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Resolve "Denial of Service via Missing HTTP Header Validation"

Jorge Moratinosrequested to merge
OCF195-denial-of-service-via-missing-http-header-validation into staging

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 denial of service vulnerability caused by missing header validation.

Description

All authenticated CAPIF endpoints crash with KeyError when the required X-Ssl-Client-Cert header is missing from requests. The default_controller.py (found in multiple services) uses direct dictionary access (request.headers["X-Ssl-Client-Cert"]) instead of the safe .get() method, causing unhandled exceptions that propagate as HTTP 500 Internal Server Error responses. This affects all operations requiring certificate-based authentication across Provider Management, Invoker Management, Service Discovery, Events, and Publish Service APIs.

Severity: HIGH

Affected Endpoints: All authenticated endpoints across CAPIF services including:

  • Provider Management: /api-provider-management/registrations
  • Invoker Management: /onboardedInvokers
  • Service Discovery: /service-apis
  • Events: /CAPIF_EVENTS_SUBSCRIPTIONS
  • Publish Service: /service-apis (publish operations)

Affected Files:

  • Multiple services' default_controller.py files (e.g., line 24 where certificate header is accessed)

Root Cause Analysis

The vulnerable code pattern in controller files:

def some_authenticated_endpoint():
    # ❌ Direct dictionary access
    cert = request.headers["X-Ssl-Client-Cert"]  # KeyError if header missing!
    
    # ... rest of authorization logic ...

When a request arrives without the certificate header:

GET /api-provider-management/registrations HTTP/1.1
Host: capif-core:8080
# Missing: X-Ssl-Client-Cert header

Python raises:

KeyError: 'X-Ssl-Client-Cert'
  File "default_controller.py", line 24, in endpoint_function
    cert = request.headers["X-Ssl-Client-Cert"]

Proof of Concept

Test Case: Access authenticated endpoint without certificate

GET /api-provider-management/registrations HTTP/1.1
Host: capif-core:8080
Content-Type: application/json

Expected: 401 Unauthorized - "Missing client certificate" Actual: 500 Internal Server Error - Service crashed

Same issue affects all authenticated operations:

  • GET requests (retrieving resources)
  • POST requests (creating registrations/subscriptions)
  • PUT/PATCH requests (updating configurations)
  • DELETE requests (removing resources)

Why this is an attack: The code uses direct dictionary access request.headers["X-Ssl-Client-Cert"] which raises a KeyError when the header is missing. An attacker can simply omit this header to crash any authenticated endpoint across all CAPIF services. Since certificate authentication is required for nearly all operations, this vulnerability effectively allows an unauthenticated attacker to bring down the entire CAPIF platform with trivial effort.

Impact Assessment

  1. Denial of Service: All authenticated operations fail with 500 errors
  2. Service Unavailability: Certificate authentication system completely broken
  3. Poor Error Handling: Users see confusing internal server errors instead of clear auth errors
  4. Easy Exploitation: No authentication required to trigger crash - just omit header

Attack Scenarios:

  • Attacker floods endpoints without certificate headers
  • Load balancer misconfiguration strips headers, breaking all requests
  • Client implementation bug forgets header, causes production outage
  • Automated testing tools trigger crashes when checking unauthenticated access

Recommended Fix

Use Safe Dictionary Access:

def some_authenticated_endpoint():
    # ✅ Safe access with default value
    cert = request.headers.get("X-Ssl-Client-Cert")
    
    if not cert:
        return make_response(
            object=ProblemDetails(
                title="Missing Authentication",
                detail="X-Ssl-Client-Cert header is required for authentication",
                status=401
            ),
            status=401
        )
    
    # ... rest of authorization logic ...

Apply fix to all controller files in:

  • TS29222_CAPIF_API_Provider_Management_API
  • TS29222_CAPIF_API_Invoker_Management_API
  • TS29222_CAPIF_Discover_Service_API
  • TS29222_CAPIF_Events_API
  • TS29222_CAPIF_Publish_Service_API

Demo or definition of done

The vulnerability is considered fixed when:

  1. All controller files use request.headers.get("X-Ssl-Client-Cert") instead of direct dictionary access
  2. Missing certificate header requests return HTTP 401 Unauthorized with descriptive error messages instead of HTTP 500
  3. All authenticated endpoints gracefully handle missing, empty, or malformed certificate headers
  4. Integration tests confirm all services properly validate header presence before processing

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)

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