Server Software Component: Web Shell

Adversaries may backdoor web servers with web shells to establish persistent access to systems. A Web shell is a Web script that is placed on an openly accessible Web server to allow an adversary to use the Web server as a gateway into a network. A Web shell may provide a set of functions to execute or a command-line interface on the system that hosts the Web server.

In addition to a server-side script, a Web shell may have a client interface program that is used to talk to the Web server (ex: China Chopper Web shell client).[1]

ID: T1505.003
Sub-technique of:  T1505
Tactic: Persistence
Platforms: Linux, Windows, macOS
System Requirements: Adversary access to Web server with vulnerability or account to upload and serve the Web shell file.
Permissions Required: SYSTEM, User
Data Sources: Application Log: Application Log Content, File: File Creation, File: File Modification, Network Traffic: Network Traffic Content, Network Traffic: Network Traffic Flow, Process: Process Creation
CAPEC ID: CAPEC-650
Version: 1.1
Created: 13 December 2019
Last Modified: 16 September 2020

Procedure Examples

ID Name Description
G0050 APT32

APT32 has used Web shells to maintain access to victim websites.[2]

G0087 APT39

APT39 has installed ANTAK and ASPXSPY web shells. [3]

S0073 ASPXSpy

ASPXSpy is a Web shell. The ASPXTool version used by Threat Group-3390 has been deployed to accessible servers running Internet Information Services (IIS).[4]

S0020 China Chopper

China Chopper's server component is a Web Shell payload.[1]

G0009 Deep Panda

Deep Panda uses Web shells on publicly accessible Web servers to access victim networks.[5]

G0074 Dragonfly 2.0

Dragonfly 2.0 commonly created Web shells on victims' publicly accessible email and web servers, which they used to maintain access to a victim network and download additional malicious files.[6][7]

G0117 Fox Kitten

Fox Kitten has installed web shells on compromised hosts to maintain access.[8][9]

G0093 GALLIUM

GALLIUM used Web shells to persist in victim environments and assist in execution and exfiltration.[10][11]

G0125 HAFNIUM

HAFNIUM has deployed multiple web shells on compromised servers including SIMPLESEESHARP, SPORTSBALL, China Chopper, and ASPXSpy.[12][13][14]

G0094 Kimsuky

Kimsuky has used modified versions of open source PHP web shells to maintain access, often adding "Dinosaur" references within the code.[15]

G0065 Leviathan

Leviathan relies on web shells for an initial foothold as well as persistence into the victim's systems. [16]

G0049 OilRig

OilRig has used web shells, often to maintain access to a victim network.[17][18][19]

G0116 Operation Wocao

Operation Wocao has used their own web shells, as well as those previously placed on target systems by other threat actors, for reconnaissance and lateral movement.[20]

S0072 OwaAuth

OwaAuth is a Web shell that appears to be exclusively used by Threat Group-3390. It is installed as an ISAPI filter on Exchange servers and shares characteristics with the China Chopper Web shell.[4]

S0598 P.A.S. Webshell

P.A.S. Webshell can gain remote access and execution on target web servers.[21]

G0034 Sandworm Team

Sandworm Team has used webshells including P.A.S. Webshell to maintain access to victim networks.[21]

S0185 SEASHARPEE

SEASHARPEE is a Web shell.[18]

S0578 SUPERNOVA

SUPERNOVA is a Web shell.[22][23][24]

G0088 TEMP.Veles

TEMP.Veles has planted Web shells on Outlook Exchange servers.[25]

G0027 Threat Group-3390

Threat Group-3390 has used a variety of Web shells.[26]

G0081 Tropic Trooper

Tropic Trooper has started a web service in the target host and wait for the adversary to connect, acting as a web shell.[27]

G0123 Volatile Cedar

Volatile Cedar can inject web shell code into a server.[28][29]

Mitigations

This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.

Detection

Web shells can be difficult to detect. Unlike other forms of persistent remote access, they do not initiate connections. The portion of the Web shell that is on the server may be small and innocuous looking. The PHP version of the China Chopper Web shell, for example, is the following short payload: [1]

<?php @eval($_POST['password']);>

Nevertheless, detection mechanisms exist. Process monitoring may be used to detect Web servers that perform suspicious actions such as running cmd.exe or accessing files that are not in the Web directory. File monitoring may be used to detect changes to files in the Web directory of a Web server that do not match with updates to the Web server's content and may indicate implantation of a Web shell script. Log authentication attempts to the server and any unusual traffic patterns to or from the server and internal network. [30]

