Detection: Linux PF_ALG Registration Outside of Boot Window

Updated Date: 2026-05-11 ID: 8a9684ae-52e6-4a57-aea4-8a9f72942564 Author: Raven Tait, Splunk Type: TTP Product: Splunk Enterprise Security

Description

The following analytic detects when the AF_ALG kernel crypto socket interface being loaded more than 300 seconds after system boot, which is a primary kernel-level indicator of Copy Fail (CVE-2026-31431) exploitation activity on Debian and Ubuntu family systems. The AF_ALG interface is required by the exploit to access the vulnerable authencesn crypto code path, and on systems where it is not auto-loaded at boot, its on-demand registration by an unprivileged process is a strong indicator of exploitation in progress.

Search

 1sourcetype="linux_messages_syslog" "NET: Registered PF_ALG protocol family"
 2
 3| rex field=_raw "kernel: \[\s*(?<uptime_seconds>[\d\.]+)\]"
 4
 5| eval uptime_seconds=tonumber(uptime_seconds)
 6
 7| where uptime_seconds > 300
 8
 9| eval uptime_readable=tostring(round(uptime_seconds/60,1)) . " minutes after boot"
10
11| rename host as dest
12
13| table _time dest uptime_seconds uptime_readable _raw
14
15| sort -uptime_seconds
16
17| `linux_pf_alg_registration_outside_of_boot_window_filter`

Data Source

Name	Platform	Sourcetype	Source
Linux Messages Syslog	Linux	`'linux_messages_syslog'`	`'/var/log/kern'`

Macros Used

Name	Value

| linux_pf_alg_registration_outside_of_boot_window_filter | search * |

linux_pf_alg_registration_outside_of_boot_window_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
T1068	Exploitation for Privilege Escalation	Privilege Escalation

Exploitation

DE.CM

CIS 10

CVE

CVE-2026-31431

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)	No

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

Implementation

To successfully implement this search, you need to have relevant kernel logs ingested with the Splunk Add-On for Unix and Linux (https://splunkbase.splunk.com/app/833).

Known False Positives

AF_ALG can be legitimately loaded after boot by on-demand LUKS volume mounts, IPsec VPN clients establishing tunnels, or OpenSSL deployments with the afalg engine enabled, making this signal most reliable on dedicated server infrastructure where these operations are handled at boot time.

Associated Analytic Story

Linux Privilege Escalation

Finding

Title	Entity Field	Entity Type	Risk Score
AF_ALG instantiated after boot on $dest$ indicating possible privilege escalation.	dest	system	50

References

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`/var/log/kern`	`linux_messages_syslog`
Integration	✅ Passing	Dataset	`/var/log/kern`	`linux_messages_syslog`

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