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Cross Site Request Forgery (CSRF)

Starting with OpenACS 5.9.1, OpenACS offers support for protecting against Cross Site Request Forgery (CSRF). In essence, this attack can cause a user’s web browser to perform an unwanted action on a trusted site for which the user is currently authenticated. The user gets a page presented, which looks harmless, but contains links or images that perform actions with the users credentials without the users consent. Note that the CSP does not protect a user against clicks on a malicious link.

CSRF protection works by ensuring that values for an action (e.g. by from a HTML form) are only accepted from a user that has received the form before. OpenACS generates by its security-procs a secure CSRF token value and provides it to a developer it in a global variable ::__csp_nonce. When requests secured with the CSRF token are received, it can be validated on the server side. Note, that this mechanism is similar to "signing" values in OpenACS.

CSRF protection concerns of two parts: add the CSRF token to the form (POST requests) or to the href, and checking the received in the queries expecting input from CSRF protected resources. The first part works technically quite similar as securing CSP via nonces. Add code to the Tcl or ADP page that outputs the global variable (the test for the token is mostly for backwards compatibility)

    <form ...>
        <if @::__csrf_token@ defined>
           <input type="hidden" name="__csrf_token" value="@::__csrf_token;literal@">

Secondly, the page contract on the receiving side has to validate the csrf token. This can be achieved by adding a call to csrf::validate to the validation part of a page contract.

ad_page_contract {
    @author ...
    @creation-date ...
} -query {
} -validate {
   csrf { csrf::validate }

In the code base of OpenACS, CSRF protection was added on several places (e.g. public pages, the list template, etc.) such the checks of OpenACS sites on vulnerability scanners improve. Technically, it would be desirable to secure more places against CSRF attacks in the future. However, it depends on the requirements of a site whether or not e.g. the API browser or search should be CSRF protected. Withe protection turned on, one cannot share e.g. a link to a search with some other user (or a search engine). A site admin has to decide, how protected/public such links should be.

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