PHISHING SCHOOL

How to Give your Phishing Domains a Reputation Boost

“Armed with the foreknowledge of my own death, I knew the giant couldn’t kill me. All the same, I preferred to keep my bones unbroken” — Big Phish

When we send out our phishing emails, we are reckoning with giants. Spamhaus, SpamAssassin, SpamTitan, Barracuda, and many more giants wish to grind your bones to bake their bread. They are big. They are scary. But they don’t catch everything. Just like Edward Bloom learned; the best way to deal with giants is to make a good first impression.

That’s how we will deal with these giants as well. Let’s talk about proper etiquette when speaking with giants.

“He Ate My Dog”

The first opportunity a mail server has to reject your email is right at the beginning of an SMTP exchange. The server can run checks on the reputation of both your sending IP, and the domain you are claiming to be in your FROM address. In general, the main controls that will get you blocked by a good secure email gateway (SEG) are some version of the following:

• Does your sending IP show up on a known SPAM list like Spamhaus?
• Is the geographical location of the sending IP in another country?
• What’s the reputation/category of the sending domain?
• What’s the age of the sending domain?
• Does SPF fail?

Of course, all of these checks are generally configurable, and not every SEG will perform all of these checks. These are just some of the most common controls I tend to expect are the root cause of a failure at this stage in the phish’s lifecycle. None of these is really all that difficult to address/bypass so by taking them into account, we can usually ensure our phishing campaigns are not blocked at this layer.

Bypassing Sender Reputation Checks

When we attempt to deliver our message, we will be connecting to a mail server for the target email’s domain, telling it who we are and who we want to send a message to, and then giving it the contents of the message. Here is an example of what that conversation looks like under the hood with data we control in blue and server responses in green:

In general, IP address blocks are easy for us to troubleshoot because if our sending IP is blocked, the server will typically just terminate the TCP connection and we won’t even be able to start this SMTP call and response. It’s pretty rare to be blocked by IP right off the bat, and typically means you’re sending from an IP that is associated with sending a lot of SPAM, or the GeoIP result for your sending IP is in a blocked country or region. If that’s the case, just try sending from another IP.

The next opportunity for the server to block us is by the reputation of the domain we supply as the sending domain. Notice in our example that the sending domain is ‘contoso.com’ and is used in both the EHLO and MAIL FROM commands. These domains typically match, but they don’t have to. Feel free to tinker with mismatching these values to see how different mail servers respond.

You can think of the EHLO and MAIL FROM commands as saying “I’m a mail server for contoso.com, and I’m going to send an email from [email protected] to one of your users”. At this point, the receiving server has an opportunity to accept or reject our claim by checking a few things:

• Is contoso.com on a known SPAM list? (never going to let it through)
• Is contoso.com explicitly whitelisted? (always going to let it through)
• How old is the contoso.com domain?
• Is contoso.com categorized? And if so, what category is it?
• Has the server ever received emails from contoso.com before?
• Does contoso.com’s MX record match your sending IP address?
• Does a reverse DNS lookup of your sending IP resolve to <something>.contoso.com?
• Does contoso.com publish an SPF record that includes your sending IP address?

Most mail servers will perform one or more of these checks to obtain hints about whether you can be trusted or not. Some of these hints, like reverse DNS records, will increase our reputation if they pass the check, but won’t really hurt our reputation too much if they fail. Others, like trying to spoof a domain from an IP that is not in that domain’s SPF record, will likely get us burned right away. Therefore, we should try to pass as many of these checks as we can, while avoiding techniques that could be clearly identified as a spoof.

So, Which Domain Should I Use?

When choosing our sending domain, we have a few options, each with its own pros and cons. We can either spoof a domain that we do not own, or buy a domain. If we buy a domain, we can either get a brand new domain of our choosing, or buy an expired domain that was previously bought by someone else. No one method is inherently better than the others, but we should have a clear understanding of how to get the most out of each technique when making our choice.

Spoofed Domains

Pros

• Free! ‘Nough said
• Makes attribution difficult. You are basically framing someone else. (Note: this could be a con if you have to go through attribution and deconfliction with the blue team)
• Probably already categorized and old enough to pass checks for newly registered domains
• Successful spoofs can look very legitimate, and therefore boost success rates

Cons

• Limited to existing domains that you can find
• You may be able to use non-existent domains, but they are easy to identify and often blocked. It’s generally better to purchase in this case.
• Most high-value domains have implemented email security settings like SPF that could make it difficult, if not impossible to spoof without detection
• We can’t set up our own email security settings on the domain that might boost our reputation (no control and no proof-of-ownership)
• We have no influence over domain category or reputation

Considerations When Spoofing Domains

Does our target mail server actually check SPF?

