CryptoPHP – A WordPress backdoor in social.png

Summary

This is a series of posts on CryptoPHP, a PHP backdoor used for spamming and blackhat SEO. It seems to come bundled with certain copies of WordPress themes from unofficial sites and resides in a file named “social.png”. It comes installed with a list of email addresses and domains to contact and communicates with a C2 server using cURL and OpenSSL for encryption. Its main purpose appears to be to facilitate the display of links and other content, sent from the C2 server. When the script determines that a web crawler (e.g., GoogleBot), and not a real user, is viewing the site, it injects links to third-party sites in hopes of being indexed.

Symptoms

CryptoPHP communicates with external servers, requiring multiple external requests. You may see the following symptoms:

  • WordPress is slow to load, especially during the first pageview
  • Error messages in your server log, possibly due to failed requests.
  • Error messages from IDS/IPS or other security software (e.g., Suhosin) indicating that someone is making calls to exec and eval.

Discovery

A few days ago, I noticed that a WordPress installation was running extremely slowly. After enabling xhprof and profiling the index page, I noticed that a single method (RoQfzgyhgTpMgdUIktgNdYvKE) was taking around 160 seconds to run. The method name (others in the stack were similarly named) and the 23 calls to curl_exec came off as immediately suspicious. I used grep to search for the file and found it under the themes folder as images/social.png.

This file was included at the bottom of a theme file, causing it to be executed on each page load.

<?php include_once(‘images/social.png’); ?>

Opening social.png in a text editor reveals obfuscated and minified code. While it looks like a mess, it’s simply renamed variables and functions with whitespace removed, and can be undone rather easily with the “Find/Replace All” feature of your favorite text editor.

Obfuscated CryptoPHP

 

How to Remove CryptoPHP or social.png

In the limited tests that I’ve done, the offending file – social.png – is the only file that is malicious. It seems to be added to the images/ directory in themes downloaded from unofficial sources. Another line in the main theme files (index.php, header.php or footer.php) includes the file.

While nothing in the file itself indicates that personal or sensitive data is being transmitted back to the server, the file allows its controllers to send commands to it. These commands are then executed by the eval and exec commands in PHP. It is theoretically possible for content, account information, etc. to be transmitted back to the controlling server.

Since the WordPress instance I was using was running on localhost, it would have been unreachable by the controlling servers. It could still phone home and download commands, but could not be controlled directly.  However, due to the possibility of sensitive data being stolen, and the evidence of storing information in the database, I’d recommend a complete re-install of WordPress and changing your admin password(s).

Coming Soon

  • Encryption methods (including a script to decrypt database contents)
  • Detailed/technical review
CryptoPHP – A WordPress backdoor in social.png

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