What is Web Shell?
A web shell is a malicious script written in popular web application languages – PHP, JSP, or ASP. They are installed on a web server operating system to facilitate remote administration. When weaponized, a web shell could allow threat actors to modify files and even access the root directory of the targeted web server. Both internet-facing and non-internet-facing servers (such as resource hosting servers) could fall victim to web shell attacks. Web shell attacks are a convenient cyber attack tactic because their execution doesn’t require additional programs. A communication channel can be simply achieved through the HTTP protocol in web browsers – this is why it’s so important to prefer HTTPS protocols.How Do Web Shell Attacks Work?
Cyber attackers first locate servers with exposures vulnerable to web shell attacks through scanning software, such as Shodan.io. Shodan surfaces all internet-connected devices, including web servers and endpoints, that could be attack vectors for hidden web servers. Once a vulnerability is discovered, cyberattackers launch a web shell attack before installing a patch for the exposure. Exploiting vulnerability CVE-2020-5902 is an example of how fast cybercriminals use exposures that facilitate web shell injections. On June 30, 2020, F5 Networks released a patch for its Traffic Management User Interface (TMUI). The vulnerability facilitated Remote Code Execution (RCE) – a type of cyber attack involving the remote injection of malicious codes into a targeted system. After publishing the vulnerability on June 30, on July 4 (just four days later), an exploit code used to abuse the exposure was discovered.
The first stage of a server infection is to penetrate the outer layer of its ecosystem. This is usually achieved by pushing corrupted web shells through file upload web pages. After this, a Local File Include (LFI) vulnerability connects the web shell to a selected web application page. There are many other web shell injection strategies, including the detection and compromise of Exposed Admin Interfaces, Cross-Site Scripting (XSS), and SQL injections. After the web shell has been installed, a backdoor is naturally established, giving cybercriminals direct remote access to the compromised web server at any time. The efficiency of back door creation with web shells is why web shell attacks are primarily used as persistence mechanisms – establishing a long-term malicious internal network presence. Because of this, data breaches and ransomware injections rarely immediately follow a web shell attack. Hackers usually establish an access channel for a future attack or reconnaissance mission.
How to Block Web Shell Injections
It’s much easier to address the vulnerabilities that facilitate web shell injection than to intercept attacks. The following suggested controls and security tools should be used to locate and remediate all possible web shell injection points in your IT ecosystem.
1. Stay Updated with the Latest Security Patches
Security vulnerabilities are the most common pathways for web shell attacks. To block these entry points, keep all web applications, Content Management Systems, web server software, and third-party software updated with the latest security patches.
2. Disable Unnecessarily Web Server Functions
2. Disable Unnecessarily Web Server Functions
If a web shell is injected, its execution could be blocked if the functions that communicate with web server scripts are disabled in php.ini.Such web server functions include: To prevent the upload of corrupted image files, the directories that facilitate such uploads should ideally be completely disabled.If such an upload mechanism is necessary, the default names of these sensitive directories should be modified to make them harder to discover. Only privileged users should have permission to access these modifications to mitigate insider threat attacks.In addition to this, specify a filter for the permitted file types that can be uploaded to your web server. WordPress plugins are common attack vectors because anyone can develop them – even cybercriminals. To secure these vectors, only install plugins from trusted developers and uninstall all unnecessary plugins.3. Modify the Names of Sensitive Directories
4. Disable All Unnecessary WordPress Plugins
5. Implement a Firewall
A Web Application Firewall (WAF) is designed to prevent web shells and malicious payloads from being injected into an ecosystem by filtering all network traffic. Like antivirus software, keeping your firewall updated with the latest cybersecurity patches is important.
6. Implement File Integrity Monitoring
A file integrity monitoring solution will compare directory updates against the timestamps of clean directory scripts. If a discrepancy is detected, the requested installation on the code directory of the targeted web server will either be blocked or activate a security alert.
7. Monitor Your Attack Surface