If not, we should be able to successfully spoof just about any domain. I’ve seen several cases where, for whatever reason, SPF checks were either turned off or misconfigured so that failing SPF did not result in rejected emails. It’s often worth a shot to try to deliver a few benign emails while intentionally failing SPF to see if the server sends back an error message or says it delivered them. While you won’t receive any bounce messages or responses from recipients (those would go to the mail server for your spoofed domain), you could try putting image links or tags for remote CSS resources in the emails that point to a web server you own and track requests for these resources as potential indicators that the emails were delivered.

Is there a similar registered domain that lacks an SPF record?

You may be able to find domains that have been registered that look very similar to your target organization’s domain, but have not been configured with any email security settings that would allow a mail server to confirm or deny a spoof attempt. In some cases, I’ve seen clients that buy up domains similar to their own to prevent typosquatting, but then forget to apply email settings to prevent spoofing. In other cases, I’ve seen opportunistic domain squatters who buy look-alike domains in case they may become valuable, and don’t bother to set up any DNS records for their domains. In either case, we can potentially spoof these domains and the mail server will have no way of knowing we don’t own them. While this might mean we are starting off with a generally low trust score, most mail servers will not outright reject messages just because the sending domain lacks a properly configured SPF record. That is because there are so many legitimate organizations that have never bothered to set one up.

Does your target organization have an expired domain listed in an ‘include’ statement in their SPF record?

Let’s say our target organization’s email domain is fabrikam.com, and we would like to spoof an internal email address ([email protected]) while sending to our target users. Consider that fabrikam.com has the following SPF record as an example:

v=spf1 mx a ip4:12.34.45.56/28 include:sendgrid.net include:imexpired.net -all

This SPF record lists the hosts/IPs that are allowed to send emails from frabrikam.com. Here is a breakdown of what each piece means:

v=spf1

This is an SPF version 1 TXT record. Used to distinguish SPF from other TXT records.

mx

The IP of the domain’s MX (mail server) record is trusted to send emails from fabrikam.com

a

the IP from doing a forward DNS lookup on the domain itself is also trusted

ip4:12.34.45.56/28

Any IP in this small range is trusted to send emails from fabrikam.com

include:sendgrid.net

any IPs or IP ranges listed in sendgrid.net’s SPF record are also trusted

include:imexpired.net

any IPs or IP ranges listed in imexpired.net’s SPF record are also trusted

-all

every other sending IP that was not listed in the previous directives is explicitly denied from sending emails claiming to be from fabrikam.com

As this demonstration shows, the ‘include’ directive in SPF allows you to trust other domains to send on your domain’s behalf. For example, emailing services like sendgrid can be configured to pass SPF checks so that sendgrid can deliver marketing emails from fabrikam.com to their customers. In the case of our example, if our client previously trusted imexpired.net to send emails, but the domain is now expired, it means we buy imexpired.net and add our own server to imexpired.net’s SPF record. We will then be able to pass SPF checks for fabrikam.com and spoof internal users.

Can we bypass SPF protections with other technical tricks?

Depending on the receiving mail server’s configuration, there may be other ways we can spoof domains that we do not own. For example, some email servers do not require SPF alignment. This means that we can have a mismatch between the domain we specify in the ‘MAIL FROM’ SMTP command, and the ‘From’ field in the message content itself. In these cases, we can use either a domain we own and can pass SPF for, or any domain without an SPF record in the ‘SMTP FROM’ command and then choose any other domain for the email address that our target user sees in their mail client. Another trick we can sometimes use is to supply what’s known as the “null sender”. To do this, we simply specify “<>” as the sender. The null sender is used for bounce messages and is sometimes allowed by spam filters for troubleshooting.

