ssh2fa/approval_server.py at main · douglangley/ssh2fa · GitHub

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#!/usr/bin/env python3
"""
PAM SSH 2FA - Approval Server
=============================

A lightweight HTTP server that handles link-based authentication approvals.
Users click an approval link in their push notification instead of typing a code.

OVERVIEW
--------
When a user authenticates via SSH with auth_method=link or auth_method=both:
1. PAM module creates an approval request file with a unique token
2. PAM sends notification containing approval link
3. User clicks link on their phone
4. This server marks the approval as granted
5. PAM module detects approval and grants SSH access

INSTALLATION
------------
This server runs as a systemd service. Install with:
    sudo ./install.sh

Or manually:
    sudo cp approval_server.py /etc/pam-ssh-2fa/
    sudo cp pam-ssh-2fa-server.service /etc/systemd/system/
    sudo systemctl daemon-reload
    sudo systemctl enable pam-ssh-2fa-server
    sudo systemctl start pam-ssh-2fa-server

CONFIGURATION
-------------
Server settings are in /etc/pam-ssh-2fa/config.ini under [server]:
    port = 9110
    url = http://your-server.example.com:9110

The 'url' is what appears in notification links. It must be reachable
from the user's phone (public IP, hostname, or Tailscale address).

SECURITY CONSIDERATIONS
-----------------------
- Tokens are cryptographically random and single-use
- Approvals expire after the configured timeout (default 5 minutes)
- Server binds to 0.0.0.0 by default (all interfaces)
- Consider firewall rules to limit access if needed
- For production, consider adding HTTPS via reverse proxy

ENDPOINTS
---------
GET /approve/<token>  - Approve an authentication request
GET /health           - Health check endpoint

AUTHOR
------
Generated by Claude (Anthropic) for SSH 2FA push notification authentication.

LICENSE
-------
MIT License - Use and modify freely.
"""

# =============================================================================
# IMPORTS
# =============================================================================

import os
import sys
import json
import time
import logging
import argparse
import configparser
from pathlib import Path
from datetime import datetime, timezone
from typing import Optional, Tuple

# Use built-in http.server for minimal dependencies
# No Flask required - keeps it simple and dependency-free
from http.server import HTTPServer, BaseHTTPRequestHandler
from urllib.parse import urlparse, parse_qs
import threading

# =============================================================================
# CONFIGURATION
# =============================================================================

# Default paths
DEFAULT_CONFIG_FILE = "/etc/pam-ssh-2fa/config.ini"
DEFAULT_APPROVALS_DIR = "/var/run/pam-ssh-2fa/approvals"
DEFAULT_PORT = 9110
DEFAULT_BIND_ADDRESS = "0.0.0.0"

# HTML templates (minimal, accessible)
HTML_SUCCESS = """<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>SSH Access Approved</title>
    <style>
        body {{
            font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
            max-width: 600px;
            margin: 50px auto;
            padding: 20px;
            text-align: center;
        }}
        .success {{
            color: #22863a;
            font-size: 24px;
            margin-bottom: 20px;
        }}
        .details {{
            color: #666;
            margin-top: 20px;
        }}
    </style>
</head>
<body>
    <div class="success">[OK] Access Approved</div>
    <p>SSH authentication for <strong>{user}</strong> has been approved.</p>
    <p>You can close this page and return to your terminal.</p>
    <div class="details">
        <p>Host: {host}</p>
        <p>From: {rhost}</p>
        <p>Time: {time}</p>
    </div>
</body>
</html>
"""

HTML_ERROR = """<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>SSH Approval Error</title>
    <style>
        body {{
            font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
            max-width: 600px;
            margin: 50px auto;
            padding: 20px;
            text-align: center;
        }}
        .error {{
            color: #cb2431;
            font-size: 24px;
            margin-bottom: 20px;
        }}
    </style>
</head>
<body>
    <div class="error">[ERROR] {title}</div>
    <p>{message}</p>
</body>
</html>
"""

HTML_HEALTH = """{"status": "ok", "timestamp": "{timestamp}"}"""


# =============================================================================
# LOGGING SETUP
# =============================================================================


def setup_logging(debug: bool = False, log_file: Optional[str] = None):
    """
    Configure logging for the approval server.

    Args:
        debug: Enable debug level logging
        log_file: Optional path to log file (also logs to stdout)
    """
    level = logging.DEBUG if debug else logging.INFO
    format_str = "%(asctime)s [%(levelname)s] %(message)s"

    handlers = [logging.StreamHandler(sys.stdout)]

    if log_file:
        try:
            log_dir = os.path.dirname(log_file)
            if log_dir and not os.path.exists(log_dir):
                os.makedirs(log_dir, mode=0o750)
            handlers.append(logging.FileHandler(log_file))
        except OSError as e:
            print(f"Warning: Could not create log file: {e}")

    logging.basicConfig(level=level, format=format_str, handlers=handlers)


# =============================================================================
# CONFIGURATION LOADER
# =============================================================================


class ServerConfig:
    """
    Configuration manager for the approval server.

    Loads settings from the main PAM 2FA config file.

    Attributes:
        port (int): Port to listen on
        bind_address (str): Address to bind to (default 0.0.0.0)
        approvals_dir (str): Directory for approval request files
        debug (bool): Enable debug logging
        log_file (str): Path to log file
    """

    def __init__(self, config_file: str = DEFAULT_CONFIG_FILE):
        """
        Initialize server configuration.

        Args:
            config_file: Path to the main config.ini file
        """
        self.port = DEFAULT_PORT
        self.bind_address = DEFAULT_BIND_ADDRESS
        self.approvals_dir = DEFAULT_APPROVALS_DIR
        self.debug = False
        self.log_file = "/var/log/pam-ssh-2fa-server.log"

        self._load_config(config_file)

    def _load_config(self, config_file: str):
        """Load configuration from INI file."""
        if not os.path.exists(config_file):
            return

        parser = configparser.ConfigParser()
        try:
            parser.read(config_file)
        except configparser.Error:
            return

        # Server settings
        if parser.has_option("server", "port"):
            try:
                self.port = int(parser.get("server", "port"))
            except ValueError:
                pass

        if parser.has_option("server", "bind_address"):
            self.bind_address = parser.get("server", "bind_address")

        # General settings
        if parser.has_option("general", "debug"):
            self.debug = parser.get("general", "debug").lower() in ("true", "yes", "1")

        if parser.has_option("server", "log_file"):
            self.log_file = parser.get("server", "log_file")

        # Approvals directory (derived from code storage dir)
        if parser.has_option("codes", "storage_dir"):
            storage_dir = parser.get("codes", "storage_dir")
            self.approvals_dir = os.path.join(storage_dir, "approvals")


# =============================================================================
# APPROVAL MANAGER
# =============================================================================


class ApprovalManager:
    """
    Manages approval request files.

    Approval files are JSON with structure:
    {
        "token": "abc123...",
        "user": "username",
        "rhost": "192.168.1.1",
        "host": "servername",
        "created": 1234567890.123,
        "expires": 1234567890.123,
        "approved": false,
        "approved_at": null
    }

    Attributes:
        approvals_dir (str): Directory containing approval files
    """

    def __init__(self, approvals_dir: str):
        """
        Initialize the approval manager.

        Args:
            approvals_dir: Directory for approval request files
        """
        self.approvals_dir = approvals_dir
        self._ensure_dir()

    def _ensure_dir(self):
        """Create approvals directory if it doesn't exist."""
        if not os.path.exists(self.approvals_dir):
            try:
                os.makedirs(self.approvals_dir, mode=0o700)
            except OSError:
                pass

    def get_approval_file(self, token: str) -> str:
        """
        Get the path to an approval file by token.

        Args:
            token: The approval token

        Returns:
            Full path to the approval file
        """
        # Sanitize token to prevent directory traversal
        safe_token = "".join(c for c in token if c.isalnum())
        return os.path.join(self.approvals_dir, f"{safe_token}.json")

    def get_approval(self, token: str) -> Optional[dict]:
        """
        Load an approval request by token.

        Args:
            token: The approval token

        Returns:
            Approval data dict, or None if not found/invalid
        """
        filepath = self.get_approval_file(token)

        if not os.path.exists(filepath):
            return None

        try:
            with open(filepath, "r") as f:
                return json.load(f)
        except (json.JSONDecodeError, IOError):
            return None

    def mark_approved(self, token: str) -> Tuple[bool, Optional[dict]]:
        """
        Mark an approval request as approved.

        Args:
            token: The approval token

        Returns:
            Tuple of (success, approval_data)
            - success: True if approval was successful
            - approval_data: The approval data dict (for display)
        """
        approval = self.get_approval(token)

        if approval is None:
            logging.warning(f"Approval not found: {token[:8]}...")
            return False, None

        # Check if already approved
        if approval.get("approved"):
            logging.info(f"Approval already granted: {token[:8]}...")
            return True, approval

        # Check if expired
        if time.time() > approval.get("expires", 0):
            logging.warning(f"Approval expired: {token[:8]}...")
            return False, None

        # Mark as approved
        approval["approved"] = True
        approval["approved_at"] = time.time()

        # Write back
        filepath = self.get_approval_file(token)
        try:
            with open(filepath, "w") as f:
                json.dump(approval, f)

            logging.info(
                f"Approval granted: user={approval.get('user')}, "
                f"rhost={approval.get('rhost')}, token={token[:8]}..."
            )
            return True, approval

        except IOError as e:
            logging.error(f"Failed to write approval: {e}")
            return False, None

    def cleanup_expired(self):
        """Remove expired approval files."""
        if not os.path.exists(self.approvals_dir):
            return

        now = time.time()
        cleaned = 0

        for filename in os.listdir(self.approvals_dir):
            if not filename.endswith(".json"):
                continue

            filepath = os.path.join(self.approvals_dir, filename)
            try:
                with open(filepath, "r") as f:
                    data = json.load(f)

                if now > data.get("expires", 0):
                    os.unlink(filepath)
                    cleaned += 1
            except (json.JSONDecodeError, IOError, OSError):
                # Remove corrupt files
                try:
                    os.unlink(filepath)
                    cleaned += 1
                except OSError:
                    pass

        if cleaned > 0:
            logging.debug(f"Cleaned up {cleaned} expired approval(s)")


# =============================================================================
# HTTP REQUEST HANDLER
# =============================================================================


class ApprovalRequestHandler(BaseHTTPRequestHandler):
    """
    HTTP request handler for approval endpoints.

    Endpoints:
        GET /approve/<token>  - Approve an authentication request
        GET /health           - Health check

    The handler accesses the ApprovalManager via server.approval_manager.
    """

    # Suppress default logging (we do our own)
    def log_message(self, format, *args):
        logging.debug(f"HTTP: {args[0]}")

    def _send_html(self, status: int, html: str):
        """Send an HTML response."""
        self.send_response(status)
        self.send_header("Content-Type", "text/html; charset=utf-8")
        self.send_header("Cache-Control", "no-store")
        self.end_headers()
        self.wfile.write(html.encode("utf-8"))

    def _send_json(self, status: int, data: str):
        """Send a JSON response."""
        self.send_response(status)
        self.send_header("Content-Type", "application/json")
        self.send_header("Cache-Control", "no-store")
        self.end_headers()
        self.wfile.write(data.encode("utf-8"))

    def do_GET(self):
        """Handle GET requests."""
        parsed = urlparse(self.path)
        path = parsed.path

        # Health check endpoint
        if path == "/health":
            self._handle_health()
            return

        # Approval endpoint: /approve/<token>
        if path.startswith("/approve/"):
            token = path[9:]  # Remove "/approve/" prefix
            self._handle_approve(token)
            return

        # Root path - simple info
        if path == "/":
            self._handle_root()
            return

        # 404 for anything else
        self._send_html(
            404,
            HTML_ERROR.format(
                title="Not Found", message="The requested page was not found."
            ),
        )

    def _handle_root(self):
        """Handle requests to root path."""
        html = """<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>PAM SSH 2FA Approval Server</title>
</head>
<body>
    <h1>PAM SSH 2FA Approval Server</h1>
    <p>This server handles SSH authentication approvals.</p>
    <p>Approval links are sent via push notification.</p>
</body>
</html>"""
        self._send_html(200, html)

    def _handle_health(self):
        """Handle health check requests."""
        timestamp = datetime.now(timezone.utc).isoformat()
        self._send_json(200, HTML_HEALTH.format(timestamp=timestamp))

    def _handle_approve(self, token: str):
        """
        Handle approval requests.