References

  1. Lee, T., Hanzlik, D., Ahl, I. (2013, August 7). Breaking Down the China Chopper Web Shell - Part I. Retrieved March 27, 2015.
  2. Lassalle, D., et al. (2017, November 6). OceanLotus Blossoms: Mass Digital Surveillance and Attacks Targeting ASEAN, Asian Nations, the Media, Human Rights Groups, and Civil Society. Retrieved November 6, 2017.
  3. Hawley et al. (2019, January 29). APT39: An Iranian Cyber Espionage Group Focused on Personal Information. Retrieved February 19, 2019.
  4. Dell SecureWorks Counter Threat Unit Threat Intelligence. (2015, August 5). Threat Group-3390 Targets Organizations for Cyberespionage. Retrieved August 18, 2018.
  5. RYANJ. (2014, February 20). Mo’ Shells Mo’ Problems – Deep Panda Web Shells. Retrieved September 16, 2015.
  6. US-CERT. (2018, March 16). Alert (TA18-074A): Russian Government Cyber Activity Targeting Energy and Other Critical Infrastructure Sectors. Retrieved June 6, 2018.
  7. US-CERT. (2017, October 20). Alert (TA17-293A): Advanced Persistent Threat Activity Targeting Energy and Other Critical Infrastructure Sectors. Retrieved November 2, 2017.
  8. CISA. (2020, September 15). Iran-Based Threat Actor Exploits VPN Vulnerabilities. Retrieved December 21, 2020.
  9. ClearSky. (2020, December 17). Pay2Key Ransomware – A New Campaign by Fox Kitten. Retrieved December 21, 2020.
  10. Cybereason Nocturnus. (2019, June 25). Operation Soft Cell: A Worldwide Campaign Against Telecommunications Providers. Retrieved July 18, 2019.
  11. MSTIC. (2019, December 12). GALLIUM: Targeting global telecom. Retrieved January 13, 2021.
  12. MSTIC. (2021, March 2). HAFNIUM targeting Exchange Servers with 0-day exploits. Retrieved March 3, 2021.
  13. Gruzweig, J. et al. (2021, March 2). Operation Exchange Marauder: Active Exploitation of Multiple Zero-Day Microsoft Exchange Vulnerabilities. Retrieved March 3, 2021.
  14. Bromiley, M. et al. (2021, March 4). Detection and Response to Exploitation of Microsoft Exchange Zero-Day Vulnerabilities. Retrieved March 9, 2021.
  15. CISA, FBI, CNMF. (2020, October 27). https://us-cert.cisa.gov/ncas/alerts/aa20-301a. Retrieved November 4, 2020.
  1. Plan, F., et all. (2019, March 4). APT40: Examining a China-Nexus Espionage Actor. Retrieved March 18, 2019.
  2. Unit 42. (2017, December 15). Unit 42 Playbook Viewer. Retrieved December 20, 2017.
  3. Davis, S. and Caban, D. (2017, December 19). APT34 - New Targeted Attack in the Middle East. Retrieved December 20, 2017.
  4. Crowdstrike. (2020, March 2). 2020 Global Threat Report. Retrieved December 11, 2020.
  5. Dantzig, M. v., Schamper, E. (2019, December 19). Operation Wocao: Shining a light on one of China’s hidden hacking groups. Retrieved October 8, 2020.
  6. ANSSI. (2021, January 27). SANDWORM INTRUSION SET CAMPAIGN TARGETING CENTREON SYSTEMS. Retrieved March 30, 2021.
  7. Tennis, M. (2020, December 17). SUPERNOVA: A Novel .NET Webshell. Retrieved February 22, 2021.
  8. Riley, W. (2020, December 1). SUPERNOVA SolarWinds .NET Webshell Analysis. Retrieved February 18, 2021.
  9. CISA. (2021, January 27). Malware Analysis Report (AR21-027A). Retrieved February 22, 2021.
  10. Miller, S, et al. (2019, April 10). TRITON Actor TTP Profile, Custom Attack Tools, Detections, and ATT&CK Mapping. Retrieved April 16, 2019.
  11. Falcone, R. and Lancaster, T.. (2019, May 28). Emissary Panda Attacks Middle East Government Sharepoint Servers. Retrieved July 9, 2019.
  12. Chen, J.. (2020, May 12). Tropic Trooper’s Back: USBferry Attack Targets Air gapped Environments. Retrieved May 20, 2020.
  13. Threat Intelligence and Research. (2015, March 30). VOLATILE CEDAR. Retrieved February 8, 2021.
  14. ClearSky Cyber Security. (2021, January). “Lebanese Cedar” APT Global Lebanese Espionage Campaign Leveraging Web Servers. Retrieved February 10, 2021.
  15. US-CERT. (2015, November 13). Compromised Web Servers and Web Shells - Threat Awareness and Guidance. Retrieved June 8, 2016.