SMTP is a purely text-based protocol. This means that there is often ambiguity in determining the beginning and end of each section of data and SMTP implementations often have parsing flaws that can be abused to bypass security features like SPF and DKIM. In 2020, Chen Jianjun et. al published a whitepaper demonstrating 18 separate such flaws they discovered and that affected even some big email providers like Google:

https://i.blackhat.com/USA-20/Thursday/us-20-Chen-You-Have-No-Idea-Who-Sent-That-Email-18-Attacks-On-Email-Sender-Authentication.pdf

https://www.usenix.org/system/files/sec20fall_chen-jianjun_prepub_0.pdf

They also published the tool they used to discover these vulnerabilities, as well as their research methodology. While they disclosed these vulnerabilities to the vendors they tested, they did not test every major SEG for all of these vulnerabilities, and you might find that some popular SEGs are still vulnerable 😉

In 2023, Marcello Salvati demonstrated a flaw in a popular email sending service that allowed anyone to pass SPF for millions of legitimate domains.

https://www.youtube.com/watch?v=NwnT15q_PS8

His DefCon talk on the subject demonstrates his research and discovery process as well. I know that there are similar flaws in some other major email sending services, but I will not be throwing them under the bus in this post. Happy hunting!

Purchasing Brand New Domains

Pros

• We can set up SPF, DMARC, DKIM to ensure we pass these checks
• We control the MX record so we can receive mail at this domain
• More granular creative control over the name

Cons

• May take some time to build a reputation before sending a campaign

Purchasing Expired Domains

Pros

• We can set up SPF, DMARC, DKIM and ensure we pass these checks
• We control the MX record so we can receive mail at this domain
• More likely to already be categorized
• You may have wayback machine results to help keep the current category
• You might salvage some freebee social media accounts

Cons

• Little control over the domain name. You can get key words but otherwise just have to get lucky on which ones become available

Considerations for any Purchased Domain

The two major advantages to using domains we own for phishing is that we can specify the mail server and mail security settings for the domain. This means that we can actually have back-and-forth conversations with phishing targets if we need to. This opens up the possibility for more individualized pretexts. By setting up our own SPF, DMARC, and DKIM records, we can ensure that we pass these checks to give our emails a high probability of passing reputation based checks.

The third benefit of owning a domain is that we can control more factors that might help us categorize the domain. Email gateways, web proxies, and DNS security appliances will often crawl new domains to determine what kind of content is posted on the domain’s main website if one exists. They then make a determination of whether to block or allow requests for that domain based on the type of site it is. Security products that include category checks like this are often configurable. For example, some organizations block social media sites while others allow employees to use social media at work. In general, most network administrators will allow certain common benign categories. A couple classic desirable categories for red teamers are ‘healthcare’ and ‘finance’. These categories have additional benefits when used for command and control (C2) traffic, though for the purposes of delivering emails, we really just need to be categorized as anything that’s not sketchy. If your domain is not categorized at all, or in a ‘bad’ category, then your campaign will be dead in the water, so here’s some tips on getting off the naughty list.

Categorizing New Domains

.US.ORG Domains

One of my favorite shortcuts for domain categorization is to register domains off the ‘us.org’ or similar top levels. Because ‘us.org’ is a domain registrar, its Bluecoat category is ‘Web Hosting’, and has been around for many years. The minute you buy one of these domains, you are automatically going to be lumped in a generally benign and frequently allowed category.

Categorize with HumbleChameleon

While Humble Chameleon (HC) was designed to bypass multi-factor authentication on phishing assessments, it also has some great features for ‘hiding behind’ benign domains. You can set HC’s ‘primary_target’ option to point at a site you would like to mimic and let the transparent proxy do the rest.

Warning: any time your domain is crawled, your server will be forwarding traffic to the impersonated site. I have seen logs on my server before where someone was scanning my site for WordPress vulnerabilities, and therefore causing my server to send the same malicious traffic to the site I was impersonating. While the chances of being sued for this are pretty low, please be aware that this technique may put you in a bit of a gray area. Choose targets wisely and keep good logs.

Categorizing with Clones and Nginx/Apache

If you’ve never used httrack to clone a site, you should really do yourself a favor and explore this absolute classic! How classic you ask? Xavier Roche first released this website cloner back in 1998 and it’s still a great utility for quickly cloning a site. Httrack saves website resources in a directory structure that is ideal for hosting with a static Nginx or Apache web server. The whole setup process can be easily scripted with Ansible to stand up clones for domain categorization.

Categorizing with Chatgpt and S3

The more modern version of static site hosting is to ask an AI bot to write your website for you, and then serve the static content using cheap cloud services like S3 buckets and content delivery networks. If you get stuck, or don’t know how to automate this process, you can just ask the robot 😉

Categorizing Expired Domains

Whenever you purchase an expired domain, you should go ahead and check its category before you buy. It’s much easier to maintain a good category than to try to re-categorize a domain, so only buy it if it’s already in a benign category. Once you buy one, you have two options to maintain the category. You can either generate some new content or re-host the site’s old content. If you want to generate new content, the AI route is probably the most efficient. However, my preferred approach is to just put the content of the old site back up on the Internet. If that content was what got the original categorization, then it should also keep it in the same category.

