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Introduction:
Public API keys are commonly embedded in client-side applications (web, mobile) to enable analytics, maps, or other third-party services without requiring user credentials. Developers assume these keys are safe because they are meant to be public – but this assumption collapses when the backend endpoint fails to enforce proper authorization. As demonstrated in a real-world bug bounty discovery, a SaaS provider’s public API key could be reused to fetch privileged tenant data, exposing admin accounts, user details, and corporate information across all customers.
Learning Objectives:
- Understand how misconfigured API endpoints can elevate a “public” key into a privilege escalation vector.
- Learn to test for authorization bypasses using public API keys with tools like Burp Suite, Postman, and cURL.
- Implement mitigations such as strict origin validation, key scoping, and least-privilege access controls for API keys.
You Should Know:
- How a Public API Key Turns Into a Data Leak – Step‑by‑Step Exploitation
The vulnerability stems from the API backend trusting the public key as sufficient proof of identity without checking what actions that key is allowed to perform. Here is how an attacker would discover and exploit this flaw:
Step 1: Identify a public API key – Look in JavaScript files, mobile app decompiled code, or network traffic. For example, using browser DevTools → Network tab, filter for `api-key` or public_key.
Step 2: Read the official API documentation – As the original finder did, search for endpoints that accept the same key. Many providers document “public key” usage across multiple endpoints, some of which may be privileged.
Step 3: Test with cURL – Send a request to a supposedly privileged endpoint (e.g., /admin/users, /tenant/all) using only the public key.
Linux / macOS / Windows (WSL or Git Bash) curl -X GET "https://api.target.com/v1/tenant/admin/users" \ -H "X-API-Key: YOUR_PUBLIC_KEY" \ -H "Content-Type: application/json"
Step 4: Observe response – If the API returns sensitive data (user lists, emails, internal IDs), you have found an authorization bypass.
Windows PowerShell alternative:
$headers = @{ "X-API-Key" = "YOUR_PUBLIC_KEY" }
Invoke-RestMethod -Uri "https://api.target.com/v1/tenant/admin/users" -Headers $headers
Mitigation: Always validate that the public key is associated with the correct tenant and has only read‑only, non‑sensitive scopes. Implement a policy that public keys cannot access any endpoint that returns PII or admin data.
- Testing API Authorization with Burp Suite & Autorize
Automated testing helps scale this discovery. The Burp Suite extension Autorize can detect endpoints where a public key (or low‑privilege token) gains unauthorized access.
Step‑by‑step guide:
- Capture a legitimate request with a full-privilege API key using Burp Proxy.
- Install Autorize (BApp Store). Configure it with two sessions: one using the public key, another using an invalid key.
- Send the original request through Autorize – it will replay it with the public key and compare responses.
- Look for “Bypass!” status – indicates the endpoint granted access with the weaker key.
- For each bypassed endpoint, manually verify using cURL to confirm the data leak.
Automation with Python:
import requests
public_key = "PUBLIC_KEY_HERE"
endpoints = ["/api/users", "/api/admin/stats", "/api/tenant/all"]
for endpoint in endpoints:
url = f"https://target.com{endpoint}"
resp = requests.get(url, headers={"X-API-Key": public_key})
if resp.status_code == 200 and ("admin" in resp.text or "email" in resp.text):
print(f"[!] Potential leak: {endpoint}")
- Linux & Windows Commands for API Key Discovery in Source Code
Linux (grep + find):
Search JavaScript files for common key patterns
find . -name ".js" -exec grep -E "(api[<em>-]?key|public[</em>-]?key|access[_-]?token)" {} \;
Extract keys from mobile APK (after unzipping)
unzip app.apk -d apk_out/
grep -r -E "[a-zA-Z0-9]{32,}" apk_out/ --include=".smali" --include=".xml"
Windows (PowerShell):
Get-ChildItem -Recurse -Include .js, .html | Select-String -Pattern "api[<em>-]?key|public[</em>-]?key" | Format-Table -AutoSize
- Hardening API Key Architecture – Scoping & Origin Validation
To prevent public keys from being abused, implement these controls in your cloud environment (AWS, Azure, GCP).
