GET vs POST request. Why POST is more secured?
The GET request is marginally less secure than the POST request. Neither offers true “security” by itself; using POST requests will not magically make your website secure against malicious attacks by a noticeable amount. However, using GET requests can make an otherwise secure application insecure.
The mantra that you “must not use GET requests to make changes” is still very much valid, but this has little to do with malicious behaviour. Login forms are the ones most sensitive to being sent using the wrong request type.
Search spiders and web accelerators
This is the real reason you should use POST requests for changing data. Search spiders will follow every link on your website, but will not submit random forms they find.
Web accelerators are worse than search spiders, because they run on the client’s machine, and “click” all links in the context of the logged in user. Thus, an application that uses a GET request to delete stuff, even if it requires an administrator, will happily obey the orders of the (non-malicious!) web accelerator and delete everything it sees
Proxy servers are likely to log GET URLs in their entirety, without stripping the query string. POST request parameters are not normally logged.
Caching proxies might retain GET responses, but not POST responses. Having said that, GET responses can be made non-cacheable with less effort than converting the URL to a POST handler.
If the user were to navigate to a third party website from the page served in response to a GET request, that third party website gets to see all the GET request parameters.
Belongs to the category of “reveals request parameters to a third party”, whose severity depends on what is present in those parameters. POST requests are naturally immune to this, however to exploit the GET request a hacker would need to insert a link to their own website into the server’s response.
This is very similar to the “proxy logs” argument: GET requests are stored in the browser history along with their parameters. The attacker can easily obtain these if they have physical access to the machine.
Browser refresh action
The browser will retry a GET request as soon as the user hits “refresh”. It might do that when restoring tabs after shutdown. Any action (say, a payment) will thus be repeated without warning.
The browser will not retry a POST request without a warning.
This is a good reason to use only POST requests for changing data, but has nothing to do with malicious behaviour and, hence, security.