Categorizing with Wayback

If the expired domain you just purchased was indexed by the Wayback Machine, then you can download the old pages with a simple Ruby script:

https://github.com/hartator/wayback-machine-downloader

This script is insanely easy to install and use, and will generally get you a nice static site clone within minutes when paired with Nginx, Apache, etc. Keep in mind that you may want to manually modify site content that might still be under copyright or trademark from the prior owner.

Social Media Account Bonus

For expired sites you’ve bought, have you ever set up a mail server and monitored for incoming emails? I have! And the results can be fascinating. What you might find is that the domain you just bought used to belong to a functioning company, and some of that company’s employees used their work email to sign up for services like social media sites. Now, the company went under, those employees don’t have access to their former work email, but you do. Once again, I need to stress that this is a legal gray area, so do what you can to give these people back their accounts, and change PII on any others you decide to keep as catphish accounts.

General Tips for any Domain

Send Some ‘Safe’ Emails First

Some SEGs will block emails from domains they have never seen before regardless of reputation. Therefore, it’s generally a good idea to send some completely benign emails with your new domain before you send out any phishing pretexts. You can address these to the ‘info@’ or similar generic mailboxes and keep the messages vague to hopefully go unnoticed/ignored by any humans on the other end. We’re just trying to prime the SEG to make a determination that emails from our domain are safe and should be allowed in the future. I’ve also heard that adding hidden links to emails with HREFs that point to your domain will also work against some SEGs, though I have not personally experimented with this technique.

Use Sendgrid, MailChimp, etc.

These services specialize in mass email delivery for their clients. They work tirelessly to keep their email servers off of block lists and help marketers deliver phishing campaigns… ahem I mean ‘advertisements’ to their targets… oops I mean ‘leads’. Once again, marketing tools are your friend when cold emailing. Keep in mind that these services don’t appreciate scammers and spammers like you, so don’t be surprised if you get too aggressive with your phishing and your account gets blocked. Yet another argument for keeping campaigns small and targeted.

Use your Registrar’s Email Service

Most domain registrars also offer hosting and email services. Some even provide a few mailboxes for free! Similar to email sending services, your registrar’s mail servers will often be delivering tons of legitimate messages for other customers and therefore have a good reputation from an IP perspective. If you take this route, be sure to let the registrar know that you are a penetration tester and which domains you will be using for what campaigns. Most I’ve worked with have an abuse email address and will honor your use case.

Use Gmail for the Long Con

Personal emails are often a blind spot for corporate email filters. Completely blocking inbound emails from Gmail, Yahoo, Outlook, etc. is a tough pill to swallow and most network administrators decide to allow them instead of answering constant requests from employees to put in an exception for this or that sender. In addition, it’s extremely difficult to apply reputation scores to individual senders. Therefore, if you can think of a pretext that would make sense to come from a Gmail account, you have a high probability of at least bypassing domain reputation checks. This can work really well for individualized campaigns where you actually have a back-and-forth conversation with the targets before link/payload delivery.

Set Up SPF, DKIM, and DMARC

For sending domains you own, it will always work in your favor to set up email security settings. Check the API documentation for your registrar and write a script to set these DNS settings.

Read the Logs!

When you do start sending emails with a domain, make sure to watch your SMTP logs carefully. They are often extremely helpful for identifying if your messages are being blocked for reasons you control. For example, I know Mimecast will frequently block emails from new senders that it hasn’t seen before, but will respond with a very helpful message explaining how to get off their ‘gray list’. The solution is simply to resend the email within the next 24 hours. If you aren’t watching the logs, you would be blocked when you could easily deliver your email.

Parting Wisdom

“What I recalled of Sunday School was that the more difficult something became, the more rewarding it was in the end.” — Big Phish

One Phish Two Phish, Red Teams Spew Phish was originally published in Posts By SpecterOps Team Members on Medium, where people are continuing the conversation by highlighting and responding to this story.

*** This is a Security Bloggers Network syndicated blog from Posts By SpecterOps Team Members - Medium authored by Forrest Kasler. Read the original post at: https://posts.specterops.io/one-phish-two-phish-red-teams-spew-phish-1a2f02010ed7?source=rss----f05f8696e3cc---4