Websites nowadays are mostly built with different resources from other origins. For example, many sites include scripts or stylesheets like jQuery or Bootstrap from a Content Delivery Network (CDN). This induces that the webmasters implicitly trust the linked external sources. But what if an attacker can force the user to load the content from an attacker controlled server instead of the genuine resource (e.g. by DNS poisoning, or by replacing files on a CDN)? A security-aware webmaster had no chance to protect his website against such an incident.
This is the point where Subresource Integrity kicks in. Subresource Integrity ensures the integrity of external resources with an additional attribute for the two HTML tags <link> and <script>. The integrity attribute contains a cryptographic hash of the external resource which should be integrated in the site. The browser then checks if the hash of the fetched resource and the hash in the HTML attribute are identical.
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" integrity="sha384-1q8mTJOASx8j1Au+a5WDVnPi2lkFfwwEAa8hDDdjZlpLegxhjVME1fgjWPGmkzs8" crossorigin="anonymous">
In the example above, resource bootstrap.min.css is checked for its integrity with its Base64 encoded SHA-384 hash. If the integrity check is positive, the browser applies the stylesheet (or executes the script in the case of a <script> tag). If the check fails, the browser must refuse to apply the stylesheet or execute the script. The user is not informed if the check has failed and a resource therefore could not be loaded. The failed request can only be seen in the developer tools of the used browser. The following image shows the error message in the Chrome developer tools.
Console in browser does inform about subresource integrity check fail.
The crossorigin attribute in the example configures the CORS request. A value anonymous indicates, that requests for this element will not have set the credentials flag and therefore no cookies would be sent. A value of use-credentials would indicate, that the request will provide cookies to authenticate.
The subresource integrity attribute is currently being reviewed before becoming a W3C standard but is already supported by Chrome 45≤ , Firefox 43≤ and Opera 32≤.
Concluding, the subresource integrity attribute offers better possibilities for webmasters to ensure the integrity of external resources. However, this attribute is not supported by older browsers and needs to be adjusted at every resource change. In the end, a security aware webmaster will keep more control with less effort by keeping all resources hosted on his own servers.
Being a security analyst at Compass Security is an interesting thing, no doubt. Besides interesting projects, there is plenty of know-how transfer and interactions between the employees. For example, each year, all security analysts come together for an event called Come’n’Hack Day. During this year’s event, they had the pleasure to perform an attack/defense hacking contest against each other.
Hacking-Lab‘s new Capture The Flag (CTF) system was used for this purpose. It was only the second time this system was used for an event, after the premiere at the European Cyber Security Challenge final last October in Lucerne.
The participants were spread on three teams: Proxy Foxes, Lucky Bucks and Chunky Monkeys. Each team owned servers with running applications, and had different tasks to perform in order to get points:
- ATTACK – Attack the other team’s applications, and steal a gold nugget.
- DEFENSE – Protect its own applications.
- CODE-PATCHING – Find and patch vulnerabilities in its own applications.
- AVAILABILITY – Keep the own applications up and running.
- JEOPARDY – Solve hacking challenges (cryptography, networking, etc.).
- POWNED – Try to exploit the other teams’ servers.
After a hard fight, the Chunky Monkeys grabbed the first place, closely followed by the Lucky Bucks:
Almost one hundred gold nuggets were stolen during the day:
All attendees enjoyed the highly eventful day. With six different ways to score points, each participant could contribute to its team’s success. This makes such a CTF occasion not only a great social event idea for security analysts but potentially for any organization having technical skilled employees (IT security officers, sysadmins and/or developers)!
“Fake President Fraud” or “CEO Fraud” is a social engineering attack where an adversary tries to convince a member of the financial department of a company to send out a payment to the attacker’s bank account. The attack can be divided into three steps.
- Establish Contact:
Typically only employees responsible for bank transfers get contacted by the adversary, as they have all needed permissions to execute payments. Therefore the criminal impersonates a CEO or any other superior who has enough authority to arrange urgent payments.
These kind of social engineering attacks work if the adversary gathers enough information about the individual he wants to impersonate. As most CEO’s are referenced on the world wide web with detailed personal information such as curriculum vitae and email address, it is easy for an attacker to gather everything he needs to fake a CEO email. Furthermore, company websites often disclose information about customers and other useful details which help an adversary to be more convincingly when requesting a payment.
