A lot of websites on the internet (not nearly enough though) have protection in place that prevents sites other then their own to post forms (log in for example) to their site. A site that does not have this protection is vulnerable to:
- Cross Site Request Forgery (CSRF): http://en.wikipedia.org/wiki/Cross-site_request_forgery
This is a major security risk that allows phishing sites to log you in to the actual website while catching your login details and a whole lot of other nasty stuff.
There could also be other protection in place to prevent you from sending a request.
Try to see if there are any API’s available instead for what you are trying to achieve.
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solved Form that logs in to a website?