Case Study: E-Commerce Microservices Architecture

Independent Application Security Assessment

Engagement Type: Bug Bounty (Managed Program)
Assessment Period: May-June 2026
Vulnerability Class: Cross-Origin Resource Sharing (CORS) Misconfiguration

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Executive Summary

This assessment identified critical architectural vulnerabilities in an e-commerce platform's microservices authentication and CORS implementation, with confirmed exploitation paths affecting user session security and cross-origin data exfiltration. The findings reveal systemic misconfigurations across multiple subdomains stemming from a centralized authentication service and token generation mechanism that prioritizes convenience over security.

Key Findings:

• CORS misconfiguration with credential leakage on multiple production subdomains
• Unauthenticated token generation enabling device identity manipulation
• Potential stored XSS/SSTI vectors in vendor intake forms
• Legacy service exposure with outdated software versions

Overall Risk Rating: High — confirmed exploitable CORS misconfiguration with direct impact to authenticated session security.

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Scope and Authorization

In Scope: Multiple subdomains across e-commerce and home services infrastructure
Out of Scope: Per program policy
Authorization: Authorized security research under bug bounty program. All testing conducted with explicit authorization within defined scope boundaries.

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Technology Stack Identified

Frontend: React + React Router.

Infrastructure: Nginx, HAProxy, Istio Envoy.

Backend: Node.js/Express, Java/Jersey (JAX-RS).

Session Management: Redis.

Authentication: JWT (HS256), Kevlar Service.

API Layer: GraphQL, REST.

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Methodology

Testing followed a structured black-box approach across five phases:

Phase 1: Reconnaissance — Subdomain enumeration, technology fingerprinting, service categorization
Phase 2: Input Validation — Type confusion testing on authentication endpoints
Phase 3: Session Management — Token analysis, device ID testing, JWT inspection
Phase 4: CORS Policy Testing — Origin reflection analysis, credential header testing
Phase 5: Legacy Service Analysis — Version identification, CVE validation

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Validated Vulnerabilities

Finding 1: Cross-Origin Resource Sharing (CORS) Misconfiguration

Severity: High
Category: Server Security Misconfiguration
CWE: CWE-942 — Permissive Cross-domain Policy with Untrusted Domains
Status: Confirmed

Technical Detail

The authentication service and internal dashboard reflect arbitrary origin values supplied in the Origin request header directly into the Access-Control-Allow-Origin response header. When combined with Access-Control-Allow-Credentials: true, this permits any attacker-controlled website to make authenticated cross-origin requests to these APIs using a victim's browser session and read the full response.

Proof of Concept:

OPTIONS /generate_token HTTP/1.1
Host: [auth-service]
Origin: https://attacker.example.com
Access-Control-Request-Method: POST

Response:

HTTP/1.1 200 OK
Access-Control-Allow-Origin: https://attacker.example.com
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: GET,PUT,POST,DELETE,OPTIONS,HEAD
Access-Control-Allow-Headers: Content-Type,Authorization,X-Requested-With
Access-Control-Max-Age: 5184000

Attack Scenario

1. Attacker hosts malicious JavaScript on attacker-controlled domain
2. Victim (logged into target platform) visits attacker's page
3. JavaScript calls token generation endpoint with victim's cookies
4. Server issues access token to attacker's page
5. Attacker uses token to call authenticated endpoints
6. Victim's cart, profile, and address data exfiltrated

Business Impact

Complete user session compromise, PII exfiltration, and potential for account takeover. Affects all authenticated users of the platform.

Affected Endpoints

• All endpoints on authentication service
• All endpoints on internal dashboard
• All endpoints on home services portal

Remediation

Replace reflected origin logic with explicit allowlist:

• Hard-code allowed origins
• Validate origin against allowlist server-side
• Review necessity of credentials for cross-origin requests

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Finding 2: Device ID Acceptance Without Validation

Severity: Low
Category: Authentication
CWE: CWE-20 — Improper Input Validation
Status: Confirmed, mitigated

Technical Detail

The token generation endpoint accepts an X-DEVICE-ID header value and reflects it directly into the JWT dId claim without server-side validation. Arbitrary values (including 0, negative numbers, non-numeric strings) are accepted.

Decoded JWT Payload:

{
  "iss": "kevlar",
  "type": "AT",
  "dId": "79365",          // Reflected from X-DEVICE-ID
  "kevId": "VICBF7F123...",
  "tId": "jeeves",
  "vs": "LO",
  "z": "CH",
  "m": false,
  "gen": 1
}

Attempts to modify device ID in subsequent requests were rejected — the JWT claim takes precedence and signature validation prevents forgery.

Business Impact

Design flaw allowing arbitrary device ID assignment, but JWT signature enforcement prevents token tampering. No user data accessible without the signing key.

