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name: sast-rce description: >- Detect Remote Code Execution (RCE) vulnerabilities in a codebase using a three-phase approach: recon (find dangerous execution sinks), batched verify (trace user input to sinks in parallel subagents, 3 sinks each), and merge (consolidate batch results). Covers OS command injection, eval-like sinks, and unsafe deserialization. Requires sast/architecture.md (run sast-analysis first). Outputs findings to sast/rce-results.md. Use when asked to find RCE, command injection, or unsafe deserialization bugs.

Remote Code Execution (RCE) Detection

You are performing a focused security assessment to find Remote Code Execution vulnerabilities in a codebase. This skill uses a three-phase approach with subagents: recon (find dangerous execution sinks), batched verify (trace whether user-supplied input reaches each sink in parallel batches of 3), and merge (consolidate batch results into the final report).

Prerequisites: sast/architecture.md must exist. Run the analysis skill first if it doesn't.

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What is Remote Code Execution

Remote Code Execution (RCE) occurs when an attacker can cause the application to execute arbitrary OS commands or application-level code that they control. This is typically the highest-severity vulnerability class, often resulting in complete server compromise.

RCE arises from three primary root causes:

1. OS Command Injection: User input is embedded unsafely into an OS command string, allowing shell metacharacters to inject additional commands.
2. Code Injection (eval-like): User input is passed to functions that interpret it as executable code (eval, exec, Function(), etc.).
3. Unsafe Deserialization: User-supplied serialized data is deserialized using a gadget-prone deserializer, triggering arbitrary code execution via crafted payloads.

What RCE IS

• Passing user input directly or indirectly into OS command execution functions with shell interpretation enabled
• Using eval(), exec(), Function(), or equivalent constructs with user-controlled strings
• Deserializing user-supplied bytes/strings with inherently unsafe deserializers (pickle, PHP unserialize, Java native serialization, Ruby Marshal, etc.)
• Using yaml.load() without a safe loader on user-supplied content
• Dynamic require()/import() with user-controlled module paths
• PHP file inclusion (include/require) with user-controlled paths

What RCE is NOT

Do not flag these as RCE:

• SSRF: Making HTTP requests to attacker-controlled URLs — different vulnerability class (no code execution)
• Path Traversal: Reading/writing arbitrary files — separate class (unless the read file is then executed/deserialized)
• SSTI: Template injection via template engines — a separate though related class; flag as SSTI, not RCE
• XSS: JavaScript execution in a victim's browser — client-side only, not server-side RCE
• SQL Injection: Injecting into database queries — different class (even if xp_cmdshell can lead to OS commands, flag it as SQLi)
• Safe subprocess list-form calls: subprocess.run(["ls", user_arg]) with a list and no shell=True — arguments are passed directly to the OS without shell expansion; not vulnerable to command injection
• Safe deserialization: json.loads(), yaml.safe_load(), xml.etree.ElementTree.parse() — these formats have no code execution semantics

Patterns That Prevent RCE

When you see these patterns, the code is likely not vulnerable:

1. Subprocess list form without shell interpretation

# Python — list args, no shell=True
subprocess.run(["convert", "-resize", size, input_file, output_file])
subprocess.Popen(["git", "clone", repo_url])

# Node.js — spawn with separate args (no shell)
child_process.spawn("ffmpeg", ["-i", inputFile, outputFile])

# Java — ProcessBuilder with list
new ProcessBuilder("ls", "-la", dir).start()

# Ruby — system() with multiple args (not a single interpolated string)
system("ffmpeg", "-i", "input.mp4", "-f", format, "output")

2. Safe deserialization formats

# Python — JSON instead of pickle
import json
data = json.loads(user_input)  # no code execution semantics

# Python — safe YAML loader
import yaml
data = yaml.safe_load(user_input)  # restricts to basic types only

# Java — Jackson without enableDefaultTyping, with concrete target type
ObjectMapper mapper = new ObjectMapper();
MyClass obj = mapper.readValue(json, MyClass.class);  # safe

