Testing for Browser cache weakness (OTG-AUTHN-006)
This article is part of the new OWASP Testing Guide v4.
Back to the OWASP Testing Guide v4 ToC: https://www.owasp.org/index.php/OWASP_Testing_Guide_v4_Table_of_Contents Back to the OWASP Testing Guide Project: https://www.owasp.org/index.php/OWASP_Testing_Project
Note: Currently, this is basically a replica of the browser cache portions of the "Testing for Logout and Browser Cache Management (OWASP-AT-007)" Test Case from OWASP v3.
In this phase we check that the server correctly instructs the browser to not cache sensitive data.
Description of the Issue
...here: Short Description of the Issue: Topic and Explanation
Black Box testing and example
The Browser "Back" Button
The first (and simplest) test consists of logging out and then hitting the 'back' button of the browser, to check whether previously displayed sensitive information can be accessed whilst unauthenticated. If by pressing the 'back' button we can access previous pages but not access new ones, then we are simply accessing the browser cache. If these pages contain sensitive data, it means that the application did not forbid the browser to cache it (by not setting the Cache-Control header). Technically, the "back" button is a history, not a cache (see http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.13). However, it is possible in practice to stop the "back" button from showing sensitive data. This can be done by:
- Deliver the page over HTTPS.
- Set Cache-Control: must-revalidate
Logging out from an application obviously does not clear the browser cache of any sensitive information that might have been stored. Therefore, another test that is to be performed is to check that our application does not leak any critical data into the browser cache. In order to do that, we can use a proxy (such as WebScarab) and search through the server responses that belong to our session, checking that for every page that contains sensitive information the server instructed the browser not to cache any data. Such a directive can be issued in the HTTP response headers:
- Cache-Control: no-cache, no-store
- Expires: 0
- Pragma: no-cache
These directives are generally robust, although additional flags may be necessary for the Cache-Control header in order to better prevent persistently linked files on the filesystem. These include:
- Cache-Control: must-revalidate, pre-check=0, post-check=0, max-age=0, s-maxage=0
HTTP/1.1: Cache-Control: no-cache
HTTP/1.0: Pragma: no-cache Expires: <past date or illegal value (e.g., 0)>
For instance, if we are testing an e-commerce application, we should look for all pages that contain a credit card number or some other financial information, and check that all those pages enforce the no-cache directive. On the other hand, if we find pages that contain critical information but that fail to instruct the browser not to cache their content, we know that sensitive information will be stored on the disk, and we can double-check that simply by looking for it in the browser cache. The exact location where that information is stored depends on the client operating system and on the browser that has been used. Here are some examples:
- Mozilla Firefox:
- Unix/Linux: ~/.mozilla/firefox/<profile-id>/Cache/
- Windows: C:\Documents and Settings\<user_name>\Local Settings\Application Data\Mozilla\Firefox\Profiles\<profile-id>\Cache
- Internet Explorer:
- C:\Documents and Settings\<user_name>\Local Settings\Temporary Internet Files
Gray Box testing and example
The methodology for testing is equivalent to the black box case, as in both scenarios we have full access to the server response headers and to the HTML code. However, with gray box testing, we may have access to account credentials that will allow us to test sensitive pages that are accessible only to authenticated users.