Detection: Linux Shell Pseudo Device Reverse Shell

Description

The following analytic detects the use of shell's /dev/tcp or /dev/udp pseudo-device feature to establish outbound network connections. This special file system interface allows opening a network socket and sending or receiving data using simple shell commands. Attackers commonly abuse this native shell capability to create reverse shells without requiring external tools such as netcat, by redirecting shell I/O over a TCP or UDP connection to an attacker-controlled host and port.

Search

 1
 2| tstats `security_content_summariesonly`
 3  count min(_time) as firstTime
 4        max(_time) as lastTime
 5
 6from datamodel=Endpoint.Processes where
 7
 8Processes.process_name IN (
 9    "bash",
10    "dash",
11    "sh",
12    "zsh"
13)
14Processes.process IN (
15    "*/dev/tcp/*",
16    "*/dev/udp/*"
17)
18
19by Processes.process Processes.vendor_product
20   Processes.user_id Processes.process_hash Processes.parent_process_name Processes.parent_process_exec
21   Processes.action Processes.dest Processes.process_current_directory Processes.process_path
22   Processes.process_integrity_level Processes.original_file_name Processes.parent_process
23   Processes.parent_process_path Processes.parent_process_guid Processes.parent_process_id
24   Processes.process_guid Processes.process_id Processes.user Processes.process_name
25
26
27| `drop_dm_object_name(Processes)`
28
29| `security_content_ctime(firstTime)`
30
31| `security_content_ctime(lastTime)`
32
33|`linux_shell_pseudo_device_reverse_shell_filter`

Data Source

Name	Platform	Sourcetype	Source
Sysmon for Linux EventID 1	Linux	'sysmon:linux'	'Syslog:Linux-Sysmon/Operational'

Macros Used

Annotations

Default Configuration

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

Implementation

The detection is based on data that originates from Endpoint Detection and Response (EDR) agents. These agents are designed to provide security-related telemetry from the endpoints where the agent is installed. To implement this search, you must ingest logs that contain the process GUID, process name, and parent process. Additionally, you must ingest complete command-line executions. These logs must be processed using the appropriate Splunk Technology Add-ons that are specific to the EDR product. The logs must also be mapped to the Processes node of the Endpoint data model. Use the Splunk Common Information Model (CIM) to normalize the field names and speed up the data modeling process.

Known False Positives

Legitimate system administrators or developers may use /dev/tcp or /dev/udp for network diagnostics or testing purposes. Filter based on known authorized users and trusted source systems.

Associated Analytic Story

• Command And Control

• Compromised Linux Host

• Linux Post-Exploitation

• Linux Privilege Escalation

• Linux Persistence Techniques

Intermediate Findings

Threat Objects

References

• https://swisskyrepo.github.io/InternalAllTheThings/cheatsheets/shell-reverse-cheatsheet/#bash-tcp

Detection Testing

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