3. Strict allowlist before command construction

# Python — allowlist for dynamic arguments
ALLOWED_FORMATS = {"png", "jpg", "webp"}
if fmt not in ALLOWED_FORMATS:
    return abort(400)
subprocess.run(["convert", infile, f"output.{fmt}"])

# Node.js — allowlist for dynamic args
const ALLOWED_COMMANDS = ['ls', 'pwd'];
if (!ALLOWED_COMMANDS.includes(cmd)) return res.status(400).end();
spawn(cmd, []);

---

Vulnerable vs. Secure Examples

OS Command Injection — Python

# VULNERABLE: shell=True with f-string
@app.route('/ping')
def ping():
    host = request.args.get('host')
    result = subprocess.run(f"ping -c 1 {host}", shell=True, capture_output=True, text=True)
    return result.stdout
# Payload: ?host=127.0.0.1;id  → executes "id"

# VULNERABLE: os.system with string formatting
def convert_image(filename):
    size = request.form.get('size')
    os.system(f"convert {filename} -resize {size} output.jpg")

# SECURE: list-form subprocess, no shell
@app.route('/ping')
def ping():
    host = request.args.get('host')
    result = subprocess.run(["ping", "-c", "1", host], capture_output=True, text=True, timeout=5)
    return result.stdout

OS Command Injection — Node.js

// VULNERABLE: exec with template literal
app.get('/search', (req, res) => {
  const query = req.query.q;
  exec(`grep -r "${query}" /var/log/app/`, (err, stdout) => {
    res.send(stdout);
  });
});
// Payload: ?q=foo" /etc/passwd "

// VULNERABLE: execSync with concatenation
function runScript(userScript) {
  return execSync('node scripts/' + userScript);
}

// SECURE: spawn with separate args
app.get('/search', (req, res) => {
  const query = req.query.q;
  const proc = spawn('grep', ['-r', query, '/var/log/app/']);
  proc.stdout.on('data', (data) => res.write(data));
  proc.on('close', () => res.end());
});

OS Command Injection — PHP

// VULNERABLE: shell_exec with user input
function generateThumbnail($file) {
    $size = $_GET['size'];
    shell_exec("convert {$file} -resize {$size} thumb.jpg");
}

// VULNERABLE: backtick operator
function checkHost() {
    $host = $_POST['host'];
    $result = `ping -c 1 $host`;
    return $result;
}

// SECURE: escapeshellarg (reduces risk — but prefer removing shell entirely)
function generateThumbnail($file) {
    $size = escapeshellarg($_GET['size']);
    $file = escapeshellarg($file);
    shell_exec("convert $file -resize $size thumb.jpg");
}

OS Command Injection — Ruby

# VULNERABLE: string interpolation in system()
get '/convert' do
  format = params[:format]
  system("ffmpeg -i input.mp4 -f #{format} output")
end

# VULNERABLE: backtick with user input
def check_dns
  `nslookup #{params[:host]}`
end

# SECURE: system() with separate args (no shell expansion)
get '/convert' do
  format = params[:format]
  ALLOWED = %w[mp4 avi mkv]
  return 400 unless ALLOWED.include?(format)
  system("ffmpeg", "-i", "input.mp4", "-f", format, "output")
end

Code Injection — Python eval/exec

# VULNERABLE: eval with user input
@app.route('/calculate')
def calculate():
    expr = request.args.get('expr')
    result = eval(expr)  # attacker can run __import__('os').system('id')
    return str(result)

# VULNERABLE: exec with user code
@app.route('/run')
def run_code():
    code = request.json.get('code')
    exec(code)  # full arbitrary code execution
    return "ok"

