Linux Auditd Copy Fail Privilege Escalation

name: Linux Auditd Copy Fail Privilege Escalation
id: dd16294f-44d2-40b4-a869-542c0b85113a
version: 2
creation_date: '2026-05-04'
modification_date: '2026-05-13'
author: Raven Tait, Nasreddine Bencherchali, Splunk
status: production
type: TTP
description: |-
    Detects the exploitation pattern associated with Copy Fail.
    Copy Fail (CVE-2026-31431) is a logic bug in the Linux kernel's authentication cryptographic template.
    It lets an unprivileged local user trigger a deterministic, controlled 4-byte write into the page cache of any readable file on the system.
    A single 732-byte Python script can edit a setuid binary and obtain root on essentially all Linux distributions shipped since 2017.
    This search relies on the auditd configuration linked in the references section. If you are using a custom configuration, make sure the Key names are the same or adapt the search accordingly.
data_source:
    - Linux Auditd Syscall
search: |-
    `linux_auditd`
    type=SYSCALL
    key IN (
        "af_alg",
        "process_creation",
        "splice_user"
    )
    | eval setuid_binary = case(
        name IN (
            "/usr/bin/chfn",
            "/usr/bin/chsh",
            "/usr/bin/fusermount3",
            "/usr/bin/gpasswd",
            "/usr/bin/mount",
            "/usr/bin/newgrp",
            "/usr/bin/passwd",
            "/usr/bin/su",
            "/usr/bin/sudo",
            "/usr/bin/umount",
            "/usr/lib/dbus-1.0/dbus-daemon-launch-helper",
            "/usr/lib/landscape/apt-update",
            "/usr/lib/openssh/ssh-keysign",
            "/usr/lib/polkit-1/polkit-agent-helper-1"
        ), name,
        exe IN (
            "/usr/bin/chfn",
            "/usr/bin/chsh",
            "/usr/bin/fusermount3",
            "/usr/bin/gpasswd",
            "/usr/bin/mount",
            "/usr/bin/newgrp",
            "/usr/bin/passwd",
            "/usr/bin/su",
            "/usr/bin/sudo",
            "/usr/bin/umount",
            "/usr/lib/dbus-1.0/dbus-daemon-launch-helper",
            "/usr/lib/landscape/apt-update",
            "/usr/lib/openssh/ssh-keysign",
            "/usr/lib/polkit-1/polkit-agent-helper-1"
        ), exe,
        true(), null()
    )
    | eval indicator = case(
        key="af_alg",        "AF_ALG socket",
        key="splice_user",   "splice syscall",
        isnotnull(setuid_binary), "setuid_exec:" . setuid_binary,
        true(),              null()
    )
    | where isnotnull(indicator)
    | stats
        dc(indicator) as unique_signals
        max(_time) as lastTime
        min(_time) as firstTime
        values(comm) as comm
        values(exe) as exe
        values(name) as name
        values(host) as dest
        values(indicator) as signals
        values(setuid_binary) as setuid_binaries
        values(pid) as pid
        values(ppid) as ppid
        values(uid) as uid
        by auid
    | where unique_signals >= 3
    | eval risk_score_factor = unique_signals * 25
    | sort - risk_score_factor
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `linux_auditd_copy_fail_privilege_escalation_filter`
how_to_implement: |-
    To implement this detection, the process begins by ingesting auditd data, that consist SYSCALL, TYPE, EXECVE and PROCTITLE events, which captures command-line executions and process details on Unix/Linux systems. These logs should be ingested and processed using Splunk Add-on for Unix and Linux (https://splunkbase.splunk.com/app/833), which is essential for correctly parsing and categorizing the data. Ensure specifically that AF_ALG socket creation and splice syscall is being monitored. The next step involves normalizing the field names  to match the field names set by the Splunk Common Information Model (CIM) to ensure consistency across different data sources and enhance the efficiency of data modeling and make sure the type=CWD record type is activate in your auditd configuration. This approach enables effective monitoring and detection of linux endpoints where auditd is deployed.
known_false_positives: No false positives have been identified at this time.
references:
    - https://xint.io/blog/copy-fail-linux-distributions
    - https://github.com/theori-io/copy-fail-CVE-2026-31431
    - https://github.com/Neo23x0/auditd/blob/master/audit.rules
finding:
    title: Privilege escalation attempt leveraging the Copy Fail vulnerability was observed on $dest$.
    entity:
        field: dest
        type: system
        score: 50
threat_objects:
    - field: exe
      type: process
analytic_story:
    - Linux Privilege Escalation
asset_type: Endpoint
cve:
    - CVE-2026-31431
mitre_attack_id:
    - T1068
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint