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PharUtil - Security-oriented utilities for Phar archives

tag: v0.5.6

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README.md

PharUtil - Security-oriented utilities for Phar archives

Command line utilities for building, signing and verifying Phar archives with OpenSSL public/private key. Additionally this library contains classes that help you distribute the code (e.g. plugins, skins) to remote PHP applications over HTTP and, thanks to signing, remove the risk of Arbitraty Remote Code Execution in those applications.

If you're not interested in remote deployment, you could always use included command line utilities to help you deal with Phar archives.

Author: Krzysztof Kotowicz - kkotowicz at gmail dot com

License: MIT

Source code: github

PEAR channel server: pear.kotowicz.net

Command line utilties

  • phar-build for building and signing Phar archives
  • phar-extract for extracting/listing contents of Phar archive
  • phar-verify for verifying signature of Phar archive
  • phar-generate-cert for generating OpenSSL certificates used to sign the Phar archives
  • phar-file-checksums for generating / checking files checksum to see if Phar archive is up-to-date with source files

All scripts support -h command line parameter to display their usage.

Installation

Dependencies

  • PHP >= 5.2.0
  • OpenSSL compiled into PHP (--with-openssl) - windows users please consult PHP manual
  • Phar 2.0.0

If you're using PHP >= 5.3.0, Phar is already bundled. For older versions you must build it from pecl.

E.g. under Ubuntu, these steps are required to build and enable the Phar extension:

$ sudo apt-get install php5-dev
$ sudo pecl install pecl/phar
$ echo "extension=phar.so" | sudo tee /etc/php5/conf.d/phar.ini

Configuring Phar

To be able to build Phar archives, you need to change php.ini setting: phar.readonly=0. Under Ubuntu, this is done by executing:

$ echo "phar.readonly=0" | sudo tee -a /etc/php5/conf.d/phar.ini

This step is optional (and discouraged) if you will only be reading Phar archives

Install the package

Install the library through PEAR installer: $ sudo pear channel-discover pear.kotowicz.net $ sudo pear install kotowicz/PharUtil-beta

Usage

Building a Phar archive

  • Generate certificates in cert/ directory (will be put in priv.pem and pub.pem) $ mkdir cert/ $ cd cert/ $ phar-generate-cert
  • Create src/ directory and copy all the files to build the archive from there
  • Build a signed phar archive $ phar-build --phar library.phar

Extracting a Phar archive

$ phar-extract library.phar output-directory

Verifying a Phar archive signature

$ phar-verify -P pub.pem http://example.com/library.phar

List a Phar archive contents

$ phar-extract -l library.phar output-directory

Using Phar archive

Just use it like a normal Phar archive

include_once 'phar://path/to/library.phar';

Using Phar-util in a build script

Thanks to Damian Bushong , Phar-Util now comes with a script used to generate file checksums in Phar archive and compare them with source file checksums to see if a Phar archive needs to be rebuilt. Thanks to using exit codes this can be easily used in your app build scripts. Use phar-file-checksums --help command for instructions.

Secure remote code deployment with PharUtil

Phar archives, though they have many superb features for a PHP developer, have a certain limitation when it comes to their security - although standard Phar file can be signed, the key used to verify the signature (i.e. the public key) must be stored alongside the archive. Moreover, the signature verification process is hardcoded and does not allow you to supply the public key by yourself.

These problems make it difficult to use Phar archives to distribute a trusted code to clients, because the code and public key have to be stored on the server. Attacker could use e.g. DNS spoofing to emulate the server and supply the code without any signature or using his own public key. Because of that, Standard Phar extension does not allow including remote (e.g. HTTP://) Phar archives to avoid the the risk of arbitrary remote code execution (see e.g. allow_url_fopen and allow_url_include discussion).

To overcome this problem, the PharUtil library recommends using a different approach:

  • the key used to verify the signature is stored on a client (shared-secret) and is never transferred over-the-wire
  • all code is downloaded to the local client sandbox and is verifed using the stored key
  • after verification the code can by safely included

For a more detailed view of different methods of including remote code see (and why should you care at all), see Hardening PHP: How to securely include remote code

Usage scenario

With PharUtil, the usage scenario is as follows:

On server (publisher):

  1. Create public / private keys with Open SSL (phar-generate-cert)
  2. On a server build a code (phar-build) and sign it using your private key.

You may use the built archive on a server like any other Phar archive (see example/local.php)

On client (consumer):

  1. Bundle a copy of public key (cert/pub.pem) for signature verification
  2. Download a Phar archive from the server
  3. Verify Phar signature using local copy of public key (to be sure that the code has been generated by trusted entity)
  4. Include and run the Phar archive in your application

4,5,6 - see example/remote.php or the example below

Mentioned files are installed in example subdirectory of PEAR package documentation (e.g. /usr/share/php/docs/PharUtil/example).

Disclaimer: For the security of given method, it is critical to never disclose the private key! This method also doesn't protect anyone from looking AT the code - the code is not encrypted, it is only signed so it cannot be changed by third party.

Using PharUtil_RemotePharVerifier on a client

Use PharUtil_RemotePharVerifier class to securely check for the Phar signature before using the archive.

<?php

// all verified Phars will be copied to lib/ directory
$verifier = new PharUtil_RemotePharVerifier('/tmp', './lib', './cert/public.pem');
try {
  $verified_file = $verifier->fetch("http://example.com/library.phar");
} catch (Exception $e) {
 // verification failed
 exit();
}

// $verified_file contains absolute filepath of a downloaded file
// with signature verified from './cert/public.pem'
include_once $verified_file;
// or
include_once 'phar://' . $verified_file . '/some/file/within.php';
// or
echo file_get_contents('phar://' . $verified_file . '/readme.txt');
?>

Contact

Krzysztof Kotowicz - kkotowicz at gmail dot com

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