Remediation

• Validate device ID format and range server-side
• Consider using a server-generated device identifier rather than client-supplied

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Finding 3: Stored Input Validation Bypass in Vendor Form

Severity: Medium (Pending Confirmation)
Category: Input Validation
CWE: CWE-20 — Improper Input Validation
Status: Pending execution confirmation

Technical Detail

Vendor intake form endpoint accepts and stores user input without proper sanitization or validation. Multiple test vectors confirmed input reaches persistent storage:

• JSON injection accepted (payload not rejected)
• SSTI patterns accepted (payload not rejected)
• Unicode-escaped HTML accepted (payload not rejected)

Business Impact

Potential stored XSS or SSTI execution in admin/moderator rendering context. Impact depends on admin dashboard rendering behavior. Payloads submitted awaiting callback confirmation.

Remediation

Implement whitelist-based input validation using schema validation library (joi, yup). Escape all user input when rendering in admin dashboards. Implement Content Security Policy headers.

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Finding 4: Legacy Service Exposure

Severity: Medium
Category: Server Security Misconfiguration
CWE: CWE-1104 — Use of Unmaintained Third Party Components
Status: Identified

Technical Detail

A legacy chat/messaging service runs Nginx version 1.10.3 (released January 2017), containing known CVEs including CVE-2017-7529 (range filter integer overflow) and CVE-2016-1247 (privilege escalation). The backend service on port 9000 was unreachable during testing.

Business Impact

Known vulnerabilities exist but exploitation appears blocked by either patching or service unavailability. The subdomain naming implies internal communication service, yet serves a default Nginx welcome page — a configuration anomaly suggesting incomplete decommissioning.

Remediation

• Upgrade Nginx to current stable release
• Audit for other legacy services with outdated versions
• Decommission or properly secure services no longer in use

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Security Controls Assessment

What Worked

✅ JWT Signature Enforcement — HMAC-SHA256 signature prevented token tampering
✅ Type Validation — Express middleware prevented NoSQL injection on auth endpoints
✅ Rate Limiting — API endpoints enforced rate limits, preventing enumeration
✅ CSRF Protection — Properly configured on state-changing operations
✅ Session Isolation — Cart ownership determined by JWT claims, not request headers

What Failed

❌ CORS Configuration — Blanket credential allowance without origin validation
❌ Input Validation — Form endpoints lacked sanitization and validation
❌ Service Lifecycle Management — Legacy service left partially deployed
❌ Configuration Management — Device ID accepted without constraints

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Attack Chain Analysis

1. Attacker registers domain (attacker.example.com)
2. Attacker crafts malicious JavaScript page
3. Attacker distributes URL via phishing/malicious ad/compromised site
4. Victim (logged into platform) visits attacker URL
5. JavaScript executes in victim's browser context
6. Attacker's JS calls token generation with victim's cookies
7. Server issues access token to attacker's page
8. Attacker uses token to call authenticated endpoints
9. Attacker exfiltrates cart contents, address data, session tokens

Impact: Complete user session compromise, PII exfiltration, account takeover potential

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Key Takeaways for Security Teams

1. Centralized Auth Requires Centralized Policy Enforcement

A central authentication service must enforce consistent security policies across all consuming services. Policy fragmentation introduces gaps.

2. CORS Misconfiguration Undermines Authentication

Origin reflection combined with credentialed requests can bypass otherwise robust authentication mechanisms. Never reflect arbitrary origins without validation.

3. Defense-in-Depth Reduces Impact

Strong JWT validation and CSRF protections significantly reduced the impact of several investigated attack paths. Multiple layers matter.

4. Legacy Services Increase Attack Surface

Outdated services increase organizational risk even when partially isolated. Incomplete decommissioning creates configuration drift.

5. Device IDs Are Not Authentication

Client-supplied identifiers should not be trusted for security-critical decisions without validation and binding to authenticated sessions.

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Recommendations

Immediate (Critical)

1. Implement CORS allowlist — Remove origin reflection, hard-code allowed origins, review credentials requirement
2. Implement input validation on vendor forms using schema validation library

Short-Term (30 Days)

1. Upgrade legacy Nginx to current stable release
2. Audit all microservice CORS policies for consistent enforcement
3. Add security headers (HSTS, X-Content-Type-Options, X-Frame-Options, CSP)

Long-Term (90 Days)

1. Implement OAuth 2.0/OpenID Connect — Replace device-scoped token model
2. Regular security audits with focus on microservice boundaries
3. Developer security training on CORS, input validation, JWT best practices

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Conclusion

The assessment identified one critical exploitable vulnerability (CORS misconfiguration) and multiple defense-in-depth weaknesses. The CORS finding represents a direct threat to user session security and data confidentiality.

The underlying causes reflect common microservices security patterns: permissive defaults, centralized policies applied without destination-specific requirements, and incomplete legacy service decommissioning.

Implementation of the recommended remediation steps would substantially improve the security posture of the authentication and API gateway layers.

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Assessment conducted under authorized bug bounty program. All testing performed ethically within defined scope boundaries.