By Scott Behrens
NeoPI was a project developed by myself and Ben Hagen to aid in the detection of obfuscated and encrypted webshells. I recently came across an article about Webacoo shell and a rewrite of this php backdoor to avoid detection from NeoPI.
Webacoo’s developer used a few interesting techniques to avoid detection. The first technique was to avoid signature based detection by using the function ‘strrev’:
This bypasses our traditional based signature detection and also lends to a few other techniques to bypass signature based detection. Another webshell surfaced after NeoPI’s release that uses similar techniques to avoid signature based detection. Another example could be the following (as seen in https://elrincondeseth.Wordpress.com/2011/08/17/bypasseando-neopi/):
We can see that just by breaking up the word, the risk of detection is highly mitigated. As I suggested at B-Sides, signature based detection is complimentary to the tests in NeoPI but by itself, ineffective. These methods described above completely thwart this tests effectiveness.
But one thing these techniques must do at some point is actually eval the code. Webacoo for example uses the following:
eval($b(str_replace(" ","","a W Y o a X …SNIP
By developing a regex that looks for eval and a variable holding the global function, we can flag this file as malicious. After running this test against a WordPress server with Webacoo’s shell, I observed the following:
Figure 1 – Webacoo identified as suspicious in eval test
NeoPI was able to detect the function and flagged it as malicious. This particular type of use of eval isn’t very common and I have really only seen it used in malware. That being said functions.php was also flagged so I imagine this test can still have many false positives and should be used to help aid in manual investigation of the files identified.
Another tweak Webacoo’s developer did was insert spaces in-between each character of a base64 encoded string. The function str_replace() is called to replace each space before the code is base64_decoded and eval’d.
In order to thwart this particular obfuscation technique, I went ahead and modified the entropy function to strip spaces within the data the function is analyzing. The screenshot below shows a scan against 1752 php files in WordPress and shows the entropy test results as flagging webacoo as potentially malicious. This increased NeoPI’s effectiveness at detecting webacoo.php but is more of a stopgap solution as the attacker can craft other junk characters to lower the shells entropy and index of coincidence. Some additional thought and research is needed on potentially looking for these complicated search and replace functions to determine if the data being obfuscated is malicious.
Figure 2 – Test results after modifying entropy code results in higher entropy of webacoo.php
The latest version of the code can be checked out at http://github.com/Neohapsis/NeoPI which includes these enhancements.
As for improving the other tests effectiveness, I am looking into the possibility of identifying base64 encodings without looking for the function name. This technique may be helpful by building a ratio of upper and lower case characters and seeing if there is a trend with files that use obfuscation.
If anyone has interesting techniques for defeating NeoPI please respond to this post and I’ll look at adding more detection mechanisms to this tool!