Linux auditd: Detect AF_ALG socket() syscall (address family 38) for crypto API abuse

Flags AF_ALG (38) socket() creations from Linux auditd while filtering common legitimate crypt/VPN tools.

FreeReviewedSigma · High · v5

Product: linux
Service: auditd
Author: Gene Kazimiarovich (SigmaHQ), DRL 1.1
Published: 2026-04-30
Updated: 2026-07-31

ATT&CK techniques

Priv Esc

Recon
Resource Dev
Initial Access
Execution
Persistence
Priv Esc
- T1068Exploitation for Privilege Escalation
Defense Evasion
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact

What it detects

This rule identifies processes creating AF_ALG sockets by matching the Linux socket() syscall argument for address family 38 via auditd. AF_ALG is the kernel crypto API interface, and attackers may use it to reach kernel code paths tied to cryptographic operations. The detection relies on auditd telemetry for the socket syscall and its recorded syscall arguments, and reduces noise by excluding a small set of known legitimate binaries.

Reporting behind it

Related detections9 linkedT1068 — drag to rearrange

Malicious EDR Evasion Driver Load from AnyDesk Directory via k.sys (via image_load)

Malicious Zemana Vulnerable Driver Load for EDR Termination (via image_load)

Suspicious EDRKillShifter Vulnerable Driver Load from User Temp Directory (via image_load)

Suspicious Process Explorer Kernel Driver Dropped as PED.sys via File Creation (via file_event)

Malicious Payload Download From copy.fail Exploit Host on Linux (via process_creation)

Suspicious su Execution From Uncommon Parent Process on Linux (via process_creation)

Possible noPac sAMAccountName Spoofing via Machine Account Rename

Possible macOS TCC Bypass via mdimport Loading Custom Spotlight Importer

Suspicious CLFS Base Log File Created in SkyPDF ProgramData Path during Zero-Day Exploitation (via file_event)

Linux auditd: Detect AF_ALG socket() syscall (address family 38) for crypto API abuse

Pivot detection · T1068 · 9 related

Changelog

v5
Candidate ingested via manual entry.2026-07-31
v4
Candidate ingested via manual entry.2026-07-31
v3
Candidate ingested via manual entry.2026-07-31
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.

linux-auditd-af-alg-address-family-38-socket-syscall-creation-474b415a

title: "Linux auditd: Detect AF_ALG socket() syscall (address family 38) for crypto API abuse"
id: de5a5192-0a45-4255-b817-50cc9fe74280
status: experimental
description: This rule identifies processes creating AF_ALG sockets by matching the Linux socket() syscall argument for address family 38 via auditd. AF_ALG is the kernel crypto API interface, and attackers may use it to reach kernel code paths tied to cryptographic operations. The detection relies on auditd telemetry for the socket syscall and its recorded syscall arguments, and reduces noise by excluding a small set of known legitimate binaries.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-31431
  - https://man7.org/linux/man-pages/man2/socket.2.html
  - https://www.kernel.org/doc/html/latest/crypto/userspace-if.html
  - https://xint.io/blog/copy-fail-linux-distributions
  - https://github.com/theori-io/copy-fail-CVE-2026-31431
  - https://github.com/torvalds/linux/blob/81d6f7807536a0436dfada07e9292e3702d2bed4/include/linux/socket.h#L245
  - https://github.com/SigmaHQ/sigma/blob/master/rules-emerging-threats/2026/Exploits/CVE-2026-31431/lnx_auditd_exploit_cve_2026_31431_af_alg_socket_creation.yml
author: Gene Kazimiarovich, Huntrule Team
date: 2026-04-30
tags:
  - attack.privilege-escalation
  - attack.t1068
  - detection.emerging-threats
  - cve.2026-31431
logsource:
  product: linux
  service: auditd
  definition: |
    Required auditd configuration:
    auditctl -a always,exit -F arch=b64 -S socket -k af_alg_detect
detection:
  selection:
    type: SYSCALL
    SYSCALL: socket
    a0: 26
  filter_main_legitimate:
    exe|endswith:
      - /cryptsetup
      - /kcapi-speed
      - /kcapi-enc
      - /kcapi-dgst
      - /charon
      - /charon-systemd
  condition: selection and not 1 of filter_main_*
falsepositives:
  - Disk encryption utilities using kernel crypto offload (cryptsetup with --use-kernel-crypto)
  - Libkcapi userspace tools for kernel crypto API benchmarking
  - StrongSwan IKE daemon with kernel-libipsec plugin
level: high
license: DRL-1.1
related:
  - id: 474b415a-8b3d-4e6a-9f12-0d5c8a7b6e94
    type: derived

Convert

This rule as a query you can run. Uses no unlock credit.

SplunkSPL