Last revision (mm/dd/yy): 04/23/2009
The Man-in-the-Browser attack is the same approach as Man-in-the-middle attack, but in this case a Trojan Horse is used to intercept and manipulate calls between the main application’s executable (ex: the browser) and its security mechanisms or libraries on-the-fly.
The most common objective of this attack is to cause financial fraud by manipulating transactions of Internet Banking systems, even when other authentication factors are in use.
A previously installed Trojan horse is used to act between the browser and the browser’s security mechanism, sniffing or modifying transactions as they are formed on the browser, but still displaying back the user's intended transaction.
Normally, the victim must be smart in order to notice a signal of such attack while he is accessing a web application like an internet banking account, even in presence of SSL channels, because all expected controls and security mechanisms are displayed and work normally.
Points of effect:
- Browser Helper Objects – dynamically-loaded libraries loaded by Internet Explorer upon startup
- Extensions – the equivalent to Browser Helper Objects for Firefox Browser
- API-Hooking – this is the technique used by Man-in-the-Browser to perform its Man-in-the-Middle between the executable application (EXE) and its libraries (DLL).
Manipulation thru DOM interface
In order to perform this attack, an attacker may progress thru the following steps:
- The Trojan infects the computer's software, either OS or Application.
- The Trojan installs an extension into the browser configuration, so that it will be loaded next time the browser starts.
- At some later time, the user restarts the browser.
- The browser loads the extension.
- The extension registers a handler for every page-load.
- Whenever a page is loaded, the URL of the page is searched by the extension against a list of known sites targeted for attack.
- The user logs in securely on to for example https://secure.original.site/.
- When the handler detects a page-load for a specific pattern in its targeted list (for example https://secure.original.site/account/do_transaction) it registers a button event handler.
- When the submit button is pressed, the extension extracts all data from all form fields through the DOM interface in the browser, and remembers the values.
- The extension modifies the values through the DOM interface.
- The extension tells the browser to continue to submit the form to the server.
- The browser sends the form, including the modified values, to the server.
- The server receives the modified values in the form as a normal request. The server cannot differentiate between the original values and the modified values, or detect the changes.
- The server performs the transaction and generates a receipt.
- The browser receives the receipt for the modified transaction.
- The extension detects the https://secure.original.site/account/receipt URL, scans the HTML for the receipt fields, and replaces the modified data in the receipt with the original data that it remembered in the HTML.
- The browser displays the modified receipt with the original details.
- The user thinks that the original transaction was received by the server intact and authorized correctly.
Related Threat Agents
- http://events.ccc.de/congress/2006/Fahrplan/attachments/1158-Subverting_Ajax.pdf - Stefano di Paola and Giorgio Fedon, Subverting Ajax, Dec, 2006.