Step 1: Restrict API key to specific endpoints – Use API gateway policies. Example AWS API Gateway with a usage plan that only allows paths like `/analytics/` but denies /admin/.
Step 2: Enforce origin/referer header checking – Reject requests that do not come from your allowed domains (though not foolproof, it adds a layer).
Step 3: Use short‑lived tokens – For client‑side, replace static public keys with temporary tokens obtained via a secure handshake.
Step 4: Implement tenant isolation – Every API key must have an explicit `tenant_id` claim. The backend should always compare the requested tenant ID with the key’s bound tenant.
Example middleware in Node.js/Express:
function authorizePublicKey(req, res, next) {
const apiKey = req.headers['x-api-key'];
const tenant = req.params.tenantId;
// Check if key is allowed to access this tenant and this endpoint
if (!isValidPublicKey(apiKey, tenant, req.path)) {
return res.status(403).json({ error: "Public key cannot access this resource" });
}
next();
}
- API Security Testing Checklist – What the Bug Bounty Hunter Looked For
Based on the original post, here is a repeatable checklist for pentesters:
- [ ] Public key presence – Does the target use client‑side public keys (e.g., for analytics, crash reporting, maps)?
- [ ] Documentation review – Read all endpoints that accept the same key; look for admin, tenant, or user listing endpoints.
- [ ] Privilege escalation test – Send the public key to every documented endpoint, noting status code changes (401 vs 200).
- [ ] Multi‑tenant enumeration – Try changing tenant identifiers in the URL (e.g., `?tenant_id=123` to
124). If the public key returns another tenant’s data, it’s a critical horizontal privilege escalation. - [ ] Responsible disclosure – As the original reporter did, notify the vendor with proof of concept and timeline.
- Real‑World Remediation: Logging & Monitoring for API Key Abuse
After the fix, implement detection rules to spot abuse of public keys.
Example Splunk / ELK query for anomalous public key usage:
api_key_type:"public" AND endpoint_path:"/admin/" AND response_code:200
Linux monitoring with auditd (for on‑prem APIs):
auditctl -w /var/log/api/access.log -p wa -k api_abuse
Windows Event Forwarding rule (PowerShell):
Monitor IIS logs for public keys accessing restricted paths Get-Content "C:\inetpub\logs\LogFiles\W3SVC1\u_ex.log" | Select-String "X-API-Key: public_" -Context 2,2
What Undecode Say:
- Public does not mean harmless – A key’s safety depends entirely on backend authorization, not on its name or intended use.
- Documentation is a hacker’s best friend – The original finder read official docs to find the privileged endpoint, not advanced tools.
- Tenant isolation must be enforced at the API layer – Without explicit tenant checks, one public key becomes a skeleton key for all customers.
- Bug bounty programs are effective – This issue was discovered in a HackerOne program and fixed quickly, demonstrating the value of responsible disclosure.
Analysis: This case highlights a classic API security anti‑pattern – trusting client‑side input (the public key) for server‑side authorization decisions. Many SaaS providers mistakenly assume that “public” keys are only used for harmless analytics, but developers often reuse the same key across internal and external endpoints. The root cause is a lack of fine‑grained access control: the API gateway or backend failed to map the key to a specific role or scope. As APIs become the backbone of modern applications, expect more such findings unless organizations adopt OAuth2’s token scoping (RFC 6749) or API key bound‑to‑principal patterns.
Prediction:
In the next 12–18 months, as API‑first architectures and serverless functions proliferate, the number of “public key privilege escalation” disclosures will rise by at least 40%. Attackers will automate scanning for public keys in JavaScript bundles and mobile apps, then feed them into fuzzers that test every documented and undocumented endpoint. Regulators (like PCI DSS v4.0 and the upcoming EU Cyber Resilience Act) will start requiring explicit tenant isolation for multi‑tenant APIs. Companies that fail to implement scope‑limited API keys will face not only data breaches but also compliance fines. The industry will shift toward short‑lived, bound tokens (e.g., using JWT with `aud` and `scope` claims) even for “public” client use cases, rendering static public keys obsolete for any privileged data access.
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IT/Security Reporter URL:
Reported By: Hossam Eldin – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