# SECURE: ast.literal_eval for safe expression parsing (literals only)
from ast import literal_eval
@app.route('/parse')
def parse():
    data = request.args.get('data')
    result = literal_eval(data)  # only parses strings/numbers/lists/dicts/bools
    return str(result)

Code Injection — JavaScript eval / Function

// VULNERABLE: eval with user input
app.post('/formula', (req, res) => {
  const formula = req.body.formula;
  const result = eval(formula);  // RCE: process.exit(), require('child_process')...
  res.json({ result });
});

// VULNERABLE: new Function() constructor
function compute(userExpression) {
  const fn = new Function('x', `return ${userExpression}`);
  return fn(42);
}

// VULNERABLE: vm.runInNewContext (sandbox escape via __proto__ pollution)
const vm = require('vm');
app.post('/eval', (req, res) => {
  const result = vm.runInNewContext(req.body.code);
  res.json({ result });
});

// SECURE: use a math expression library (no arbitrary code)
const { evaluate } = require('mathjs');
app.post('/formula', (req, res) => {
  const result = evaluate(req.body.formula);  // sandboxed math expressions only
  res.json({ result });
});

Unsafe Deserialization — Python pickle

# VULNERABLE: deserializing user-supplied pickle data
@app.route('/load', methods=['POST'])
def load_session():
    data = request.get_data()
    session = pickle.loads(data)  # attacker controls __reduce__ → RCE
    return jsonify(session)

# VULNERABLE: base64-encoded pickle from cookie
@app.route('/profile')
def profile():
    session_cookie = request.cookies.get('session')
    data = base64.b64decode(session_cookie)
    user = pickle.loads(data)  # crafted cookie → arbitrary code at deserialization
    return render_template('profile.html', user=user)

# SECURE: use JSON (no code execution semantics)
@app.route('/profile')
def profile():
    session_cookie = request.cookies.get('session')
    user = json.loads(base64.b64decode(session_cookie))
    return render_template('profile.html', user=user)

Unsafe Deserialization — Java

// VULNERABLE: ObjectInputStream.readObject() on user-supplied stream
@PostMapping("/deserialize")
public ResponseEntity<?> deserialize(@RequestBody byte[] data) throws Exception {
    ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(data));
    Object obj = ois.readObject();  // gadget chains (Commons Collections, Spring, etc.) → RCE
    return ResponseEntity.ok(obj);
}

// VULNERABLE: Jackson with enableDefaultTyping
ObjectMapper mapper = new ObjectMapper();
mapper.enableDefaultTyping();  // attacker specifies arbitrary class type in JSON → RCE
MyData data = mapper.readValue(userJson, MyData.class);

// SECURE: Jackson with concrete type, no enableDefaultTyping
ObjectMapper mapper = new ObjectMapper();
MyData data = mapper.readValue(userJson, MyData.class);  // safe with concrete target type

Unsafe Deserialization — PHP

// VULNERABLE: unserialize() with user input
function loadProfile() {
    $data = base64_decode($_COOKIE['profile']);
    $user = unserialize($data);  // PHP object injection → POP chain → RCE
    return $user;
}

// VULNERABLE: unserialize from POST body
$obj = unserialize($_POST['data']);

// SECURE: json_decode instead
function loadProfile() {
    $data = base64_decode($_COOKIE['profile']);
    $user = json_decode($data, true);  // no code execution semantics
    return $user;
}

Unsafe Deserialization — Ruby Marshal

# VULNERABLE: Marshal.load with user-supplied data
post '/restore' do
  data = Base64.decode64(params[:state])
  object = Marshal.load(data)  # arbitrary Ruby object graph → RCE via gadgets
  object.process
end

# SECURE: use JSON
post '/restore' do
  data = JSON.parse(Base64.decode64(params[:state]))
  # work with plain data structures only
end

Unsafe Deserialization — Node.js

// VULNERABLE: node-serialize (known RCE via IIFE in serialized string)
const serialize = require('node-serialize');
app.post('/r