C99 is a very popular PHP based web-shell. There are numerous C99 variants which infect vulnerable web application to give hackers a GUI. The shell lets the attacker take control of the server and also browse the file system, upload, edit, delete, view files and even change file permissions amongst other dangerous actions.
Cleanup can be done by deleting the malicious web shell file from your server, Search for the string c99sh in all files (including images). Also review the raw access logs for HTTP POST requests. You may be able to identify the possible infected files.
Shell c99 php for
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An attacker can take advantage of vulnerabilities such as Unrestricted File Uploads, SQL injection (SQLi), Remote File Inclusion (RFI), unsecured FTP etc to upload the php.cmdshell.c99 malicious script. These vulnerabilities may exist in your website code, or any plugins/themes used.
The c99 shell is a somewhat notorious piece of PHP malware. C99 shell is often uploaded to a compromised web application to provide an interface to an attacker. The c99 shell allows an attacker to hijack the web server process, allowing the attacker to issue commands on the server as the account under which PHP is running. The c99 shell allows an attacker to browse the filesystem, upload, view, and edit files as well as move files, delete files, and even change permissions, all as the web server. Finding the c99 shell on your system is pretty solid evidence of a compromise.
Luckily, if you find the c99 shell on your system, you can usually recreate much of the attack using log files. If the attacker never manages to gain root access every request to c99 will be logged as a normal web request. Because c99 uses GET URL variables for many of its options so it is possible to recreate an attackers footprints by looking through web server access logs. Unfortunately many of the operations within c99 utilize arguments passed via form posts, which are not logged, and so you may not be able to find a complete command history.
There are several versions of c99 shell floating around online. This is a relatively recent version, culled form an incident response (i.e. in the wild). Feel free to take a look, there are many signatures in the file that can be used to write defensive countermeasures.
/*Encoder : AROHA PHPencoder ver. 1.04WEB : */?>//add php tags before usage/********************************************************************************************************* c99shell.php v.1.0 beta (?? 21.05.2005)
In the meantime, we've been able to confirm that our existing rules for the use of an installed C99 shell work well; we suggest that customers concerned about this sort of traffic consider enabling SIDs 16613 - 16628, 18686 - 18690, and 22917 - 22936. We'd love to hear your feedback on the rules, so don't be shy about dropping us a note if you see anything around them.
The c99 shell is a somewhat infamous piece of PHP malware. C99 shell is commonly uploaded to a compromised internet application to supply an interface to an aggressor. The c99 covering permits an opponent to pirate the web server procedure, permitting the enemy to issue commands on the web server as the account under which PHP is running. The c99 covering allows an assailant to surf the filesystem, upload, sight, and edit documents in addition to action files, delete documents, and also change approvals, all as the internet server. Locating the c99 covering on your system is rather solid proof of a compromise.
IBM Security is always looking for high-volume anomalies that might signify a new attack trend. One example is webshells, which are scripts (such as PHP, ASPX, etc.) that perform as a control panel graphical user interface (GUI). An attacker could utilize a webshell to gain system-level access to a vulnerable server.
Although we see many attempts to push malicious PHP code on a daily basis, the increased volume of this particular webshell is startling compared to other types of webshell activity IBM MSS tracks: a 45 percent increase from February through March.
Webshells are considered post-exploitation tools. Before the webshell can be used in an attack, a vulnerability must be found on a target Web application. One way to accomplish this is by first uploading the webshell through a file upload page (e.g., a submission form on a company website) and then using a Local File Include (LFI) weakness in the application to include the webshell in one of the pages.
Next, the attacker will call the new form from the browser. This will enable the attacker to execute shell commands on the server as well as push additional files that can be used for other nefarious actions.
What is very clear is that this particular variant of the C99 webshell is focusing on WordPress CMS vulnerabilities. Our data showed a distinct upward trend in alert volume tied specifically to this C99 variant. IBM MSS detected almost 1,000 attacks utilizing this specific webshell code beginning in February 2016.
We brought this issue forward to draw attention to one of the easiest methods for attackers to achieve unauthorized access to your network. Prevention goes a long way to limiting the risk exposure. The following recommendations can help mitigate the latest variant of the C99 webshell:
Hello Aspiring Hackers. In this article we will learn about the infamous C99 shell. In our previous tutorial RFI hacking for beginners we learnt what is remote file inclusion vulnerability and how hackers use this vulnerability to upload files into the web server. In that tutorial, we uploaded a C99 php shell, which is the most popular shell used in RFI hacking. Today we will see further on how hackers upload shell and hack a website. We have successfully uploaded a shell in the above post.
Let us go to the path where we uploaded our shell as shown below. You should see something as shown below. This is our PHP shell. As you can see, it already shows lot of information about our target system like OS, the web server software, version etc. It also shows all the files in our folder or directory where we uploaded our shell as shown below.
If you are a website admin, always keep a backup of your website as hackers can sometimes delete the entire website and databases. It is also a good thing to scan your web server for any malicious files since I have seen in many instances that people often restore the website deleted but still keep the shell intact.
The attacker can then access the webshell from a browser and start executing shell commands on the server. The webshell also allows malicious actors to upload files that can be used to perform various actions.
According to IBM Security, the variant of the C99 webshell leveraged in these attacks has also been used by Hmei7, an Indonesian hacker whose Zone-H account shows that he has defaced more than 150,000 websites from all across the world.
Webshells are typically uploaded to WordPress websites via vulnerabilities in the content management system (CMS) or third-party plugins, which also seems to be the case in the attacks observed by IBM.
Administrators can protect their websites against these C99 webshell attacks by ensuring that their WordPress installations are not plagued by any vulnerabilities, installing security plugins, changing defaults and customizing their installations as much as possible, changing the name of the uploads folder, and scanning all files when they are uploaded to the website.
A web shell is a shell-like interface that enables a web server to be remotely accessed, often for the purposes of cyberattacks.[1] A web shell is unique in that a web browser is used to interact with it.[2][3]
A web shell could be programmed in any programming language that is supported on a server. Web shells are most commonly written in the PHP programming language due to the widespread usage of PHP for web applications. However, Active Server Pages, ASP.NET, Python, Perl, Ruby, and Unix shell scripts are also used, although these languages are less commonly used.[1][2][3]
Using network monitoring tools, an attacker can find vulnerabilities that can potentially allow delivery of a web shell. These vulnerabilities are often present in applications that are run on a web server.[2]
An attacker may also modify (spoof) the Content-Type header to be sent by the attacker in a file upload to bypass improper file validation (validation using MIME type sent by the client), which will result in a successful upload of the attacker's shell.
A web shell is usually installed by taking advantage of vulnerabilities present in the web server's software. That is why removal of these vulnerabilities is important to avoid the potential risk of a compromised web server.
Using web shells, adversaries can modify the .htaccess file (on servers running the Apache HTTP Server software) on web servers to redirect search engine requests to the web page with malware or spam. Often web shells detect the user-agent and the content presented to the search engine spider is different from that presented to the user's browser. To find a web shell a user-agent change of the crawler bot is usually required. Once the web shell is identified, it can be deleted easily.[2]
Analyzing the web server's log could specify the exact location of the web shell. Legitimate users/visitor usually have different user-agents and referers, on the other hand, a web shell is usually only visited by the attacker, therefore have very few variants of user-agent strings.[2]
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