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Introduction:
Email-based verification remains a cornerstone of online security, yet most systems blindly trust any address from a non‑free domain. Bug bounty hunters have long exploited this blind spot: when generic domains like Gmail or Yahoo are blocked, a personally owned domain (e.g., yourname.com) is often treated as an institutional or organizational email, bypassing filters entirely. This technique recently earned a researcher a bounty on OpenAI, proving that even AI giants fall prey to fundamental email validation flaws.
Learning Objectives:
- Understand how email filtering distinguishes “free” vs. “custom” domains and why that creates a critical security gap.
- Learn to set up a custom domain with proper SPF/DKIM records to evade detection during authorised penetration tests.
- Execute practical email injection tests using Linux, Windows, and common security tools to validate or mitigate such bypasses.
You Should Know:
- Email Filtering Bypass Explained – Why Custom Domains Slip Through
Most web applications flag disposable or free email providers (Gmail, Yahoo, Outlook) to prevent abuse, credential stuffing, or spam. However, custom domains—even those registered minutes ago—are frequently whitelisted because they appear legitimate (e.g.,[email protected]). Attackers leverage this by registering cheap domains and routing emails through their own mail servers.
Step‑by‑step guide to test the bypass (authorised testing only):
- Register a domain (e.g., `testlab.xyz` from Namecheap or GoDaddy).
- Set up email hosting (use Cloudflare Email Routing, ImprovMX, or a VPS with Postfix).
3. Configure SPF/DKIM (optional but increases deliverability):
- SPF record: `v=spf1 mx ~all`
– DKIM: generate keys and add as a TXT record.
- Send a test email from your custom domain to the target’s sign-up form (e.g., OpenAI’s bug bounty report portal).
Linux commands to validate domain reputation:
Check if domain is listed on any free email blocklist dig +short txt _spf.google.com Example SPF query nslookup -type=txt yourdomain.com Test SMTP relay (if you control a mail server) telnet smtp.yourdomain.com 25 EHLO test MAIL FROM:<a href="mailto:admin@yourdomain.com">admin@yourdomain.com</a> RCPT TO:<a href="mailto:victim@target.com">victim@target.com</a> DATA Subject: Bypass test This is a test. . QUIT
Windows PowerShell equivalent:
Send-MailMessage -SmtpServer smtp.yourdomain.com -From "[email protected]" -To "[email protected]" -Subject "Filter Test" -Body "Checking custom domain bypass"
- Testing OpenAI’s Bug Bounty Program – Responsible Disclosure Workflow
The referenced bounty on OpenAI likely involved using a custom domain to submit a vulnerability report or create a test account where free domains were disallowed. Here’s how to systematically test such logic without violating terms.
Step‑by‑step guide for authorised bug bounty testing:
- Read the program’s scope (OpenAI’s bug bounty on BugCrowd allows testing on `.openai.com` and
.chatgpt.com). - Identify all registration or contact forms that accept email input.
- Attempt registration with a free domain – confirm it’s blocked (e.g., “email not allowed”).
- Repeat with your custom domain – if successful, document the bypass.
- Check for downstream impact (e.g., password reset, account takeover, or delivery of sensitive notifications).
6. Craft a proof‑of‑concept (PoC) script:
import smtplib
from email.mime.text import MIMEText
def send_custom_domain_email(domain, to_email):
msg = MIMEText("This email originates from a custom domain that bypasses your filter")
msg['Subject'] = "Bypass demonstration"
msg['From'] = f"researcher@{domain}"
msg['To'] = to_email
Replace with your SMTP server
with smtplib.SMTP('smtp.yourdomain.com', 25) as server:
server.send_message(msg)
print(f"Email sent from {domain} to {to_email}")
Example usage – authorised test against a staging server
send_custom_domain_email("testlab.xyz", "[email protected]")
- Linux & Windows Recon Commands – Verify Domain Filtering Logic
To understand what a target blocks, you must first map their email validation rules. Use these commands to gather intelligence (ethically and within scope).
Linux:
Find the target's mail server (often reveals filtering)
dig +short mx openai.com
Check if free domains are in their SPF reject list
dig +short txt gmail.com | grep -i "spf"
Use curl to test API endpoints that accept email
curl -X POST https://target.com/api/register \
-H "Content-Type: application/json" \
-d '{"email":"[email protected]","password":"test123"}'
Windows (CMD & PowerShell):
nslookup -type=mx openai.com nslookup -type=txt gmail.com
Test email validation via Invoke-WebRequest
$body = @{email="[email protected]"} | ConvertTo-Json
Invoke-RestMethod -Uri "https://target.com/check-email" -Method POST -Body $body -ContentType "application/json"
- Configuring SPF, DKIM, and DMARC to Appear Legitimate – A Hacker’s Guide
Why stop at simple bypass? Adding proper email authentication increases the chance your email is delivered to the inbox, not spam. Defenders can also use this knowledge to block misconfigured domains.
