verifier mismatch means the verifier sent during the token exchange doesn't match the code challenge originally sent during authorization — since...
Rate Limiting Per IP Failing Behind Cloudflare/Reverse Proxy (Incorrect Client IP)
Application-level rate limiting that seems to treat every visitor as the same client, or that's trivially bypassable, almost always means your framework is...
SSH Host Key Verification Failed Error Fix After Server IP Change
SSH's host key warning after a server's IP address changes, or after a genuine reinstall, is the client correctly detecting that the server's identity...
API Key Header Leaked in Browser Network Tab, How to Route via Proxy Backend
An API key visible in the browser's Network tab is exposed to anyone with access to developer tools — which is effectively any user of your site — regardless...
CORS Preflight Response Invalid HTTP Status Code 405 on OPTIONS Verb
A 405 Method Not Allowed specifically on the automatic OPTIONS preflight request means your server (or a framework/router within it) has no route configured to...