Detection: HTTP Rapid POST with Mixed Status Codes

Updated Date: 2025-10-14 ID: c8c987d6-3a1a-4555-9a52-eea0741b6113 Author: Raven Tait, Splunk Type: Anomaly Product: Splunk Enterprise Security

Description

This detection identifies rapid-fire POST request attacks where an attacker sends more than 20 POST requests within a 5-second window, potentially attempting to exploit race conditions or overwhelm request handling. The pattern is particularly suspicious when responses vary in size or status codes, indicating successful exploitation attempts or probing for vulnerable endpoints.

Search

1`nginx_access_logs` http_method="POST"
2| bin _time span=5s 
3| rename dest_ip as dest 
4| stats count, values(status) as status_codes, values(bytes_out) as bytes_out, values(uri_path) as uris by _time, src_ip, dest, http_user_agent 
5| where count>20 
6| table _time, dest, src_ip, count, status_codes, bytes_out, http_user_agent 
7| `http_rapid_post_with_mixed_status_codes_filter`

Data Source

Name	Platform	Sourcetype	Source
Nginx Access	N/A	`'nginx:plus:kv'`	`'/var/log/nginx/access.log'`

Macros Used

Name	Value
nginx_access_logs	`(sourcetype="nginx:plus:kv" OR sourcetype="nginx:plus:access")`
http_rapid_post_with_mixed_status_codes_filter	`search *`

http_rapid_post_with_mixed_status_codes_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
T1595	Active Scanning	Reconnaissance

Command and Control

Delivery

Reconnaissance

DE.AE

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 Risk Event	True

This configuration file applies to all detections of type anomaly. These detections will use Risk Based Alerting.

Implementation

This analytic necessitates the collection of web data, which can be achieved through Splunk Stream or by utilizing the Splunk Add-on for Apache Web Server. No additional configuration is required for this analytic.

Known False Positives

False positives may be present if the activity is part of diagnostics or testing. Filter as needed.

Associated Analytic Story

HTTP Request Smuggling

Risk Based Analytics (RBA)

Risk Message:

A potential attempt to perform request smuggling against a web server was detected. The source IP is $src_ip$ and the destination is $dest$.

Risk Object	Risk Object Type	Risk Score	Threat Objects
dest	system	40	src_ip

References

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`nginx:plus:kv`	`nginx:plus:kv`
Integration	✅ Passing	Dataset	`nginx:plus:kv`	`nginx:plus:kv`

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