Step‑by‑step to set up authenticating records (on a VPS with Postfix + OpenDKIM):
1. Install Postfix and OpenDKIM (Ubuntu/Debian):
sudo apt install postfix opendkim opendkim-tools
2. Generate DKIM keys:
sudo opendkim-genkey -D /etc/dkimkeys/ -d yourdomain.com -s default sudo chown opendkim:opendkim /etc/dkimkeys/default.
3. Add TXT records to your domain DNS:
- DKIM: `default._domainkey.yourdomain.com` → value from `default.txt`
– SPF: `yourdomain.com` → `v=spf1 mx -all` (hard fail) - DMARC: `_dmarc.yourdomain.com` → `v=DMARC1; p=reject; rua=mailto:[email protected]`
4. Test authentication:
sudo opendkim-testkey -d yourdomain.com -s default -vvv
Now send an email that passes all checks – most filters will treat it as a trusted corporate sender.
- Automating Email Injection Tests – Using Burp Suite and Custom Payloads
For web applications, email injection often happens at the parameter level. Burp Suite’s Intruder can fuzz the `email=` field with a list of custom domains.
Step‑by‑step automation:
1. Capture a registration request in Burp Suite.
2. Send to Intruder (position the email value).
- Load a payload list of custom domains (e.g.,
evilcorp.net,securemail.io,testlab.xyz). - Set a Grep‑Extract rule for “success” or error messages.
- Run the attack and look for HTTP 200 responses with “account created”.
Alternatively, use a simple bash loop with curl:
for domain in $(cat custom_domains.txt); do
curl -X POST https://target.com/register \
-d "email=user@$domain&password=pass" \
-w "%{http_code} - $domain\n" \
-s -o /dev/null
done
- Mitigating Such Bypasses for Defenders – How to Stop Custom Domain Abuse
If you’re securing an application, don’t just block free domains – implement layered email validation.
Step‑by‑step defensive measures:
- Check domain age – reject domains registered in the last 30 days (WHOIS API).
- Verify MX records – if no mail exchanger exists, treat as suspicious.
- Implement email OTP – send a code before activating any privileged action.
- Use allowlists – only accept emails from known corporate domains (if applicable).
- Monitor for pattern abuse – same domain used across multiple accounts.
6. Linux command for defenders to analyse logs:
Extract all custom domains from signup attempts
grep -oE "[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}" /var/log/auth.log | \
awk -F'@' '{print $2}' | sort | uniq -c | sort -nr
- Real‑World Case Studies – OpenAI, Meta, and PayPal Bounties
Multiple bug bounty programs have paid for email filter bypasses:
– OpenAI (the post above): Custom domain allowed report submission where free domains were blocked.
– Meta (2023): Researchers used `.xyz` domains to bypass Facebook’s anti‑spam filters during account creation.
– PayPal (2022): Custom domain emails bypassed KYC email verification, leading to an account takeover chain.
In each case, the root cause was the same: trust in the absence of a free provider flag, rather than active verification of domain legitimacy.
What Undercode Say:
- Key Takeaway 1: Email filtering that relies solely on a blocklist of free providers is fundamentally broken. A custom domain costs less than $5 and can bypass most enterprise defences within minutes.
- Key Takeaway 2: For defenders, domain age checks, MX record validation, and mandatory OTP are cheap, effective countermeasures. For bug bounty hunters, this technique remains a high‑yield, low‑effort discovery vector that platforms like OpenAI continue to reward.
Analysis: The posted LinkedIn bounty on OpenAI highlights a systemic issue – even AI‑first companies overlook basic input validation. As automation and API registrations grow, email trust boundaries will be attacked more frequently. The custom domain trick works because it exploits a cognitive bias: “only real companies own domains.” In reality, anyone can own one. Until email verification moves toward cryptographic identity (e.g., BIMI, VMC) or behavioural analysis, this bypass will outlive most patches.
Prediction:
Over the next 18 months, we will see a surge in “domain reputation” services that assign risk scores based on registration date, historical abuse, and MX configuration. Simultaneously, AI‑powered email filters will learn to distinguish human‑registered custom domains from legitimate corporate ones by analyzing sending patterns and language models. However, attackers will adapt by aging domains and mimicking corporate communication styles – turning email validation into an escalating AI arms race. Bug bounty payouts for such bypasses will triple as organisations realise their current filters are a sieve, not a shield.
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