$kernel->infect(): Creating a cryptovirus for Symfony2 apps

MALICIOUS CRYPTOGRAPHY
IN SYMFONY APPS Raul Fraile

WHO AM I?
• PHP/Symfony2 developer at
• PHP 5.3 Zend Certified Engineer
• Symfony Certified Developer
• BS in Computer Science. Ms(Res) student in
Computing Technologies.
• Open source: Distill, LadybugPHP

https://leanpub.com/symfony-selfstudy

• Cryptovirology studies how to use
cryptography to design malicious
software.
• Closely related to ransomware and
private information retrieval.
• A fundamental twist in cryptography.
CRYPTOVIROLOGY

CREATING OUR
OWN CRYPTOVIRUS
…for fun and profit!

• This is not a real virus, just a proof of concept.
• I chose Symfony just because is my favourite
framework. It can be applied to any other
PHP framework.
• We assume that the virus is already in the
target computer.
• NOT AN EXPERT

#1 Get public key from the hacker server
GET public_key
Hacker serverApp server
OUR CRYPTOVIRUS

#2 Infect the Symfony 2.x app
app[_dev].php
bootstrap.php.cache
Kernel events
OUR CRYPTOVIRUS

#3 Use the public key to encrypt data
app[_dev].php
bootstrap.php.cache
Kernel events
Database
User uploads
Logs
…
OUR CRYPTOVIRUS

#4 Pay to get the private key to decrypt data
GET private_key
OUR CRYPTOVIRUS

#3 (b) Intercept user/passwords and save them encrypted
app[_dev].php
bootstrap.php.cache
Kernel events
raul
Submit
User
*****Password
OUR CRYPTOVIRUS

#4 (b) Get user/password pairs using a backdoor
GET users
OUR CRYPTOVIRUS

• Public key (asymmetric) cryptography requires
two different keys: public and private.
• Based on one-way functions (trapdoors), which
are easy to compute in one direction, but
believed to be difficult to find its inverse.
• Most used one-way functions: integer
factorization, discrete logarithm and elliptic
curves.
PUBLIC KEY CRYPTOGRAPHY

WANT SEND image.jpg
Alice
A A
Bob
B B
B
image.jpg
101101001011001

p = 115307171677547
q = 190761112638809
n = p * q
= 21996124364443030184426121523
Having p and q, calculate n Having n, calculate p and q
Multiplication Factorization
SlowFast
not in Polinomial time
n = 21996124364443030184426121523
= p * q
= …
= 115307171677547 * 190761112638809

• Open Source toolkit for SSL/TLS, as well
as a full-strength general purpose
cryptography library.
• PHP extension: php-openssl.
OPENSSL

$config = array(
"digest_alg" => "sha512",
"private_key_bits" => 4096,
"private_key_type" => OPENSSL_KEYTYPE_RSA,
);
// Create the private and public key
$resource = openssl_pkey_new($config);
// Extract the private key
openssl_pkey_export($resource, $privKey);
// Extract the public key
$pubKey = openssl_pkey_get_details($res);
$pubKey = $pubKey[“key"];
PHP + OPENSSL

$data = “Creating a cryptovirus for Symfony2 apps”;
// Encrypt the data
openssl_public_encrypt($data, $encrypted, $pubKey);
// Decrypt the encrypted data
openssl_private_decrypt($encrypted, $decrypted,
$privKey);
PHP + OPENSSL

kernel.request
Request
Response
kernel.controller
kernel.view kernel.response
kernel.terminate
kernel.exception
KERNEL EVENTS

• kernel.request is dispatched as soon as
the request arrives. Listeners can return a
Response and “end” the execution.
• kernel.controller is dispatched once the
controller has been resolved. Listeners
can manipulate the Controller callable.
KERNEL EVENTS

• kernel.view is dispatched only if the
Controller does not return a Response
object.
• kernel.response allows to modify or
replace the Response object after its
creation.
KERNEL EVENTS

• kernel.exception is dispatched if there is
an uncaught exception. Last chance to
convert an Exception object into a
Response object.
• kernel.terminate is dispatched once the
response has been sent. Allows to run
expensive post-response jobs.
KERNEL EVENTS

• The bootstrap.php.cache file is created
to improve performance, reducing IO
operations and autoload lookups.
• Just a copy&paste of common classes
and interfaces that will be used for sure.
BOOTSTRAP FILE

{
"name": "symfony/framework-standard-edition",
"scripts": {
"post-install-cmd": [
...,
“SensioBundleDistributionBundleComposer
ScriptHandler::buildBootstrap",
...
],
"post-update-cmd": [
...,
“SensioBundleDistributionBundleComposer
ScriptHandler::buildBootstrap",
...
]
}
}
BOOTSTRAP FILE

