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In a campaign reported in 2021, an actor Sygnia researchers called TG1021—and CSO called Praying Mantis—exploited several weaknesses in public-facing ASP.NET applications to compromise IIS servers. The attackers then used custom malware designed to run inside the IIS worker process, take instructions through incoming web requests, and support credential theft and movement through victims’ networks. Sygnia assessed the actor as likely government-sponsored; that sponsorship has not been established as fact. CSO’s July 27, 2021 report describes the campaign.
How the IIS attacks worked
Deserialization turns stored or transmitted data back into objects an application can use. If an application deserializes attacker-controlled data without properly validating it, an attacker may be able to make the server process malicious instructions. The Praying Mantis reporting describes several distinct routes into ASP.NET applications—not one universal IIS flaw.
| Application or component | Reported prerequisite | Execution or pivot mechanism | Potential observable artifact |
|---|---|---|---|
| Checkbox Survey, versions 6 and earlier | Unsafe handling of the custom _VSTATE value. The report says the application’s manual handling of this data meant ASP.NET ViewState MAC protection was ignored. |
Attacker-controlled data was deserialized, enabling arbitrary code execution, according to CERT/CC analysts quoted by CSO. | Suspicious requests involving the custom value; subsequent IIS process or module activity. CSO’s report |
| ASP.NET ViewState | A stolen or exposed application machineKey. An attacker who knows the key can create ViewState data with a valid MAC; this is a separate path from Checkbox Survey’s custom handling. |
A forged ViewState can pass integrity validation and trigger unsafe processing. Mandiant describes this technique in a separate APT41 campaign; that does not make APT41 the Praying Mantis actor. Mandiant’s APT41 analysis | Unexpected ViewState-bearing requests, exposed or exfiltrated configuration files, and suspicious IIS activity. ACSC’s advisory |
| ASP.NET session state stored in MSSQL | An attacker able to place malicious serialized session objects in a shared session-state database. | The objects could be used to reach other IIS servers that consumed that shared state, creating a pivot through the database. | Unexpected session-state database access or activity involving multiple IIS applications or servers. CSO’s report |
| Telerik UI for ASP.NET AJAX, including CVE-2019-18935 | A vulnerable Telerik deployment susceptible to exploitation. | The 2021 reporting includes Telerik among the routes used against IIS. Separately, CISA, FBI, and MS-ISAC reported that multiple actors exploited CVE-2019-18935 on a U.S. federal civilian agency’s IIS server from November 2022 through early January 2023, enabling remote code execution. | The agencies reported malicious DLL uploads, some disguised as PNG files. Their advisory does not attribute this later activity to Praying Mantis. CISA, FBI, and MS-ISAC advisory |
ViewState MAC validation helps protect ViewState integrity on up-to-date .NET installations, but it cannot protect an application if an attacker obtains the relevant machine key. The Australian Cyber Security Centre also warned that attackers had targeted previously compromised organizations, raising the possibility that configuration files and keys were taken during earlier access. ACSC advisory, May 22, 2020
What the malware did inside IIS
NodeIISWeb took instructions through web requests
The report says the attackers reflectively loaded a malicious DLL and NodeIISWeb into w3wp.exe, the IIS worker process. Reflective loading can run a DLL without first writing it to disk. NodeIISWeb hooked IIS input-validation functions and inspected incoming HTTP requests for commands encoded in expected cookie names and values. Because control could arrive through inbound requests, defenders could not rely only on detecting a continuous stream of outbound command-and-control traffic.
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NodeIISWeb could also forward TCP, HTTP, and SQL traffic and load additional modules. The report notes a trade-off: the reflective payload was lost when its parent process restarted, reducing persistence even as it limited some disk evidence.
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Other modules supported access and movement
Sygnia’s account describes ExtDLL.dll as a backdoor capable of file operations, system-information collection, DLL execution, code injection, and token manipulation. Related modules could run PowerShell scripts without starting a PowerShell process, forward HTTP traffic, assist with privilege escalation and Active Directory mapping, or return custom responses to confirm exploitation.
The operators also modified login pages to collect credentials, ran tools including SharpHound and PowerSploit from memory, and used compromised domain credentials to access internal SMB shares. These activities show why investigation should include the application, the IIS worker process, authentication activity, and likely lateral-movement paths—not just the original exploit request.
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How to detect suspected memory-resident IIS malware
Memory residency and request-driven control can leave fewer obvious artifacts than a conventional executable and persistent outbound connection. A single absence—such as no suspicious file on disk—does not rule out compromise. The 2021 report recommends using its indicators of compromise, scanning internet-facing IIS servers with suitable YARA rules, and actively hunting for suspicious IIS activity. Its published report is the source for the campaign-specific indicators and rules.
- Review web-server requests for unusual serialized data, suspicious ViewState activity, or unexpected cookie names and values, especially when followed by unusual application behavior.
- Investigate unexpected DLL activity or other anomalous behavior associated with
w3wp.exe, including activity that has no corresponding DLL file on disk. - Check for altered login pages, unexpected module loading, unexplained PowerShell-related activity, and unusual access to SQL or internal SMB resources.
- Correlate IIS and application logs with database access, identity events, and network traffic. Request-driven control means that inbound requests may be more revealing than steady outbound C2 connections.
- Use the indicators and YARA rules published with the CSO report as leads, not as a guarantee that every related compromise will match them.
What IIS operators should change
Patch and validate deserialization paths
- Patch affected applications and frameworks, including vulnerable Telerik deployments, and review whether public-facing applications deserialize user-controlled or otherwise untrusted data.
- Validate ViewState and any application-specific equivalents such as Checkbox Survey’s
_VSTATE. Do not assume ASP.NET’s MAC setting protects custom code that handles serialized data outside the normal validation path. - For the ASP.NET settings addressed in the 2021 report, verify
enableViewStateMacisTrue,aspnet:AllowInsecureDeserializationisFalse, andAspNetEnforceViewStateMacis1. Check these against the requirements and supported configuration of the application and .NET version in use.
Protect machine keys and session-state databases
- Protect ASP.NET machine keys as secrets, restrict who can read them, and rotate them routinely or after suspected exposure. A valid MAC does not help if an attacker can generate it with a stolen key.
- Limit session-state database access to legitimate network locations, use least-privilege SQL permissions, and separate session-state databases between IIS servers or applications where practical.
- Run applications under designated, low-privilege application-pool identities rather than accounts with unnecessary access to other services or systems.
Investigate the full access chain
If you find suspicious IIS activity, preserve relevant logs and evidence before restarting processes or making changes that could erase volatile artifacts. Determine which application and deserialization path may have been exposed, assess whether machine keys or credentials could have been accessed, and check for use of compromised identities against databases, directory services, and SMB shares. The campaign report’s indicators and hunt guidance are available in CSO’s coverage of Sygnia’s findings.
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What the later Telerik advisory does—and does not—show
The March 2023 CISA, FBI, and MS-ISAC advisory is evidence that CVE-2019-18935 continued to be exploited on IIS systems: the agencies documented activity by multiple actors, including an APT actor, against a U.S. government server in late 2022 and early 2023. It is not evidence that Praying Mantis conducted that later campaign. The groups of activity should remain distinct unless a source explicitly connects them.
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