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Introduction:
Unauthenticated Remote Code Execution (RCE) and business logic flaws that bypass payment controls are among the most dangerous vulnerabilities in production applications. While automated scanners miss these custom, hand‑crafted flaws, manual security auditing—as highlighted by Mattéo B. at m‑sec.tech—uncovers exposed admin interfaces, logic abuse, and zero‑day RCE paths that put your entire infrastructure at risk.
Learning Objectives:
- Understand how to manually discover unauthenticated RCE vulnerabilities using command injection and deserialization attacks.
- Learn to identify and exploit business logic flaws that bypass payment gateways and privilege controls.
- Implement mitigation strategies including hardened input validation, session management, and cloud configuration reviews.
You Should Know:
- Unauthenticated RCE – Command Injection in Web Parameters
Many production apps trust user input in debug endpoints, file uploads, or legacy APIs. Manual testers look for places where input is passed to system shells without sanitization.
Step‑by‑step guide to test for command injection (Linux/Windows):
1. Identify a suspect parameter (e.g., `?ping=127.0.0.1`, `?file=report.txt`).
2. Inject a time‑based payload to confirm execution:
- Linux: `; sleep 5`
- Windows: `& timeout 5`
3. Extract system information to prove RCE:
- Linux: `; whoami; id; cat /etc/passwd`
- Windows: `& whoami & ipconfig`
4. Use curl for blind injection (example against a vulnerable API):curl -X POST "https://target.com/api/debug" -d "cmd=ping%20-c%204%20127.0.0.1%20%3B%20whoami"
- Mitigation: Use allow‑lists for commands, never pass raw input to `exec()` or
system(), and run web apps in containerized environments with no shell access.
2. Business Logic Flaws – Bypassing Payment Controls
Logic flaws are design errors that let attackers manipulate workflows (e.g., negative quantities, currency tampering, or replaying success responses).
Step‑by‑step guide to test payment bypass:
- Intercept the checkout request using Burp Suite or OWASP ZAP.
- Modify price parameters – change `price=99.99` to `price=0.00` or
price=-10. - Tamper with quantity – set `quantity=-1` to trigger refund logic that increases store credit.
- Replay a successful payment response – capture a legitimate `payment_status=success` response and replay it for a different order ID.
- Mitigation: Never trust client‑side price/quantity fields. Re‑calculate totals on the backend, implement idempotency keys, and bind each payment session to a unique, server‑generated token.
3. Exposed Admin Interfaces Without Access Controls
Many production apps leave admin panels (e.g., /admin, /manager, /api/swagger) open to the internet with no IP restriction or authentication.
Step‑by‑step guide to discover and harden admin exposures:
- Discover hidden admin paths using directory brute‑forcing:
Linux (using gobuster) gobuster dir -u https://target.com -w /usr/share/wordlists/dirb/common.txt -x .php,.asp,.js
Windows (using Invoke-WebRequest loop) foreach ($path in @("admin","cms","dashboard")) { Invoke-WebRequest -Uri "https://target.com/$path" -Method GET } - Check for missing authentication – directly browse to discovered paths; if no login prompt appears, it’s exposed.
- Test for default credentials –
admin:admin,root:root. - Hardening steps:
- Enforce multi‑factor authentication (MFA) on all admin endpoints.
- Restrict access by source IP (e.g., only from VPN subnet).
- Use a separate, non‑guessable URL path (e.g.,
/9f7a2e-dashboard) combined with client certificates.
4. Manual Source Code Auditing for Custom Vulnerabilities
Automated scanners miss logic unique to your application. Manual code review focuses on authentication bypass, IDOR, and unsafe deserialization.
Step‑by‑step guide for a focused code audit:
1. Search for dangerous functions in the codebase:
- PHP:
eval(),system(), `unserialize()`
– Python:eval(),exec(), `pickle.loads()`
– Java:Runtime.exec(), `ObjectInputStream.readObject()`
– Node.js:eval(), `child_process.exec()`
2. Check for direct object references – find where parameters like `?user_id=123` are used without ownership validation.
- Review authentication bypass patterns – look for logic like
if (user.role == "admin" || debug_mode == true).