- Request Payment Transaction:
The attacker often uses email (spear phishing) or phone calls (vishing) to contact his target. Whereby a phone call only works if the victim does not know the voice of the impersonated superior, an attack over email has no such restrictions.
The request itself is about an urgent payment to a foreign bank account and uses a variety of pretexts such as acquisitions or customer projects. For this step to succeed, the criminal uses different elements to convince the target to be compliant to his request and send out the payment:
- Authority: Asking the target as an authority adds a strong argument to every request. Jonathan J. Rusch writes “People are highly likely, in the right situation, to be highly responsive to assertions of authority, even when the person who purports to be in a position of authority is not physically present.” One of the most impressive experiment in social psychology history, which demonstrated the blind obedience to authority figures, is the Stanley Milgram experiment.
- Valorization: The “fact” that the CEO or a superior has “chosen” this specific employee implies that he trusts him. The feeling of being trusted makes the body release oxytocin, often referred as the “love hormone”. This hormones facilitates trust and attachment between individuals. This is an additional factor that helps the attacker to quickly build feelings of rapport and to convince the target to be compliant to his request and send out the payment.
- Secrecy: In order to avoid that a target verifies the authenticity and validity of an order, attackers often label the request as “STRICTLY CONFIDENTIAL” or insert statements like “this project is still secret and its success depends on this transaction”.
- Pressure: Shifting all the responsibility for the success or failure of a project to the target’s shoulder, the attacker put a lot of pressure on him. This will induce the victim to be more compliant and execute the request.
- Urgency: Urgency and authority is a good combination to convince the target to perform the payment as fast as possible. The attacker creates a false sense of urgency in order to get the target to make a rushed judgment or a rash decision. Example email:
- Transfer Money:
If the attacker manages to convince the targeted employee to send out the payment, the money gets transferred to the foreign bank’s account.
Now the question arises what a company can do to avoid a “Fake President Fraud”. Different organizational and technical steps can be done to mitigate the risk of an incident.
- If possible email or phone communication should not be allowed when authorizing large payments. Therefore face-to-face meetings should be mandatory.
- Develop and communicate guidelines and processes of how payment transactions need to be handled.
- If a transaction does not fit the defined process it should be necessary that the employee asks feedback questions which verify that it is a authorized request.
- Employees should participate a special social engineering training to learn how to avoid getting manipulated by an attacker. The goal of the course should be to convey how an attacker thinks, arise a general awareness for social engineering attacks and explain that such attacks are not limited to one communication channel (e.g. email, SMS, WhatsApp, personal).
- Only publish as little financial and personal information in social media and company websites as necessairy. This makes it harder for an adversary to prepare his attack.
- A two-step (4-eyes) verification process when sending out large amounts of money to foreign bank accounts helps mitigating the damage an attack. It is important that the companies culture is good enough that the second person actually really reviews the payment and can challenge the first person without fears for both of them of being in a “Big Brother” system.
- Do not allow employees to use their personal email address for business purpose. This avoids that a compromised private email account allows someone to gain business data access.
- Do not allow that an email from an external mail server is accepted by your mail server when the email address contains your domain name.
- Always use email signatures/encryption at least when sending mails with confidential and/or sensitive content (e.g. S/MIME, PGP).
- Mark emails from external mail-servers with a tag inside the subject (e.g. [EXTERNAL]). This should be done when the emails enter the company’s mail server. With conditional formatting mails can be marked in red if the mail originates outside the company.
- To avoid an Outlook WebAccess from being hacked and used to perform a CEO Fraud, a strong authentication method should be used in addition of a strong password policy (e.g. client certificates, two factor authentication or access restriction to VPN traffic only).
- Attackers may register and use similar looking domains for a CEO fraud attack. To be aware of this, the company should check if there are any similar existing domains (e.g. .co instead of .com top-level domain) and blacklist them on their mail server.
- Additionally the company should try to register all similar looking domains to make it harder for an attacker to register a domain for his attack which looks slightly different then the domain of the company.
Even if the described attack is a real threat, exploited by attackers around the world every day, there are some effective steps to mitigate the risk of an incident as shown above. Especially an awareness training where the employee is taught when to be suspiciously and how attackers try to manipulate people is recommended, as this is the best protection against different types of social engineering attacks.