# ASP.NET Core Security in 2026: 11 Production Mistakes Developers Still Make (and How to Fix Them) If you are building APIs in ASP.NET Core, security mistakes usually don't come from "not knowing security." They come from shipping fast, copy-pasting old patterns, and skipping boring guardrails. This ASP.NET Core security checklist focuses on the eleven mistakes that actually cause production incidents - and the concrete fix for each. As of early 2026, with .NET 10 projects gaining traction and AI-assisted coding speeding up delivery, the gap between feature velocity and security hygiene is getting wider. If you want more than a checklist, the full secure-API setup - auth, rate limiting, validation, and safe error handling wired into one running project - is on [Patreon](https://www.patreon.com/CodingDroplets), ready to run and adapt. Most of these controls are also built step by step inside the [ASP.NET Core Web API: Zero to Production course](https://aspnetcoreapi.codingdroplets.com/), so you can see them working together rather than as isolated tips. [![ASP.NET Core Web API: Zero to Production](https://newsletter.codingdroplets.com/images/aspnet-core-api-course-banner-1.jpg)](https://aspnetcoreapi.codingdroplets.com/) ## Why This Matters Now * Attack surfaces are wider (mobile, SPA, public APIs, third-party webhooks) * Teams are shipping more frequently, often with AI-generated code that looks correct but skips guardrails * Misconfiguration, not exotic cryptography, still causes the majority of preventable incidents If you fix the eleven items below, you dramatically reduce real-world risk. They map closely to the [OWASP API Security Top 10](https://owasp.org/API-Security/), which is the reference worth bookmarking alongside this list. --- ## 1) Returning Raw Exceptions to Clients **Mistake:** Returning stack traces or internal exception messages in API responses. Every leaked exception hands an attacker a map of your internals - framework versions, file paths, and SQL fragments. **Fix:** Use centralized exception handling middleware (or `IExceptionHandler` on .NET 8+) and return safe, consistent error contracts via Problem Details. Never expose `exception.Message` on a 500. Related video from Coding Droplets: %[https://www.youtube.com/watch?v=n3TV4Lqqt4k] --- ## 2) Weak JWT Token Implementation **Mistake:** Long-lived access tokens, poor signing key management, and no revocation strategy. A leaked long-lived token is a leaked account until it expires. **Fix:** * Short-lived access tokens (around 15 minutes) * Proper refresh token rotation with reuse detection * Store refresh tokens securely and revoke on suspicious activity For a deeper walkthrough, see [JWT authentication with refresh tokens in ASP.NET Core](https://codingdroplets.com/jwt-authentication-with-refresh-tokens-in-aspnet-core-web-api). Related video: %[https://www.youtube.com/watch?v=vTXXdm44IdQ] --- ## 3) No API Versioning Strategy **Mistake:** Breaking old clients after endpoint changes. This is a security and reliability problem - clients stuck on broken versions get pushed toward insecure workarounds. **Fix:** Use explicit versioning (URL, header, or media type) from day one, and document each version in OpenAPI/Swagger. Related video: %[https://www.youtube.com/watch?v=ql1hJEgTi8Q] --- ## 4) Insecure Deployment Without Proper TLS/SSL **Mistake:** Delayed SSL setup or manual certificate renewals that silently fail. Expired or missing TLS opens the door to interception and downgrade attacks. **Fix:** Enforce HTTPS, add HSTS, automate certificate renewal, and monitor expiry alerts. See [HSTS in ASP.NET Core](https://codingdroplets.com/aspnet-core-hsts-security) for why redirection alone is not enough. Related video: %[https://www.youtube.com/watch?v=yyLyYPylD9w] --- ## 5) Misusing HTTP Semantics (Especially GET Bodies) **Mistake:** Sending request bodies with GET and assuming all clients and proxies will behave. Non-standard semantics lead to caching and proxy bugs that are hard to reason about under load. **Fix:** Keep GET idempotent and body-free. Use query params for filters, and POST for complex payloads. --- ## 6) Skipping Model Validation and Input Normalization **Mistake:** Trusting incoming DTOs blindly. Unvalidated input is the root of injection, overflow, and mass-assignment bugs. **Fix:** * Enforce server-side validation (FluentValidation or Data Annotations) * Normalize inputs (trim, canonicalize) * Reject invalid payloads early with clear 400 responses --- ## 7) Over-Permissive CORS Configuration **Mistake:** `AllowAnyOrigin` combined with `AllowCredentials`, and broad wildcard setups. This effectively lets any site call your authenticated API from a user's browser. **Fix:** Restrict by exact origin, method, and headers per environment. Treat a wildcard CORS policy on a credentialed API as a bug. --- ## 8) Over-Fetching Sensitive Data **Mistake:** Returning full entities when clients only need a subset, quietly exposing fields like password hashes, internal flags, or PII. **Fix:** Use response DTOs and minimize payloads. It is a performance win and a security win at once. --- ## 9) Missing AuthZ Checks at the Business Layer **Mistake:** Assuming `[Authorize]` at the controller level is enough. That confirms *who* the user is, not *whether they may touch this specific resource* - the gap behind most broken-object-level-authorization incidents. **Fix:** Add resource-level authorization checks in services or handlers as a second safety layer. --- ## 10) No Rate Limiting / Abuse Protection **Mistake:** Public endpoints with unlimited request rates, inviting brute force, scraping, and accidental self-inflicted overload. **Fix:** Configure ASP.NET Core rate limiting policies by route, user, or IP, and add anomaly monitoring. Remember it is an app-layer control, not DDoS protection. --- ## 11) Poor Observability for Security Events **Mistake:** Logs exist, but there is no actionable security telemetry. You find out about the breach from someone else. **Fix:** * Structured logs with correlation IDs * Alerting on auth failures and token anomalies * Audit trails for high-risk operations --- ## Final Thoughts Most API security failures are not advanced cryptography failures. They are architecture and process failures. Fixing these eleven issues gives you a strong baseline for secure, scalable ASP.NET Core APIs in 2026. Treat this list as a recurring review item, not a one-time pass. ## FAQ **What is the most common ASP.NET Core API security mistake?** Returning raw exceptions to clients. It is trivial to fix with centralized exception handling and Problem Details, yet it routinely leaks stack traces, framework versions, and SQL fragments that hand attackers a head start. Never expose `exception.Message` on a 500 response. **Is `[Authorize]` enough to secure an ASP.NET Core API?** No. `[Authorize]` confirms the caller is authenticated and, with policies, that they hold a role or claim. It does not confirm they are allowed to act on a specific resource. Add resource-level authorization checks in your services or handlers to close broken-object-level-authorization gaps. **How do I configure CORS securely in ASP.NET Core?** Never combine `AllowAnyOrigin` with `AllowCredentials`. Restrict to an explicit allow-list of origins, methods, and headers, and vary it per environment so production is tighter than development. A wildcard CORS policy on a credentialed API should be treated as a defect. **Does rate limiting protect against DDoS?** No. The built-in ASP.NET Core rate limiter is an application-layer control for brute force and abuse. Volumetric DDoS must be absorbed earlier, at a WAF, CDN, or cloud edge. Use both, and treat them as separate layers. * * * ## About the Author **Celin Daniel** is Co-founder of Coding Droplets with 13+ years of hands-on experience building, shipping, and operating .NET and ASP.NET Core systems in production. The guidance here comes from real projects and production incidents, not theory. - Website: [codingdroplets.com](https://codingdroplets.com/) - GitHub: [github.com/codingdroplets](http://github.com/codingdroplets/) - YouTube: [youtube.com/@CodingDroplets](https://www.youtube.com/@CodingDroplets)