Privilege Escalation via API Endpoint Trailing Slash Vulnerability

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In a recent bug bounty discovery, a security researcher earned $3,000 for identifying a critical privilege escalation vulnerability in an API endpoint. The issue stemmed from inconsistent access control between two similar endpoints:

– `api/v4/endpoint/old` → 403 Forbidden
– `api/v4/endpoint/new` → 200 OK

In another variation:

– `api/v4/endpoint/new/` → 403 Forbidden
– `api/v4/endpoint/new` → 200 OK

The presence (or absence) of a trailing slash (/) determined whether access was granted, allowing unauthorized privilege escalation.

You Should Know:

1. Testing for Trailing Slash Vulnerabilities

Use `curl` or `Burp Suite` to test endpoints with and without trailing slashes: 

curl -X GET "https://target.com/api/v4/endpoint/new" -H "Authorization: Bearer token" 
curl -X GET "https://target.com/api/v4/endpoint/new/" -H "Authorization: Bearer token"

2. Automated Scanning with ffuf

Fuzz API endpoints for access control flaws:

ffuf -w wordlist.txt -u "https://target.com/api/v4/FUZZ" -H "Authorization: Bearer token" -mc 200

3. Detecting Misconfigured Servers

Check for Nginx/Apache misconfigurations that may cause inconsistent behavior: 

nikto -h https://target.com -id token

4. Windows Equivalent (PowerShell)

Test API endpoints using PowerShell:

Invoke-WebRequest -Uri "https://target.com/api/v4/endpoint/new" -Headers @{"Authorization"="Bearer token"}

5. Mitigation Steps for Developers

  • Normalize URLs before processing.
  • Implement strict role-based access control (RBAC).
  • Use API gateways to enforce consistency.

What Undercode Say:

This vulnerability highlights the importance of input sanitization and consistent access control in API development. Attackers often exploit subtle differences in endpoint handling, such as trailing slashes, case sensitivity, or parameter parsing.

Relevant Commands for Further Testing:

 Check for open directories 
dirb https://target.com/api/v4/ /usr/share/wordlists/dirb/common.txt

Test HTTP methods 
nmap --script http-methods -p 443 target.com

Monitor API traffic 
tcpdump -i eth0 'port 443' -w api_traffic.pcap

Expected Output:

  • 403 Forbidden vs. 200 OK discrepancies.
  • Unauthorized access to high-privilege endpoints.

Prediction:

As APIs grow in complexity, similar logic flaws (path traversal, inconsistent sanitization) will remain a top bug bounty target. Automation tools like `Burp Suite` and `ffuf` will increasingly detect these issues, forcing developers to adopt stricter validation mechanisms.

Relevant URLs:

Expected Output:

A detailed analysis of API vulnerabilities, practical exploitation commands, and mitigation strategies for security researchers and developers.

IT/Security Reporter URL:

Reported By: Mohammedalqi Bugbounty – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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