Automating cybercrime with sentry MBA

The most commonly used attack tool for credential stuffing


Sentry MBA is an automated attack tool used by cybercriminals to take over user accounts on major websites. With Sentry MBA, criminals can rapidly test millions of usernames and passwords to see which ones are valid on a targeted website. The tool has become incredibly popular — the Shape Security research team sees Sentry MBA attack attempts on nearly every website we protect.

In the past, cybercriminals had to master arcane web technologies to launch online attacks. Sentry MBA has a point-and-click graphical user interface, online help forums, and vibrant underground marketplaces to enable large numbers of individuals to become cybercriminals. These individuals no longer need advanced technical skills, specialized equipment, or insider knowledge to successfully attack major websites.

Sentry MBA features advanced capabilities that help attackers elude common web application defenses. For example, the tool can bypass preventative controls (such as IP blacklists or rate limiting) by using proxies to spread the attack across a large number of IP addresses. Sentry MBA can also bypass detective controls (such as referrer checks that ensure visitors were sent to the login page from another, expected page) by spoofing the “referer” header value.

Using Sentry MBA in Credential Stuffing Attacks

A list of usernames and passwords is at the heart of every Sentry MBA attack. In Sentry MBA parlance, these are called “combo” lists. If the combo list has credentials that were valid on another website (e.g. via publicized breaches at eBay, Sony, and Ashley Madison or through phishing techniques), the attack is called “credential stuffing.” Credential stuffing works because Internet users routinely reuse passwords across web accounts. Verizon’s 2015 data breach report cites the use of stolen credentials as the most common attack action used against web applications today.

Credential stuffing attacks are difficult to stop because they target online user interface elements — like login pages — that are open to all Internet traffic by design. In one such attack, cybercriminals using Sentry MBA targeted the stored-value card program at a large retail corporation. Automation accounted for over 91% of the traffic on the company’s login page. Even though the company had implemented established best practices for online security, online fraud losses still exceeded $25M a year.

1. Targeting and Attack Refinement

Before it can test account credentials, Sentry MBA must be configured to understand the targeted login page. A Sentry MBA “config” file contains, among other items, the url for the website’s login page, field markers to help navigate form elements, and rules for valid password constructions. A number of forums offer a wide variety of working configurations for various websites. Here’s a screenshot from one such forum:

Once the attacker has a basic working configuration, Sentry MBA offers tools to optimize and test the attack setup against the live target website. For example, the tool can be configured to recognize certain keywords associated with a website’s responses to successful and unsuccessful login attempts. If the targeted page includes a CAPTCHA test, Sentry MBA can defeat the CAPTCHA with its optical character recognition module or with an optional database containing thousands of possible CAPTCHA images and answers. This YouTube video demonstrates the process of tuning Sentry MBA to solve CAPTCHA.

2. Automated Account Checking

Once the site configuration is optimized, attackers need only add their “combo” file and a “proxy” file to Sentry MBA to commence their attack.

Combo files are simply lists of usernames and passwords. The Darknet and open web offer many options for acquiring stolen lists of usernames and passwords. Fresh lists are often sold at a premium but other lists can be freely downloaded. Here’s a screenshot of combo lists available for sale on a public website:

Proxy files are lists of computers used by Sentry MBA to send login attempts to a targeted site. These lists, like combo lists, are readily available on the open web and the Dark net. Proxies make attack detection and defense far more difficult. Specifically, they undercut two common application defense strategies: IP reputation filtering and rate limiting.

How do proxies defeat IP reputation filtering (also known as “blacklisting”)? Sentry MBA proxies are compromised computers that are typically used without authorization. Cybercriminals constantly work to gain access to fresh proxies that can help evade blacklists. An analysis of a proxy-based attack at one Shape customer revealed that 60% of the proxies used in the attack were new each day. Blacklists can’t be updated quickly enough to detect compromised computers and stop them from joining attacks.

Proxies also help attackers evade defenses that rely on rate limiting. These defenses fail because the attacker’s login attempts appear to come from a large number of different computers. Sentry MBA can be tuned to ensure that no individual proxy sends too many requests, which makes rate limiting ineffective.

With a working configuration, combos, and proxies, the attack can commence. Sentry MBA coordinates proxies and collects information on which credentials successfully open an account. It even checks for what assets are available in an account and notifies the attacker when hits occur. After all the credentials in the combo list have been checked, Sentry MBA’s role in the attack is over.

3. Monetization

Once the cybercriminal has working credentials, they need a way to make money from their victims.Monetization strategies can take many forms. The retailer mentioned earlier was defrauded when cybercriminals transferred stored-value gift card balances out of compromised accounts to cards controlled by the cybercriminal. Sites such as,, and make it easy to liquidate these fraudulent cards for cash or merchandise.

Many other monetization schemes are available to cybercriminals, including extortion, account funds transfers, and acquisition of credit card information, to name a few. As cybercriminals find increasingly creative ways to monetize attacks, more and more websites become attractive targets.


The rise of Sentry MBA illustrates the pivotal role automation plays in cybercrime and highlights how cybercrime is increasingly compartmentalized and commoditized. The “script kiddies” have grown up and now have easy access to powerful automation tools. Sentry MBA and the underground marketplaces that have grown up around it are one example of how online attacks have evolved into a mass-market endeavor for legions of cybercriminals across the globe.

Download (PDF)

Anatomy of a Sentry
MBA Attack

An attack using Sentry MBA has three phases:

  1. Targeting and attack refinement.
  2. Automated account checking.
  3. Monetization. Sentry MBA (and the criminal ecosystem that supports it) dramatically improve criminal productivity during the first two phases.

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