Testing for Browser cache weakness (OWASP-AT-007)
This article is part of the OWASP Testing Guide v4 (the current status is:DRAFT).
OWASP Testing Guide v4 Table of Contents [DRAFT] At the moment the The entire OWASP Testing Guide v3 can be downloaded here.
In this phase we check that the application correctly instructs the browser to not remember sensitive data.
Description of the Issue
Browsers can store information for purposes of caching and history. Caching is used to improve performance, so that previously displayed information doesn't need to be downloaded again. History mechanisms are used for user convenience, so the user can see exactly what they saw at the time when the resource was retrieved.
If sensitive information is displayed to the user (such as their address, credit card details, Social Security Number, or username), then this information could be stored for purposes of caching or history, and therefore retrievable through examining the browser's cache or by simply pressing the browser's "Back" button.
Black Box testing and example
Technically, the "Back" button is a history, not a cache (see http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.13). The cache and the history are two different entities, however, they share the same weakness that we are trying to prevent, that of presenting previously displayed sensitive information. The first (and simplest) test consists of entering sensitive information into the application, 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 it is not an authentication issue, but a browser history issue. If these pages contain sensitive data, it means that the application did not forbid the browser to store it. Authentication does not necessarily need to be involved in the testing. For example, when a user enters their email address in order to sign up to a newsletter, this information could be retrievable if not properly handled. The "Back" button can be stopped from showing sensitive data. This can be done by:
- Delivering the page over HTTPS.
- Setting Cache-Control: must-revalidate
Here we check that the application does not leak any sensitive 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 the 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.