        Args:
            token: The approval token from the URL
        """
        if not token:
            self._send_html(
                400,
                HTML_ERROR.format(
                    title="Invalid Request", message="No approval token provided."
                ),
            )
            return

        # Get the approval manager from the server
        manager = self.server.approval_manager

        # Attempt to mark as approved
        success, approval = manager.mark_approved(token)

        if success and approval:
            # Format the response with approval details
            approved_time = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
            self._send_html(
                200,
                HTML_SUCCESS.format(
                    user=approval.get("user", "unknown"),
                    host=approval.get("host", "unknown"),
                    rhost=approval.get("rhost", "unknown"),
                    time=approved_time,
                ),
            )
        else:
            # Check why it failed
            approval_data = manager.get_approval(token)
            if approval_data is None:
                self._send_html(
                    404,
                    HTML_ERROR.format(
                        title="Invalid Link",
                        message="This approval link is invalid or has already been used.",
                    ),
                )
            elif time.time() > approval_data.get("expires", 0):
                self._send_html(
                    410,
                    HTML_ERROR.format(
                        title="Link Expired",
                        message="This approval link has expired. Please try connecting again.",
                    ),
                )
            else:
                self._send_html(
                    500,
                    HTML_ERROR.format(
                        title="Server Error",
                        message="An error occurred processing your request. Please try again.",
                    ),
                )


# =============================================================================
# APPROVAL SERVER
# =============================================================================


class ApprovalServer(HTTPServer):
    """
    HTTP server for handling approval requests.

    Extends HTTPServer to include the ApprovalManager instance.
    """

    def __init__(self, bind_address: str, port: int, approval_manager: ApprovalManager):
        """
        Initialize the approval server.

        Args:
            bind_address: Address to bind to (e.g., "0.0.0.0")
            port: Port to listen on
            approval_manager: ApprovalManager instance for handling approvals
        """
        super().__init__((bind_address, port), ApprovalRequestHandler)
        self.approval_manager = approval_manager


# =============================================================================
# CLEANUP THREAD
# =============================================================================


class CleanupThread(threading.Thread):
    """
    Background thread that periodically cleans up expired approvals.

    Runs every 60 seconds to remove expired approval files.
    """

    def __init__(self, approval_manager: ApprovalManager):
        super().__init__(daemon=True)
        self.approval_manager = approval_manager
        self.running = True

    def run(self):
        """Run the cleanup loop."""
        while self.running:
            try:
                self.approval_manager.cleanup_expired()
            except Exception as e:
                logging.error(f"Cleanup error: {e}")

            # Sleep for 60 seconds, checking for stop every second
            for _ in range(60):
                if not self.running:
                    break
                time.sleep(1)

    def stop(self):
        """Stop the cleanup thread."""
        self.running = False


# =============================================================================
# MAIN
# =============================================================================


def main():
    """Main entry point for the approval server."""
    parser = argparse.ArgumentParser(
        description="PAM SSH 2FA Approval Server",
        formatter_class=argparse.RawDescriptionHelpFormatter,
        epilog="""
This server handles link-based SSH authentication approvals.
Run as a systemd service for production use.

Examples:
  %(prog)s                      Run with default settings
  %(prog)s --port 9110          Run on specific port
  %(prog)s --debug              Enable debug logging
        """,
    )

    parser.add_argument(
        "--config",
        "-c",
        default=DEFAULT_CONFIG_FILE,
        help=f"Path to config file (default: {DEFAULT_CONFIG_FILE})",
    )

    parser.add_argument(
        "--port", "-p", type=int, help="Port to listen on (overrides config file)"
    )

    parser.add_argument("--bind", "-b", help="Address to bind to (default: 0.0.0.0)")

    parser.add_argument(
        "--debug", "-d", action="store_true", help="Enable debug logging"
    )

    args = parser.parse_args()

    # Load configuration
    config = ServerConfig(args.config)

    # Command line overrides
    if args.port:
        config.port = args.port
    if args.bind:
        config.bind_address = args.bind
    if args.debug:
        config.debug = True

    # Setup logging
    setup_logging(debug=config.debug, log_file=config.log_file)

    # Create approval manager
    approval_manager = ApprovalManager(config.approvals_dir)

    # Start cleanup thread
    cleanup_thread = CleanupThread(approval_manager)
    cleanup_thread.start()

    # Create and start server
    server = ApprovalServer(config.bind_address, config.port, approval_manager)

    logging.info(f"PAM SSH 2FA Approval Server starting")
    logging.info(f"Listening on {config.bind_address}:{config.port}")
    logging.info(f"Approvals directory: {config.approvals_dir}")

    try:
        server.serve_forever()
    except KeyboardInterrupt:
        logging.info("Shutting down...")
    finally:
        cleanup_thread.stop()
        server.shutdown()

    logging.info("Server stopped")


if __name__ == "__main__":
    main()