Web shell explained

A web shell is a shell-like interface that facilitates remote access to a web server, commonly exploited for cyberattacks.^[1] Unlike traditional shells, it is accessed via a web browser, making it a versatile tool for malicious activities.^[2] ^[3]

Web shells can be coded in any programming language supported by a server, with PHP being the most prevalent due to its widespread use in web applications. Other languages, such as Active Server Pages, ASP.NET, Python, Perl, Ruby, and Unix shell scripts, are also employed.^[1] ^[2] ^[3]

Attackers identify vulnerabilities often in web server application using network monitoring tools, which can be exploited to deploy a web shell.^[2]

Once installed, a web shell allows attackers to execute shell commands, perform privilege escalation, and manage files by uploading, deleting, downloading, or executing them on the server.^[2]

General usage

Web shells are favored in cyberattacks for their versatility and elusiveness.^[4] Common applications include:

Data theft^[4]
Infecting visitors through watering hole attacks^[5]
Website defacement by altering files with malicious intent
Launching DDoS attacks^[2]
Relaying commands to inaccessible network segments^[2]
Serving as a command and control hub, such as in botnet systems or to compromise external networks^[2]

Web shells enable hackers to extract data, corrupt systems, and deploy more damaging malware. The threat intensifies when compromised servers are used to infiltrate additional systems. They are also employed in cyber espionage targeting sectors like government, finance, and defense. A notable example is the "China Chopper" web shell.^[6]

Delivery of web shells

Web shells are deployed by exploiting vulnerabilities in web applications or weak server configurations, including:^[2] ^[4]

SQL injection
Flaws in applications and services (e.g., web server software like NGINX or content management systems like WordPress)^[7] ^[8]
File processing and upload vulnerabilities (mitigated by restricting file types)^[8]
Remote file inclusion (RFI) and local file inclusion (LFI) vulnerabilities
Remote code execution
Exposed administration interfaces^[2]

Attackers may also spoof the Content-Type header during file uploads to bypass weak file validation, enabling shell deployment.

Example

The following is a basic PHP web shell that executes a shell command and displays the output:

With a filename of example.php, the command to display the [[Passwd#Password file|/etc/passwd]] file could be:

https://example.com/example.php?x=cat%20%2Fetc%2Fpasswd

This executes the command cat /etc/passwd. Such risks can be mitigated by disabling PHP shell functions to prevent arbitrary command execution.

Prevention and mitigation

Preventing web shell installation requires addressing server vulnerabilities. Key measures include:^[2] ^[3]

Regularly updating applications and the host server's operating system to patch known bugs
Implementing a demilitarized zone (DMZ) between web-facing servers and internal networks
Securing web server configurations^[2]
Closing unused ports and services^[2]
Validating user input to limit local and remote file inclusion vulnerabilities^[2]
Using a reverse proxy to restrict administrative URLs to legitimate sources^[2]
Conducting frequent vulnerability scans (though ineffective against zero-day attacks)^[2]
Deploying a firewall^[2]
Disabling directory browsing
Avoiding default passwords^[2]

Detection

Web shells are challenging to detect due to their modifiability, often evading antivirus software.^[2] ^[9]

Indicators of a web shell include:^[2] ^[3]

Unusually high web server activity from downloading/uploading^[2] ^[9]
Files with abnormal timestamps (e.g., newer than last modification)^[9]
Unknown files on the server
Suspicious references (e.g., cmd.exe or [[eval]])
Unusual connections in server logs

For instance, a PNG file with POST parameters or dubious logins between DMZ servers and internal subnets may signal a web shell.^[2] ^[10] ^[11] ^[12]

Web shells may include disguised login forms, such as fake error pages.^[2] ^[13] ^[14] ^[15]

Attackers can modify the .htaccess file (on Apache HTTP Server) to redirect search engine queries to malware or spam pages, often tailoring content based on user-agent detection. Identifying the shell may require altering the crawler's user-agent, after which it can be easily removed.^[2]

Analyzing server logs can pinpoint the web shell's location, as legitimate users typically have diverse user-agents and referers, while attacker access is more uniform.^[2]

