Detection: Linux Telnet Authentication Bypass

Updated Date: 2026-05-13 ID: 6e0913d4-5461-487c-9dce-6d22ef2c0f03 Author: Raven Tait, Splunk Type: TTP Product: Splunk Enterprise Security

Description

Detects an authentication bypass in telnet tracked as CVE-2026-24061. An attacker can supply a specifically crafted USER environment variable (-f root) that is passed to /usr/bin/login. Because this input isn't sanitized an attacker can force the system to skip authentication and login directly as root.

Search

 1
 2| tstats `security_content_summariesonly` count min(_time) as firstTime max(_time) as lastTime FROM datamodel=Endpoint.Processes
 3  WHERE Processes.process_name = "login" Processes.parent_process_name = "telnetd" Processes.process = "* -p *" Processes.process = "* -f root*"
 4  BY Processes.action Processes.dest Processes.original_file_name
 5     Processes.parent_process Processes.parent_process_exec Processes.parent_process_guid
 6     Processes.parent_process_id Processes.parent_process_name Processes.parent_process_path
 7     Processes.process Processes.process_exec Processes.process_guid
 8     Processes.process_hash Processes.process_id Processes.process_integrity_level
 9     Processes.process_name Processes.process_path Processes.user
10     Processes.user_id Processes.vendor_product
11
12| `drop_dm_object_name(Processes)`
13
14| `security_content_ctime(firstTime)`
15
16| `security_content_ctime(lastTime)`
17
18| `linux_telnet_authentication_bypass_filter`

Data Source

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

Macros Used

Name	Value
security_content_summariesonly	`summariesonly=`summariesonly_config `allow_old_summaries=`oldsummaries_config `fillnull_value=`fillnull_config``
linux_telnet_authentication_bypass_filter	`search *`

linux_telnet_authentication_bypass_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
T1548	Abuse Elevation Control Mechanism	Privilege Escalation

Exploitation

DE.CM

CIS 10

CVE

CVE-2026-24061

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 Finding (Notable)	Yes
Rule Title	`%name%`
Rule Description	`%description%`
Notable Event Fields	user, dest
Creates Intermediate Finding (Risk Event)	Yes

TTP detections generate a Finding (Notable) and may generate Intermediate Findings (Risk Events) for associated entities.

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

It is rare for the telnetd to spawn login process with these arguments.

Associated Analytic Story

Telnetd CVE-2026-24061

Finding

Title	Entity Field	Entity Type	Risk Score
An instance of $parent_process_name$ spawning $process_name$ with CommandLine $process$ was identified on endpoint $dest$ by user $user$ related to an authentication bypass in telnetd.	user	user	50

Intermediate Findings

Message	Entity Field	Entity Type	Risk Score
An instance of $parent_process_name$ spawning $process_name$ with CommandLine $process$ was identified on endpoint $dest$ by user $user$ related to an authentication bypass in telnetd.	dest	system	50

Threat Objects

Field	Type
process_name	process_name
process	process
parent_process	parent_process

References

https://www.safebreach.com/blog/safebreach-labs-root-cause-analysis-and-poc-exploit-for-cve-2026-24061/

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`Syslog:Linux-Sysmon/Operational`	`sysmon:linux`
Integration	✅ Passing	Dataset	`Syslog:Linux-Sysmon/Operational`	`sysmon:linux`

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: 5