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Introduction:
In the dynamic world of bug bounty hunting, a single misconfigured API endpoint can be a treasure trove of vulnerabilities. A recent disclosure by a cybersecurity researcher highlights how a simple fuzzing technique, followed by an elementary bypass, led to a critical information disclosure flaw, netting a $200 reward and exposing sensitive operational data.
Learning Objectives:
- Understand the methodology for effective API endpoint fuzzing using modern tools.
- Learn how to identify and exploit weak authorization mechanisms on API routes.
- Master the technique of using character encoding to bypass simplistic security filters.
You Should Know:
1. Choosing Your Fuzzing Arsenal
The initial discovery phase in this case relied on a robust fuzzing tool and a comprehensive wordlist. The researcher specifically mentioned using ffuf, a fast web fuzzer written in Go, and wordlists from the `SecLists` project.
Verified Command: Basic FFUF Fuzzing Command
ffuf -w /usr/share/seclists/Discovery/Web-Content/api/common.txt -u https://target.com/api/v1/FUZZ -mc 200,403 -H "Authorization: Bearer YOUR_TOKEN"
Step-by-step guide:
-w: Specifies the path to the wordlist. Here, we use a common API endpoints list from SecLists.-u: The target URL, with `FUZZ` acting as the placeholder where words from the list are inserted.-mc: Tells `ffuf` to only display responses with these HTTP status codes (200 for success, 403 for forbidden).-H: Used to add headers, like an API key or Bearer token, if required for initial access. This helps find endpoints that are supposed to be protected but might not be.
2. Interpreting Fuzzing Results: The 403 Goldmine
A `403 Forbidden` response, while indicating access denial, is a significant finding. It confirms the endpoint exists and is protected, making it a prime target for bypass attempts. Unlike a 404 Not Found, a `403 signals a potential logic flaw in the protection mechanism.
Verified Command: Grep for Interesting Status Codes
ffuf -w common.txt -u https://target.com/api/v1/FUZZ -mc all -s | grep -E "(403|401|500)"
Step-by-step guide:
- This command runs a broader fuzzing scan (
-mc allcaptures all status codes) and then pipes (|) the output togrep.
– `grep -E` uses extended regular expressions to filter for lines containing403, `401` (Unauthorized), or `500` (Internal Server Error). These are the most promising status codes for further investigation.
- The Bypass: Character Encoding as a WAF Killer
The core of this vulnerability was a bypass using URL encoding. The protection logic was likely checking for the literal string"metrics", but not its encoded equivalent.
Verified Command: Manual Bypass with cURL
curl -H "Authorization: Bearer token" https://target.com/api/v1/metric%73
Step-by-step guide:
- This uses `cURL` to send a GET request to the encoded endpoint.
– `%73` is the URL-encoded representation of the lowercase letter's'. - The server’s security filter (e.g., a Web Application Firewall or simple path validation) sees
/metric%73, while the application layer often decodes this back to `/metrics` after the security check has passed, granting access.
4. Automating Encoding Bypasses
Manually testing every character is inefficient. A more advanced approach is to use a tool that can automatically generate and test encoded payloads.
Verified Command: Ffuf with Multiple Encoding
ffuf -w /usr/share/seclists/Discovery/Web-Content/api/common.txt -u https://target.com/api/v1/FUZZ -e .json,.asp,.php,%20,%2e,%73 -mc 200,302
Step-by-step guide:
- The `-e` (extensions) flag is a powerful feature. Here, we provide a list of extensions and encodings to append to every word in the wordlist.
– `%20` (space), `%2e` (dot), and `%73` (s) are automatically tried. So, for the word “metrics”, `ffuf` will also test/metrics%73,/metrics.%73, etc.
5. Expanding the Attack Surface: Fuzzing Parameters
Once a critical endpoint like `/metrics` is found, the next step is to fuzz for parameters, as they might be unprotected or require different bypasses.
Verified Command: Fuzzing GET Parameters
ffuf -w /usr/share/seclists/Discovery/Web-Content/burp-parameter-names.txt -u "https://target.com/api/v1/metrics?FUZZ=test" -mc 200 -fs 0
Step-by-step guide:
- This command fuzzes the parameter names for the discovered `/metrics` endpoint.
– `?FUZZ=test` replaces the parameter name.
– `-fs 0` (filter by size) is used here to ignore responses of 0 bytes, which are common for invalid parameters, helping to clean up the output.
6. Deep-Dive Analysis with Burp Suite
While CLI tools are fast, Burp Suite offers deep analysis. After finding the endpoint with ffuf, send it to Burp Repeater for manual testing.
Verified Tutorial: Bypass in Burp Repeater
- Intercept a request to a known endpoint with Burp Proxy.
2. Send the request to Repeater.
- Change the path to the protected endpoint (e.g.,
/api/v1/metrics). Observe the `403` response. - In the Repeater tab, right-click on the request and select “Convert Selection.” Choose “URL” > “URL-encode key characters”. This will automatically encode the path.
- Send the request again. If it returns a
200, you’ve confirmed the bypass.
7. Mitigation: Hardening API Endpoints
From a defensive perspective, this vulnerability stems from inconsistent parsing layers. Mitigation requires a security-first design.
Verified Code Snippet: Unified Path Normalization (Python/Flask Example)
from flask import request, abort import urllib.parse @app.before_request def normalize_and_authorize(): Normalize the path before security checks normalized_path = urllib.parse.unquote(request.path).lower() Define protected endpoints protected_endpoints = ['/api/v1/metrics', '/api/v1/admin'] Check authorization against the normalized path if normalized_path in protected_endpoints: if not is_authorized(request): abort(403) Forbidden def is_authorized(request): Logic to validate API key, JWT, etc. return True or False
Step-by-step guide:
- This Flask middleware (
@app.before_request) runs before every request.
– `urllib.parse.unquote(request.path)` decodes any URL encoding first. - The security logic then checks this normalized, decoded path against a list of protected endpoints.
- This ensures that
/metrics,metric%73, and `metric%53` (capital S) are all treated identically and blocked before reaching the application logic.
What Undercode Say:
- Simplicity is Key: The most effective attacks are often not complex zero-days but simple bypasses of flawed logic. The $200 reward was earned not by advanced memory corruption exploits, but by understanding how different application layers parse input.
- Automation is Non-Negotiable: The initial find was powered by automated fuzzing. Manual testing alone is insufficient for comprehensive coverage in modern web applications with vast API surfaces.
This case is a classic example of a “weakness in security control” rather than a direct flaw in business logic. The endpoint was correctly identified as sensitive and a control (a path filter) was put in place. However, the control was defeated because its logic did not mirror the application’s own path parsing logic. This creates a tiny gap between the security layer and the application layer, a gap that bug bounty hunters and attackers are exceptionally skilled at exploiting. Defenders must ensure that all input, including paths, headers, and parameters, is normalized and canonicalized before any security decision is made.
Prediction:
This specific bypass technique, while simple, points to a broader systemic issue: the proliferation of API endpoints and the inconsistent implementation of security controls across microservices and serverless architectures. As organizations continue to decompose monoliths into APIs, we predict a significant rise in such “parsing layer” vulnerabilities. Security filters (WAFs, API gateways) that operate on raw, un-normalized requests will increasingly fail, leading to mass data leakage events. The future of API security will hinge on integrated, code-level security that normalizes and validates all input within the application logic itself, rendering these layer-bypass attacks obsolete.
🎯Let’s Practice For Free:
IT/Security Reporter URL:
Reported By: Francisco Hernando – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
