A common use of curl is to fetch or test an HTTP or HTTPS endpoint from the command line, for example to check whether a service responds or to send an API request. In security operations, monitor its use because it can also retrieve payloads or exfiltrate data if misused. Restrict where it can run.
What curl does in real work
In practice, curl is a command-line client for making HTTP and HTTPS requests from a shell. Practitioners use it to validate that a service is reachable, inspect headers, test authentication flows, submit JSON to an API, or quickly compare what an endpoint returns before and after a change. That makes it useful both for troubleshooting and for lightweight integration checks.
Because curl speaks common web protocols, it fits naturally into day-to-day operations. A simple request can confirm whether a reverse proxy, load balancer, or application endpoint is returning the expected status code. More advanced requests can add custom headers, bearer tokens, cookies, or request bodies, which is why it is often the fastest way to reproduce an API issue outside a browser or application client.
curl is also useful for seeing the difference between a service being “up” and actually behaving correctly. A successful TCP connection does not prove the response is valid, authenticated, or safe to trust. With curl, operators can check redirects, content types, response payloads, and failure modes directly, which helps separate network reachability problems from application-level faults.
Common examples and operational uses
A typical example is checking a health endpoint during deployment. Another is sending a POST request with JSON to test an API route, or using a token in an Authorization header to confirm that the service accepts the expected credentials. curl can also download files, follow redirects, or call internal services during incident response and automation scripts.
The same flexibility makes curl a common tool in CI jobs, runbooks, and ad hoc diagnostics. Teams often use it to verify certificates, test proxy settings, confirm response timing, or compare environments such as development, staging, and production. If a request works in one place but not another, curl helps isolate whether the issue is DNS, TLS, routing, headers, or server logic.
For that reason, curl is best thought of as a precise probe rather than a full application client. It does not hide the details of the request, so it is valuable when you need to know exactly what is being sent and received. That transparency is one reason security teams also use it when validating exposed services or reproducing suspicious traffic patterns.
Why curl also matters to defenders
curl is legitimate tooling, but it can also be used to retrieve sensitive content, stage payloads, or move data out of an environment if a system is already compromised. NIST Cybersecurity Framework 2.0 is a useful lens here because curl use should be observable within broader governance, detection, and response processes rather than treated as ordinary background noise.
Failure mechanism: The risk is not the utility itself, but unchecked execution on a system that can reach sensitive services or external networks. If an attacker gains shell access, curl can become a fast way to test internal endpoints, pull credentials from metadata or local files, or exfiltrate data over an allowed protocol.
Impact: That can turn a small foothold into broader exposure, especially where egress controls, command logging, or endpoint monitoring are weak. In environments that rely on API calls, curl can also blur the line between normal administration and suspicious activity, so defenders need visibility into where it runs and what it is allowed to reach.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP API Security Top 10 and MITRE ATT&CK address the attack and risk surface, while NIST CSF 2.0 sets the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST CSF 2.0 | ID.AM-01 — Physical devices and systems within the organization are inventoried | curl use should be tied to known hosts and managed endpoints |
| Recommendation — Inventory hosts that can run curl and review their permitted network paths. | ||
| OWASP API Security Top 10 | API2 — Broken Authentication | curl is often used to test API auth flows and token handling |
| Recommendation — Validate API authentication behavior with controlled curl requests. | ||
| MITRE ATT&CK | T1105 — Ingress Tool Transfer | curl can retrieve payloads or stage tools during compromise |
| Recommendation — Detect and restrict suspicious command-line retrieval from exposed hosts. | ||
Practitioner Guidance
What to verify: Confirm whether the host running curl is allowed to reach the target service and whether the request matches an approved operational pattern. If the command includes secrets, tokens, or production endpoints, treat it as a higher-risk action and review who launched it, from where, and for what purpose. OWASP API Security Top 10 is relevant when curl is being used to exercise API authentication or authorization paths.
What to prioritize: Focus first on execution context, network reachability, and logging, because those determine whether curl is a routine diagnostic tool or a viable abuse path. Restrict where it can run, limit outbound access from systems that do not need it, and make sure command-line activity is covered by your monitoring and incident review process. MITRE ATT&CK Enterprise Matrix helps anchor curl-related abuse to credential access, discovery, and exfiltration behaviors.
Practitioner takeaway: curl is routine, but on the wrong host it becomes a highly capable transport for testing, retrieval, and exfiltration. Treat it as a controlled diagnostic utility, not an automatically trusted one.