Webserver logs: Webshell ReGeorg indicators in POST URI query with null Referer/User-Agent

Flags HTTP POST requests with null referer/user-agent and ReGeorg-like URI query parameters in web logs.

FreeReviewedSigma · High · v2

Category: webserver
Author: Cian Heasley (SigmaHQ), DRL 1.1
Published: 2020-08-04
Updated: 2026-07-31

ATT&CK techniques

Persistence

Recon
Resource Dev
Initial Access
Execution
Persistence
- T1505.003Web Shell
Priv Esc
Defense Evasion
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact

What it detects

This rule flags HTTP POST requests where the URI query contains specific parameter strings associated with ReGeorg webshell activity while the request has null Referer and null User-Agent. Attackers using webshells often blend command-and-control parameters into legitimate-looking web requests, so these distinctive query fragments combined with missing browser headers can be a useful signal. It relies on webserver log fields including the URI query, HTTP method, Referer, and User-Agent.

Reporting behind it

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Webserver logs: Webshell ReGeorg indicators in POST URI query with null Referer/User-Agent

Pivot detection · T1505.003 · 9 related

Changelog

v2
Candidate ingested via manual entry.2026-07-31
v1
No changelog recorded for this version.2026-07-30

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.

webserver-log-detection-of-regeorg-webshell-http-post-probing-uri-query-2ea44a60

title: "Webserver logs: Webshell ReGeorg indicators in POST URI query with null Referer/User-Agent"
id: 57048b7f-8412-40b3-86e0-fa7188b8e8b6
status: test
description: This rule flags HTTP POST requests where the URI query contains specific parameter strings associated with ReGeorg webshell activity while the request has null Referer and null User-Agent. Attackers using webshells often blend command-and-control parameters into legitimate-looking web requests, so these distinctive query fragments combined with missing browser headers can be a useful signal. It relies on webserver log fields including the URI query, HTTP method, Referer, and User-Agent.
references:
  - https://community.rsa.com/community/products/netwitness/blog/2019/02/19/web-shells-and-netwitness-part-3
  - https://github.com/sensepost/reGeorg
  - https://github.com/SigmaHQ/sigma/blob/master/rules/web/webserver_generic/web_webshell_regeorg.yml
author: Cian Heasley, Huntrule Team
date: 2020-08-04
modified: 2023-01-02
tags:
  - attack.persistence
  - attack.t1505.003
logsource:
  category: webserver
detection:
  selection:
    cs-uri-query|contains:
      - cmd=read
      - connect&target
      - cmd=connect
      - cmd=disconnect
      - cmd=forward
  filter:
    cs-referer: null
    cs-user-agent: null
    cs-method: POST
  condition: selection and filter
falsepositives:
  - Web applications that use the same URL parameters as ReGeorg
level: high
license: DRL-1.1
related:
  - id: 2ea44a60-cfda-11ea-87d0-0242ac130003
    type: derived

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