Posted in Application Security
You built a feature that lets users submit a URL so your backend can fetch data from it. Have you considered…
samuel watua Soita — Full Stack Software Engineer
You built a feature that lets users submit a URL so your backend can fetch data from it. Have you considered what happens when someone submits an internal server address?
This is where a seemingly harmless feature can become a serious security vulnerability.
Imagine building:
- A URL preview service.
- A webhook testing tool.
- An image importer.
- A document fetcher.
- An integration that retrieves external API resources.
Your backend receives a URL and makes an HTTP request.
But what if the submitted URL points to an internal service or cloud metadata endpoint?
The server might unintentionally access resources that should never be exposed to external users.
This is known as Server-Side Request Forgery (SSRF).
A safer implementation should consider:
1. Allowlisting trusted destination hosts where practical.
2. Blocking private, loopback and link-local IP ranges.
3. Validating DNS resolution and rechecking destinations after redirects.
4. Restricting outbound network access.
5. Controlling supported protocols and ports.
6. Applying request timeouts and response-size limits.
7. Never assuming a URL is safe simply because it starts with HTTPS.
This is especially important for applications that integrate with third-party services or allow user-controlled URLs.
Your backend should not become a proxy into infrastructure that users were never supposed to reach.
Developers: do you validate user-submitted URLs before your server makes outbound requests?
#Cybersecurity #SSRF #OWASP #ApplicationSecurity #BackendEngineering #API Security #CloudSecurity #NodeJS #WebSecurity #SecureCoding #DevSecOps #SoftwareArchitecture
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Topics: #devsecops, #owasp, #nodejs, #api, #websecurity, #cloudsecurity, #applicationsecurity, #ssrf, #backendengineering, #softwarearchitecture