HTTP Rapid POST with Mixed Status Codes

name: HTTP Rapid POST with Mixed Status Codes
id: c8c987d6-3a1a-4555-9a52-eea0741b6113
version: 5
creation_date: '2025-10-21'
modification_date: '2026-05-13'
author: Raven Tait, Splunk
status: production
type: Anomaly
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.
data_source:
    - Nginx Access
search: |-
    `nginx_access_logs` http_method="POST"
      | bin _time span=5s
      | rename dest_ip as dest
      | 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
      | where count>20
      | table _time, dest, src_ip, count, status_codes, bytes_out, http_user_agent
      | `http_rapid_post_with_mixed_status_codes_filter`
how_to_implement: 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.
references:
    - https://portswigger.net/web-security/request-smuggling#what-is-http-request-smuggling
    - https://portswigger.net/research/http1-must-die
    - https://www.vaadata.com/blog/what-is-http-request-smuggling-exploitations-and-security-best-practices/
    - https://www.securityweek.com/new-http-request-smuggling-attacks-impacted-cdns-major-orgs-millions-of-websites/
intermediate_findings:
    entities:
        - field: dest
          type: system
          score: 20
          message: A potential attempt to perform request smuggling against a web server was detected. The source IP is $src_ip$ and the destination is $dest$.
threat_objects:
    - field: src_ip
      type: ip_address
analytic_story:
    - HTTP Request Smuggling
asset_type: Web Server
mitre_attack_id:
    - T1071.001
    - T1190
    - T1595
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: web
security_domain: network