• Virus deﬁnitions. Antivirus software scans
files to find matches. Useful for known
malware (up-to-date antivirus).
• Heuristics allow antivirus software to identify
new or modified malware, even without virus
definition files. Based on system calls,
network packets, kernel events…
ANTIVIRUS

unlink(__FILE__);
REMOVING ITSELF

$originalCode = "phpinfo();";
// encode with base65 n times
$encoded = $originalCode;
$times = 5;
for ($i=0; $i<$times;$i++) {
$encoded = base64_encode($encoded);
}
// generate hidden code
$code = sprintf('eval(%s"%s"%s);',
str_repeat('base64_decode(', $times),
$encoded,
str_repeat(')', $times)
);
$code = gzdeflate($code);
var_dump($code); // K-K??HJ,N53...
GZIP + BASE64

eval(gzinflate($code));
GZIP + BASE64

• Polymorphic code is code that uses a
polymorphic engine to mutate while
keeping the original algorithm intact.
• Makes it difficult for antivirus software to
recognise the code as it constantly
changes.
• Emulation (sandbox) may be used.
POLYMORPHIC CODE

echo 'Hello world!';
echo 'Hello' . ' ' . 'world!';
printf('Hello world!');
file_put_contents('php://stdout', 'Hello world!');
printf('%c%c%c%c%c%c%c%c%c%c%c%c',
0x48, 0x65, 0x6c, 0x6c, 0x6f, 0x20,
0x77, 0x6f, 0x72, 0x6c, 0x64, 0x21
);
POLYMORPHIC CODE

All of them print “Hello world!”, but using
different code which generate different
AST/opcodes.
POLYMORPHIC CODE

Op Operands
1 ECHO Hello+world%21'
2
CONCAT Hello', '+'
CONCAT ~0, 'world%21'
ECHO ~1
3
SEND_VAL Hello+world%21'
DO_FCALL printf'
4
SEND_VAL php%3A%2F%2Fstdout'
SEND_VAL Hello+world%21'
DO_FCALL ﬁle_put_contents'
5
SEND_VAL %25c%25c%25c…%25c%25c’
SEND_VAL 72
SEND_VAL …101, 108, 108, 111, 32, 119, 111, 114, 108, 100, 33
DO_FCALL printf'
POLYMORPHIC CODE

• The goal would be to create a
polymorphic engine that generates
different code in each infection
randomly.
• Really difficult to get random numbers in
computers, as they can be predictable.
POLYMORPHIC CODE

rand() mt_rand()
RANDOM NUMBERS

• Computational methods are not considered
true random number generators. In practice,
they are sufficient for most tasks.
• Physical methods use physical phenomenon
expected to be random. For example,
atmospheric noise (random.org), radioactive
decay, radio noise or even a coin flipping.
RANDOM NUMBERS

• Before the infection: security measures,
restrictive permissions, disable php-
openssl if we don’t need it,
allow_url_fopen, read-only code…
• Once the app has been infected, we want
to know it as soon as possible, checking
its integrity.
PROTECTING US

Hash functions create a fixed-length digest
from data of arbitrary length.
Easy to compute.
Infeasible to generate a message that has a
given hash.
Infeasible to modify a message without
changing the hash.
Infeasible to ﬁnd two different messages
with the same hash.
HASH FUNCTIONS

Tiny changes in source generate (with high
probability) big changes in the digest.
HASH FUNCTIONS

• md5() is not collision resistant. It is
possible to create two files that share the
same checksum.
• We can include the checksum of the
whole project in the build process and
check it regularly.
HASH FUNCTIONS

use SymfonyComponentFinderFinder;
$finder = new Finder();
$finder->in(__DIR__ . ‘/project')
->files()
->name('*.php');
$hashes = '';
foreach ($finder as $file) {
$hashes .= sha1($file->getContents());
}
// hash of the whole project
$hash = sha1($hashes);
HASH FUNCTIONS

• The PHAR extension provides a way to
put entire PHP applications into a single
ﬁle.
• Equivalent to Java JAR files.
• PHAR files can contain a signature
(checksum) of the included files.
PHAR SIGNATURES

Stub
Manifest
File contents
Signature
Actual contents of the files
Describes the contents of the files:
filename, size, timestamp, CRC32…
Phar Signature in MD5, SHA1,
SHA256 or SHA512
__HALT_COMPILER();
Usually contains loader functionality
PHAR SIGNATURES

Malicious Cryptography. Exposing Cryptovirology

slideshare.net/raulfraile/steganography-hiding-your-secrets-with-php

THANK YOU!
https://github.com/raulfraile/cryptosymfony

$kernel->infect(): Creating a cryptovirus for Symfony2 apps

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