Proposers

Mujtahid Akon (SyNSec Lab, Pennsylvania State University)
Syed Md Mukit Rashid (SyNSec Lab, Pennsylvania State University)
Syed Rafiul Hussain (SyNSec Lab, Pennsylvania State University)

We are researchers from SyNSec Lab, Pennsylvania State University. While working on a fuzzer for REST APIs, we used CAPIF as one of the targets and found several issues ranging from authorization bypass to denial of service vulnerabilities. This report discusses a critical authorization bypass vulnerability.

Description

The CAPIF implementation contains a critical authorization bypass vulnerability in certificate validation logic. The validate_user function (located in multiple services including services/TS29222_CAPIF_API_Invoker_Management_API/api_invoker_management/core/validate_user.py and services/TS29222_CAPIF_Discover_Service_API/service_apis/core/validate_user.py) implements a fail-open pattern where authorization succeeds when a valid certificate is not found in the expected role collection. This allows attackers with valid certificates from ANY role (provider, invoker, or admin) to access and manipulate resources belonging to OTHER roles, completely bypassing the intended access control model.

Severity: CRITICAL

Affected Endpoints: All authenticated endpoints across CAPIF services including:

API Invoker Management: /onboardedInvokers/{onboardingId} (DELETE, PUT, PATCH)

The following endpoints may also be vulnerable:

API Provider Management: /api-provider-management/registrations/{registrationId}
Service Discovery: /service-apis
Events API: /CAPIF_EVENTS_SUBSCRIPTIONS

Affected Files:

services/TS29222_CAPIF_API_Invoker_Management_API/api_invoker_management/core/validate_user.py (lines 18-26)
services/TS29222_CAPIF_Discover_Service_API/service_apis/core/validate_user.py (similar implementation)
Similar patterns in other CAPIF services

Root Cause Analysis

The vulnerable code in validate_user.py:

class ControlAccess(Resource):

    def validate_user_cert(self, api_invoker_id, cert_signature):

        cert_col = self.db.get_col_by_name(self.db.certs_col)

        try:
            my_query = {'invoker_id':api_invoker_id}
            cert_entry = cert_col.find_one(my_query)

            if cert_entry is not None:
                if cert_entry["cert_signature"] != cert_signature:
                    prob = ProblemDetails(title="Unauthorized", detail="User not authorized", cause="You are not the owner of this resource")
                    prob = serialize_clean_camel_case(prob)
                    return Response(json.dumps(prob, cls=CustomJSONEncoder), status=401, mimetype="application/json")

        except Exception as e:
            exception = "An exception occurred in validate invoker"
            current_app.logger.error(exception + "::" + str(e))

The vulnerability is in the logic flow:

When cert_entry is None (certificate not found in the collection), the function returns nothing (implicitly None)
The caller interprets this as authorization success (fail-open behavior)
When cert_entry exists but signatures don't match, it returns 401 (correct)
Problem: Missing explicit denial when certificate is not in the expected role collection

This means:

Provider certificate not in invoker collection → Returns None → Treated as success → Provider can access invoker resources ✓
Invoker certificate not in provider collection → Returns None → Treated as success → Invoker can access provider resources ✓

Proof of Concept

Invoker deletes Provider resource

DELETE /api-provider-management/registrations/f7d3c5a1... HTTP/1.1
X-Ssl-Client-Cert: <invoker-certificate>

Expected: 403 Forbidden (invoker cannot delete provider registration) Actual: 200 OK - Provider registration deleted by invoker

Why this is an attack: The invoker presents a valid certificate that is NOT in the provider certificate collection, causing the validation function to return None (fail-open). This allows the invoker to delete a critical provider resource they should have no access to. An attacker can systematically delete all provider registrations in the system, causing complete service disruption and preventing legitimate API providers from offering their services.

Impact Assessment

Complete Access Control Bypass: Any authenticated user can access/modify/delete resources across all roles
Data Integrity Violation: Attackers can modify other users' configurations, redirect callbacks, steal API access
Service Disruption: Malicious deletion of registrations, subscriptions, or published APIs
Privilege Escalation: Low-privilege invoker can perform admin-level operations

Attack Scenarios:

Invoker redirects provider's notification callbacks to attacker-controlled server
Provider deletes competitors' API invoker registrations
Any role can enumerate and access all resources in the system
Mass deletion of registrations causing complete service outage

Recommended Fix

Option 1: Explicit Denial When Certificate Not Found (Recommended)

class ControlAccess(Resource):

    def validate_user_cert(self, api_invoker_id, cert_signature):

        cert_col = self.db.get_col_by_name(self.db.certs_col)

        try:
            my_query = {'invoker_id':api_invoker_id}
            cert_entry = cert_col.find_one(my_query)

            if cert_entry is None:
                # ✅ FAIL-CLOSED: Explicitly deny when not found
                prob = ProblemDetails(title="Unauthorized", detail="User not authorized", cause="Certificate not found in expected role")
                prob = serialize_clean_camel_case(prob)
                return Response(json.dumps(prob, cls=CustomJSONEncoder), status=403, mimetype="application/json")
            
            if cert_entry["cert_signature"] != cert_signature:
                prob = ProblemDetails(title="Unauthorized", detail="User not authorized", cause="You are not the owner of this resource")
                prob = serialize_clean_camel_case(prob)
                return Response(json.dumps(prob, cls=CustomJSONEncoder), status=401, mimetype="application/json")

        except Exception as e:
            exception = "An exception occurred in validate invoker"
            current_app.logger.error(exception + "::" + str(e))
            return internal_server_error(detail="Internal error during authorization", cause=str(e))

Key Changes:

Add explicit 403 response when cert_entry is None
Add explicit error handling in exception block
Return proper error responses instead of None

Demo or definition of done

The vulnerability is considered fixed when:

The validate_user function explicitly denies access when certificates are not found in the expected role collection
Cross-role access attempts (e.g., provider accessing invoker resources) result in HTTP 403 Forbidden responses
Integration tests confirm that each role can only access resources within their authorized scope
All affected services (API Provider Management, API Invoker Management, Service Discovery, Events, Publish Service) implement the corrected authorization logic

References

OWASP API Security Top 10 - API1:2023 Broken Object Level Authorization

Disclosure Timeline

2026-01-20: Vulnerability discovered
2026-02-06: Submitted as part of research paper to USENIX Security Conference
2026-05-06: Public disclosure (90 days)

Draft: Resolve "Critical Authorization Bypass Vulnerability in CAPIF Certificate Validation"