Cybercriminals are using the most sophisticated techniques to bypass the security controls in various organization such as IT, medical, manufacturing industries, energy sectors, even government entities.
Sometimes developers are creating a backdoor for a legitimate purpose such as maintenance and easy accessibility during the technical issue via a remote location.
But the hackers are using it for completely malicious purposes especially creating and injecting an advanced backdoor to the target system using various advanced obfuscation techniques into the vulnerable server.
This technique will help them to perform an attack to gain control over the target and upload malicious payloads to steal the various sensitive data also mining the cryptocurrencies.
There are various types of backdoor which can be written in various languages, for an example if the backdoor was written in ASP then it can run on .net based servers and if it’s written in PHP then it will run on the servers that run on PHP.
In this case, the organization should learn how to protect your web applications from vulnerabilities such as a backdoor, SQL injection types of attack with the best WAF solutions, and about Incapsula backdoor shell protection.
PHP Based Evasion Techniques
There is a different method that is used by attackers to evade the detection, mask known functions or PHP keywords are mainly used by many of the PHP based backdoors.
The first method is Character reordering where attacker used to place and embedded backdoor code in well-known “404 Not Found”message and the keyword “_POST” is written in the plain site.
Line 1 – the backdoor code turns off all error reporting to avoid detection in case of an error.
Line 3- the “default” parameter is defined -a random combination of characters.
line 4 – the “about” parameter is defined when the code reorders these characters and turns them to upper case to build the keyword “_POST”.
Link 5 – keyword is used in lines 5-12 to check if the HTTP request to this page was done via the POST method and whether it contained the “lequ” parameter.
According to Incapsula, If so, the backdoor uses the “eval” function to run the code that was sent in the parameter “lequ”. Thus, the backdoor reads the value from a parameter in a post request without ever using the keyword “$_POST”.
Other than this, some of the other attacks are used by hackers to hide their malicious code and evade detection.
- Hiding known PHP function using string manipulations (replacement, concatenation, reverse, shift and split)
- Using obscure parameter names, like random characters or combinations of the characters O and 0 which are visually similar
- Encoding the backdoor, or part of its code with base64 encoding
- Using compression as a mean to hide the backdoor code
- Obfuscating base64 encoded text by manipulating the text in order to avoid simple decoding
- Obfuscating requests sent to the backdoor after it was uploaded by using the “preg_replace” function on the input.
Protection from these kindly of obfustication techniques Strong web-application firewall such as Incapsula CDN identifying the malicious threats using several layers of security policies is highly recommended for any organization.
Mask Known Functions or PHP Keywords
In some case, the Attacker used to hide known functions or PHP keywords in order to evade detection.
Here the some of known functions and keywords include:
- PHP Superglobals – predefined global variables of PHP
- $_POST – array containing parameters of the post request (see example 1)
- $_REQUEST – array containing parameter of post or get request (see example 5)
- $_FILES – array containing items uploaded in a post request (see example 6)
- $_COOKIE – array containing variables passed via HTTP cookie (see example 4)
- String manipulation functions
- str_replace – given three strings, replaces all occurrences of the first string with the second string in the third string(see example 4)
- preg_replace – same as str_replace but with regular expression searches (see example 3)
- strtr – similar to str_replace, but replaces characters using a map (see example 6)
- substr – returns a specified portion of a given string (see example 6)
- strtoupper – turns all letters to uppercase (see example 1)
- ‘.’ operator – concatenates characters or strings (see example 2)
- str_rot13 – shifts every letter by 13 places in the alphabet (see example 3)
- strrev – reverses a string (see example 5)
- eval – executes a given PHP code (see example 1)
- create_function – creates a new function given the function’s parameters and code (see example 2)
- base64_decode – decodes base64 encoded text (see example 4)
- gzinflate – decompresses compressed data (see example 5)
- chr – returns a single byte string from a number using ascii encoding (or another mapping) (see example 2)
Command line evasion and obfuscation are the most used technique among many numbers of advance level attacks which are increased by attackers with their phishing and Malware attacks which create a powerful backdoor.
Mitigations – Advanced Backdoor
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If the backdoor was already uploaded on an infected server, it is possible to block the communication between the attacker and the backdoor which will stop the backdoor from working and alerts the server admin, so the backdoor can be removed.
Placing a web-application firewall can filter out the malicious Backdoor shell and isolate the further attack.
Implement the highly recommended Imperva Incapsula backdoor shell protection.
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