Notes and References

Web site: How can web shells be used to exploit security tools and servers?. SearchSecurity. 2018-12-21. https://web.archive.org/web/20190328065900/https://searchsecurity.techtarget.com/answer/How-can-web-shells-be-used-to-exploit-security-tools-and-servers. 2019-03-28. live.
Web site: Web Shells – Threat Awareness and Guidance. US Department of Homeland Security. www.us-cert.gov. 9 August 2017. 20 December 2018. https://web.archive.org/web/20190113062745/https://www.us-cert.gov/ncas/alerts/TA15-314A. 13 January 2019. live.
Web site: What is a Web shell?. admin. 3 August 2017. malware.expert. 20 December 2018. https://web.archive.org/web/20190113003907/https://malware.expert/general/what-is-a-web-shell/. 13 January 2019. live.
Web site: Russian Government Cyber Activity Targeting Energy and Other Critical Infrastructure Sectors – US-CERT. www.us-cert.gov. 16 March 2018. 20 December 2018. https://web.archive.org/web/20181220172706/https://www.us-cert.gov/ncas/alerts/TA18-074A. 20 December 2018. live.
Web site: The Definitive Guide about Backdoor Attacks - What are WebShell BackDoors. Makis Mourelatos. WordPress Security Engineer at FixMyWPWC Athens 2016. W. P.. Support. Security. Aficionado. Wannabe. Kitesurfer. 16 October 2017. fixmywp.com. 20 December 2018. https://web.archive.org/web/20190113003929/https://fixmywp.com/security/what-are-web-shell-backdoors.php. 13 January 2019. live.
Hannousse. Abdelhakim. Yahiouche. Salima. 2021-09-01. Handling webshell attacks: A systematic mapping and survey. Computers & Security. 108. 102366. 10.1016/j.cose.2021.102366. 0167-4048. subscription.
Web site: Got WordPress? PHP C99 Webshell Attacks Increasing. 14 April 2016. 21 December 2018. https://web.archive.org/web/20181229190535/https://securityintelligence.com/got-wordpress-php-c99-webshell-attacks-increasing/. 29 December 2018. live.
Web site: Equifax breach was 'entirely preventable' had it used basic security measures, says House report. 10 December 2018. 21 December 2018. https://web.archive.org/web/20181220232937/https://techcrunch.com/2018/12/10/equifax-breach-preventable-house-oversight-report/. 20 December 2018. live.
Web site: Breaking Down the China Chopper Web Shell - Part I. FireEye. 20 December 2018. https://web.archive.org/web/20190113182308/https://www.fireeye.com/blog/threat-research/2013/08/breaking-down-the-china-chopper-web-shell-part-i.html. 13 January 2019. live.
Web site: Intrusion Detection and Prevention Systems. 2018-12-22. https://web.archive.org/web/20190113003930/https://ws680.nist.gov/publication/get_pdf.cfm?pub_id=901146. 2019-01-13. live.
Web site: Five signs an attacker is already in your network. Kasey Cross. 16 June 2016. Network World. 22 December 2018. https://web.archive.org/web/20190113003925/https://www.networkworld.com/article/3085141/network-security/five-signs-an-attacker-is-already-in-your-network.html. 13 January 2019. live.
Web site: Traffic Analysis for Network Security: Two Approaches for Going Beyond Network Flow Data. 15 September 2016. 2018-12-22. https://web.archive.org/web/20161114084537/https://insights.sei.cmu.edu/sei_blog/2016/09/traffic-analysis-for-network-security-two-approaches-for-going-beyond-network-flow-data.html. 2016-11-14. live.
Web site: Hackers Hiding Web Shell Logins in Fake HTTP Error Pages. BleepingComputer. 21 December 2018. https://web.archive.org/web/20180726011308/https://www.bleepingcomputer.com/news/security/hackers-hiding-web-shell-logins-in-fake-http-error-pages/. 26 July 2018. live.
Web site: Hackers Hiding Web Shell Logins in Fake HTTP Error Pages. 24 July 2018. ThreatRavens. 17 February 2019. https://web.archive.org/web/20190113062731/https://threatravens.com/hackers-hiding-web-shell-logins-in-fake-http-error-pages/. 13 January 2019. live.
Web site: Hackers Hiding Web Shell Logins in Fake HTTP Error Pages. cyware.com. 22 December 2018. https://web.archive.org/web/20190113003916/https://cyware.com/news/hackers-hiding-web-shell-logins-in-fake-http-error-pages-f9f1b47e. 13 January 2019. live.