Detection: HTTP Request to Reserved Name on IIS Server

Updated Date: 2025-10-17 ID: 1e45e6a8-110b-4886-b815-8d69cf35bf0a Author: Raven Tait, Splunk Type: TTP Product: Splunk Enterprise Security

Description

Detects attempts to exploit a request smuggling technique against IIS that leverages a Windows quirk where requests for reserved Windows device names such as "/con" trigger an early server response before the request body is received. When combined with a Content-Length desynchronization, this behavior can lead to a parsing confusion between frontend and backend.

Search

1
2| tstats count min(_time) as firstTime max(_time) as lastTime from datamodel=Web where Web.url IN ("/con", "/prn", "/aux", "/nul", "/com1","/com2","/com3","/com4", "/com5","/com6","/com7") by Web.src, Web.dest, Web.http_user_agent, Web.url, Web.status, Web.http_method 
3| `drop_dm_object_name("Web")` 
4| `security_content_ctime(firstTime)` 
5| `security_content_ctime(lastTime)` 
6| `http_request_to_reserved_name_on_iis_server_filter`

Data Source

Name	Platform	Sourcetype	Source
Suricata	N/A	`'suricata'`	`'suricata'`

Macros Used

Name	Value
security_content_ctime	`convert timeformat="%Y-%m-%dT%H:%M:%S" ctime($field$)`
http_request_to_reserved_name_on_iis_server_filter	`search *`

http_request_to_reserved_name_on_iis_server_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK

+ Kill Chain Phases

+ NIST

+ CIS

- Threat Actors

ID	Technique	Tactic
T1071.001	Web Protocols	Command And Control
T1190	Exploit Public-Facing Application	Initial Access

Command and Control

Delivery

DE.CM

CIS 13

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Setting	Value
Disabled	true
Cron Schedule	`0 * * * *`
Earliest Time	`-70m@m`
Latest Time	`-10m@m`
Schedule Window	`auto`
Creates Notable	Yes
Rule Title	`%name%`
Rule Description	`%description%`
Notable Event Fields	user, dest
Creates Risk Event	True

This configuration file applies to all detections of type TTP. These detections will use Risk Based Alerting and generate Notable Events.

Implementation

To implement this analytic, ensure proper logging is occurring with IIS, Apache, or a Proxy server and that these logs are being ingested into Splunk. The analytic was written against Suricata. The proper TA will need to be enabled and should be mapped to CIM and the Web datamodel. Ingestion of the data source is required to utilize this detection. In addition, if it is not mapped to the datamodel, modify the query for your application logs to look for requests the same URI and investigate further.

Known False Positives

False positives are not expected on IIS servers, as the detection is based on the presence of web requests to reserved names, which is not a common page to be accessed by legitimate users. Modify the query as needed to reduce false positives or hunt for additional indicators of compromise.

Associated Analytic Story

HTTP Request Smuggling

Risk Based Analytics (RBA)

Risk Message:

Known scripting tool was used against a web request. The source IP is $src$ and the destination is $dest$.

Risk Object	Risk Object Type	Risk Score	Threat Objects
dest	system	31	src

References

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`suricata`	`suricata`
Integration	✅ Passing	Dataset	`suricata`	`suricata`

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 1