Adversaries may install malicious components that run on Internet Information Services (IIS) web servers to establish persistence. IIS provides several mechanisms to extend the functionality of the web servers. For example, Internet Server Application Programming Interface (ISAPI) extensions and filters can be installed to examine and/or modify incoming and outgoing IIS web requests. Extensions and filters are deployed as DLL files that export three functions: <code>Get{Extension/Filter}Version</code>, <code>Http{Extension/Filter}Proc</code>, and (optionally) <code>Terminate{Extension/Filter}</code>. IIS modules may also be installed to extend IIS web servers.(Citation: Microsoft ISAPI Extension Overview 2017)(Citation: Microsoft ISAPI Filter Overview 2017)(Citation: IIS Backdoor 2011)(Citation: Trustwave IIS Module 2013)

Adversaries may install malicious ISAPI extensions and filters to observe and/or modify traffic, execute commands on compromised machines, or proxy command and control traffic. ISAPI extensions and filters may have access to all IIS web requests and responses. For example, an adversary may abuse these mechanisms to modify HTTP responses in order to distribute malicious commands/content to previously comprised hosts.(Citation: Microsoft ISAPI Filter Overview 2017)(Citation: Microsoft ISAPI Extension Overview 2017)(Citation: Microsoft ISAPI Extension All Incoming 2017)(Citation: Dell TG-3390)(Citation: Trustwave IIS Module 2013)(Citation: MMPC ISAPI Filter 2012)

Adversaries may also install malicious IIS modules to observe and/or modify traffic. IIS 7.0 introduced modules that provide the same unrestricted access to HTTP requests and responses as ISAPI extensions and filters. IIS modules can be written as a DLL that exports <code>RegisterModule</code>, or as a .NET application that interfaces with ASP.NET APIs to access IIS HTTP requests.(Citation: Microsoft IIS Modules Overview 2007)(Citation: Trustwave IIS Module 2013)(Citation: ESET IIS Malware 2021)
IIS Components
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1505/004  external_id: T1505.004     source_name: Microsoft ISAPI Extension Overview 2017  description: Microsoft. (2017, June 16). ISAPI Extension Overview. Retrieved June 3, 2021.  url: https://docs.microsoft.com/en-us/previous-versions/iis/6.0-sdk/ms525172(v=vs.90)     source_name: Microsoft ISAPI Filter Overview 2017  description: Microsoft. (2017, June 16). ISAPI Filter Overview. Retrieved June 3, 2021.  url: https://docs.microsoft.com/en-us/previous-versions/iis/6.0-sdk/ms524610(v=vs.90)     source_name: IIS Backdoor 2011  description: Julien. (2011, February 2). IIS Backdoor. Retrieved June 3, 2021.  url: https://web.archive.org/web/20170106175935/http:/esec-lab.sogeti.com/posts/2011/02/02/iis-backdoor.html     source_name: Trustwave IIS Module 2013  description: Grunzweig, J. (2013, December 9). The Curious Case of the Malicious IIS Module. Retrieved June 3, 2021.  url: https://www.trustwave.com/en-us/resources/blogs/spiderlabs-blog/the-curious-case-of-the-malicious-iis-module/     source_name: Microsoft ISAPI Extension All Incoming 2017  description: Microsoft. (2017, June 16). Intercepting All Incoming IIS Requests. Retrieved June 3, 2021.  url: https://docs.microsoft.com/en-us/previous-versions/iis/6.0-sdk/ms525696(v=vs.90)     source_name: Dell TG-3390  description: Dell SecureWorks Counter Threat Unit Threat Intelligence. (2015, August 5). Threat Group-3390 Targets Organizations for Cyberespionage. Retrieved August 18, 2018.  url: https://www.secureworks.com/research/threat-group-3390-targets-organizations-for-cyberespionage     source_name: MMPC ISAPI Filter 2012  description: MMPC. (2012, October 3). Malware signed with the Adobe code signing certificate. Retrieved June 3, 2021.  url: https://web.archive.org/web/20140804175025/http:/blogs.technet.com/b/mmpc/archive/2012/10/03/malware-signed-with-the-adobe-code-signing-certificate.aspx     source_name: Microsoft IIS Modules Overview 2007  description: Microsoft. (2007, November 24). IIS Modules Overview. Retrieved June 17, 2021.  url: https://docs.microsoft.com/en-us/iis/get-started/introduction-to-iis/iis-modules-overview     source_name: ESET IIS Malware 2021  description: Hromcová, Z., Cherepanov, A. (2021). Anatomy of Native IIS Malware. Retrieved September 9, 2021.  url: https://i.blackhat.com/USA21/Wednesday-Handouts/us-21-Anatomy-Of-Native-Iis-Malware-wp.pdf     source_name: Unit 42 RGDoor Jan 2018  description: Falcone, R. (2018, January 25). OilRig uses RGDoor IIS Backdoor on Targets in the Middle East. Retrieved July 6, 2018.  url: https://researchcenter.paloaltonetworks.com/2018/01/unit42-oilrig-uses-rgdoor-iis-backdoor-targets-middle-east/     