Did You Just Get an Email from “support.yourcompany.com”? Think Again.
Imagine this: Your customer gets an email from billing.yourcompany.com asking for payment details. It looks legit. The email address checks out.
🚨 But wait… Your company never sent that email!
This is subdomain spoofing—one of the most overlooked security threats in email authentication. Attackers exploit unprotected subdomains of your domain (like help.yourdomain.com or info.yourdomain.com) to send phishing emails. And if you're not using DMARC properly, you might never even know it's happening.
Scary, right? 😨
Let’s fix that.
🔍 How Does Subdomain Spoofing Work?
Attackers take advantage of misconfigured or unprotected subdomains to send fake emails that look like they’re from your brand.
Here’s how:
1️⃣ They find a subdomain without proper DMARC protection. 2️⃣ They use it to send phishing emails to your customers or employees. 3️⃣ Because the email “looks real,” people fall for it—compromising accounts, sending money, or leaking sensitive data.
And the worst part? Since it’s a subdomain, even your main domain’s DMARC policy might not protect it!
🚦 How to Check if Your Subdomains Are Vulnerable
First things first—let’s check if your subdomains are properly secured. Run this command in your terminal:
shCopyEditnslookup -type=TXT _dmarc.yourdomain.com
If you see “NXDOMAIN” or no DMARC record, it means your subdomain is not protected.
You can also use a free DMARC checker tool to scan for missing policies.
🛡️ The DMARC Fix: Securing Your Subdomains Against Spoofing
1️⃣ Apply a DMARC Policy to Every Subdomain
By default, DMARC does NOT inherit the policy from your main domain. This means each subdomain needs its own protection.
✅ Solution: Publish a wildcard DMARC record:
txtCopyEdit_dmarc.yourdomain.com TXT "v=DMARC1; p=reject; rua=mailto:dmarc-reports@yourdomain.com"
This ensures that ANY subdomain under yourdomain.com follows the DMARC policy.
2️⃣ Use a Strict DMARC Policy (Not Just “None”)
A "p=none" policy is like putting a lock on your door but leaving the key under the mat. Attackers can still spoof your subdomain!
✅ Better Approach: Change this:
txtCopyEditv=DMARC1; p=none
To this:
txtCopyEditv=DMARC1; p=reject; sp=reject
🚨 That “sp=reject” part is critical! It tells email servers to reject spoofed emails sent from ANY subdomain under your main domain.
3️⃣ Enable DMARC Reports to Catch Attackers in the Act
Wouldn’t it be great if you could see exactly who’s trying to spoof your domain? That’s where DMARC reports come in.
✅ Solution: Set up an email to receive DMARC reports:
txtCopyEditv=DMARC1; p=reject; rua=mailto:dmarc-reports@yourdomain.com; ruf=mailto:dmarc-forensics@yourdomain.com
This will give you real-time insights into spoofing attempts, so you can act fast.
4️⃣ Lock Down SPF and DKIM for Extra Security
DMARC is powerful, but it works best with SPF and DKIM.
🔹 SPF Setup: Make sure your SPF record includes only authorized mail servers and is strict:
txtCopyEditv=spf1 include:_spf.google.com -all
🔹 DKIM Setup: Enable DKIM signing for all outgoing emails. Run this check to see if it’s active:
shCopyEditnslookup -type=TXT default._domainkey.yourdomain.com
If nothing shows up, you need to enable DKIM in your email provider settings.
🚀 Final Thoughts: Don’t Let Subdomain Spoofing Ruin Your Reputation
Attackers love exploiting subdomains because they’re often unprotected. Don’t make it easy for them!
✅ Action Plan Recap:
✔️ Check for missing DMARC records on subdomains ✔️ Apply a strict DMARC policy with “sp=reject” ✔️ Set up DMARC reports to monitor spoofing attempts ✔️ Lock down SPF and DKIM for extra protection
You’ve worked hard to build trust in your brand. Don’t let subdomain spoofing destroy it. Signup & try free today! 🚀
