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To log in with cURL, fetch the login page, preserve its cookies, submit the form’s actual fields—including hidden CSRF or state values—and reuse the resulting cookies for protected pages. The exact endpoint and parameters depend on the website. cURL can handle HTTP requests and cookies, but it does not run a page’s JavaScript or complete interactive challenges such as CAPTCHA, WebAuthn, or MFA.
Identify what kind of login the site uses
Before sending credentials, determine whether the server expects an HTML form submission or HTTP authentication. These are different mechanisms: a command that works for one will not necessarily work for the other.
Browser-style form login
Most website logins use a form: the browser sends a POST containing credentials and sometimes hidden values, then retains cookies that represent the session. The curl project’s authentication guide notes that most modern websites do not use HTTP authentication for ordinary logins; they ask users to log in on webpages and then maintain a session with cookies. The project’s guide to HTTP POST describes the practical task as identifying the data to submit and the target URL.
HTTP authentication
If the server challenges a protected URL with HTTP Basic authentication, use -u or --user. For other HTTP authentication methods, cURL can be asked to select a method offered by the server with --anyauth. These options do not submit a normal website form.
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Log in with a website form and keep the session
Replace the example domain, form action, field names, and token below with the values from the site you are authorized to access. The commands assume a URL-encoded form and a shell compatible with backslash line continuation.
- Fetch the login page and save its cookies.
curl -sS -c cookies.txt https://example.com/login -o login.html
This writes response cookies tocookies.txtand saves the HTML locally. The cookie jar matters because a site may issue a session cookie before the form is submitted. - Inspect the HTML form.
Openlogin.htmland locate the form’sactionandmethod, the names of the username and password fields, and any hidden inputs. A hidden CSRF token or state value may be required. Use the form’s actual destination, not an assumed URL. - Submit the form while reading and updating the same cookie jar.
curl -sS -L -b cookies.txt -c cookies.txt
--data-urlencode 'username=USER'
--data-urlencode 'password=PASS'
--data-urlencode 'csrf_token=TOKEN'
https://example.com/session
Replacehttps://example.com/sessionwith the form action and replace each example field with the exact field name and value the site expects. Include every required hidden input. The-boption sends cookies already in the jar;-csaves cookies received in the response.-Lfollows redirects. - Request a protected page using the jar.
curl -sS -b cookies.txt https://example.com/account
Check that the response is the expected account page, not a login page returned with a successful HTTP status.
For a simple form without hidden inputs, omit the token parameter. For a site that expects multipart form data, use -F or --form instead of --data-urlencode; follow the site’s actual form encoding requirements.
Handle hidden tokens, encodings, and redirects
CSRF and state values
Many sites require a token that is generated when the login page loads. Fetch the page first, retain its cookies, extract the current token, then submit that token with the same session. A stale token, a missing hidden field, or cookies from a different session can cause a 403 response or an invalid-form error. Official cURL guidance notes that browser-like scripts may need to extract hidden values and send them with the POST.
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Choose the request encoding the form expects
--data-urlencode 'name=value'is useful for URL-encoded fields, especially values containing spaces or reserved characters.--data 'name=value'sends form data without the same automatic URL-encoding behavior; use it only when the values and expected encoding make that appropriate.--form 'name=value'sends multipart form data. Use it when the form or endpoint expectsmultipart/form-data.
Know what redirect following does
With -L (also called --location), cURL follows redirects after the login request. For 301, 302, and 303 responses, cURL can change a POST into a GET; 307 and 308 preserve the method. If the final page is unexpected, inspect the response headers and redirect destination rather than assuming the form submission succeeded. Avoid --location-trusted unless you have a specific, safe reason to use it: it can allow credentials and other secrets to be forwarded across hosts.
Use cURL for HTTP Basic authentication
If the endpoint uses HTTP Basic authentication, a request looks like this:
curl -u 'USER:PASS' https://example.com/protected
Use the authentication mechanism the server actually requests. The -u option is not a substitute for posting a username and password to a website’s HTML form. If the server offers a choice of HTTP authentication methods, --anyauth lets cURL select from methods the server offers; --basic forces Basic authentication.
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Verify that you are actually logged in
A command can complete without a network error and still leave you anonymous. Confirm the result with more than the shell exit status: inspect the HTTP response, the final URL after redirects, or a marker that appears only on the authenticated page.
- Use
-ito include response headers in the output when diagnosing a request. - Use
-D headers.txtto save response headers for inspection. - Check whether the server returns the expected protected content rather than redirecting back to the login form.
- For subsequent requests, keep using
-b cookies.txt. If the site rotates cookies during login, ensure the login command also writes updated cookies with-c cookies.txt.
Troubleshoot common cURL login failures
| Symptom | Likely cause | What to check or change |
|---|---|---|
| 401 Unauthorized | The request uses the wrong authentication method, or credentials are rejected. | Determine whether the endpoint expects HTTP authentication, a form POST, a bearer token, or another scheme. -u only handles HTTP authentication. |
| 403 Forbidden or invalid form | A hidden field, CSRF token, or original session cookie is missing or stale. | Fetch the login page again, keep its cookie jar, and submit all required hidden values from that page with the matching session cookie. |
| Redirect loop or unexpected destination | The form action or redirect host may be wrong; the server may also expect different handling after the POST. | Inspect headers with -i or -D headers.txt, verify the form action, and account for POST-to-GET changes on 301/302/303 redirects. |
| Login appears to work, but the next page is anonymous | The follow-up request did not send the same session cookies, or cookie domain/path rules do not match. | Use the same jar for both requests: -b cookies.txt -c cookies.txt on the login request and -b cookies.txt on the protected request. Confirm the protected URL is within the cookie’s scope. |
| HTML response contains a challenge or never reaches the account page | The flow may depend on JavaScript or an interactive security step. | Use the site’s documented API or an approved browser automation flow for JavaScript, CAPTCHA, WebAuthn, or MFA challenges. cURL does not execute browser JavaScript or complete interactive prompts. |
Protect credentials and session cookies
Use HTTPS so credentials and cookies are encrypted in transit. Avoid putting reusable passwords directly in shell commands when possible: command text may remain in shell history or be visible in process listings. Treat cookies.txt as a secret because it may grant access to the account; keep it out of shared directories and delete it when no longer needed. Do not send credentials to a host you have not verified, and do not use --location-trusted casually because it may forward secrets to another host.
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Example cURL request for a publicly accessible page:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webp
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Can cURL log in when a website requires MFA?
Not reliably for interactive MFA. Use the site’s documented API or an approved browser automation flow when a challenge must be completed.
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Does cURL save login cookies automatically?
Only when you tell it to write them, such as with -c cookies.txt; send them on later requests with -b cookies.txt.
Can I use cURL to capture a screenshot of a logged-in page?
cURL retrieves HTTP responses; it does not render a page as a browser screenshot. Use a browser automation flow for authenticated pages or a screenshot service for pages accessible without interactive login.
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