4. Use grep on Linux to scan quickly:
grep -rn "eval|system|unserialize" ./src/
5. Mitigation: Replace dangerous functions with safe APIs, enforce strict type checking, and implement object‑level authorization for every endpoint.
- API Security – JWT Manipulation and Parameter Pollution
Modern apps rely on REST/GraphQL APIs. Manual testers exploit weak JWT validation, mass assignment, and parameter pollution.
Step‑by‑step guide to test API security:
- Decode a JWT token (using `jwt_tool` or Python):
import jwt token = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..." decoded = jwt.decode(token, options={"verify_signature": False}) print(decoded) - Try algorithm confusion – change header `{“alg”:”none”}` and remove signature.
- Test for mass assignment – add unexpected fields like `{“email”:”[email protected]”, “is_admin”: true}` to a profile update request.
- Parameter pollution – send duplicate parameters `?id=1&id=2` to bypass input filters.
- Mitigation: Validate JWT algorithm and signature strictly; reject
alg:none. Use explicit allow‑lists for writable fields; reject duplicate parameters or process them deterministically.
6. Cloud Hardening – Misconfigured IAM and Storage
Production apps hosted on AWS/Azure often leak credentials or expose storage. Manual audits uncover these before attackers do.
Step‑by‑step guide to audit cloud misconfigurations:
- Check for public S3 buckets (Linux using AWS CLI):
aws s3 ls s3://target-bucket/ --no-sign-request
- Look for hardcoded secrets in frontend JS files or GitHub repos:
grep -r "AKIA" . AWS access keys pattern
- Test IAM privilege escalation – if you have low‑privilege keys, try:
aws iam list-attached-user-policies --user-name lowprivuser aws sts assume-role --role-arn "arn:aws:iam::123:role/admin"
- Hardening actions:
- Enforce S3 bucket policies that deny public access.
- Use secrets manager (AWS Secrets Manager, Azure Key Vault).
- Apply least‑privilege IAM roles and regularly rotate credentials.
7. Exploitation Mitigation – WAF Bypass and Monitoring
Even with a WAF, manual testers find bypasses using encoding, HTTP verb tampering, or SQLi with logic conditions.
Step‑by‑step guide to test WAF bypass and set up proper monitoring:
- Try case variation – `SeLeCt` instead of
SELECT. - Use double encoding – `%2527` instead of
%27.
3. Inject with line wrapping – `/!50000SELECT/`.
- Monitor for these attempts – set up fail2ban or ModSecurity with custom rules that log anomalies:
Linux – tail web server logs for injection attempts tail -f /var/log/apache2/access.log | grep -E "(\%27|UNION|SELECT|xp_cmdshell)"
- Mitigation: Use parameterised queries everywhere; deploy a WAF with regular rule updates and enable request anomaly scoring; implement real‑time alerting on authentication failures and shell command patterns.
What Undercode Say:
- Manual auditing finds what scanners miss – Automated tools are blind to business logic and context‑dependent flaws like payment bypasses and exposed admin panels.
- RCE and logic flaws share a root cause – Over‑trust of client‑supplied data, whether in price fields or command parameters, enables catastrophic compromise.
Manual security testing is not a luxury but a necessity for production applications. The techniques shown here—command injection, parameter tampering, JWT manipulation, cloud misconfiguration checks—are exactly what ethical hackers use to uncover zero‑day vulnerabilities. Organizations must shift from relying solely on CVE databases and SAST/DAST tools to regular, human‑led audits. Integrating these manual checks into CI/CD pipelines and training developers to think like attackers reduces risk dramatically. As Mattéo B. demonstrates at m‑sec.tech, custom, hand‑crafted vulnerabilities are the true threat to modern web, mobile, and cloud environments.
Prediction:
Within two years, regulatory frameworks (e.g., DORA, NIS2) will mandate manual penetration testing for business logic and authentication bypass in all critical production applications. AI‑powered scanners will improve but still fail against novel logic abuse, creating a sustained demand for human security auditors. Companies that ignore manual testing will face a surge in supply‑chain attacks and payment fraud, driving insurance premiums to require proof of custom auditing. Meanwhile, attack automation will shift toward semi‑manual “low‑code” exploit kits that mimic human testers, forcing defenders to adopt continuous, context‑aware monitoring.
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Reported By: Matteo B – Hackers Feeds
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