The PoC targets `/etc/passwd`: it finds the offset of the running user's UID field and overwrites it with `0000`, then invokes `su` to obtain a root shell.

---

## Detection Limitations

> **Read this section before deploying any rules below.**

This exploit has two properties that significantly limit detection coverage:

**1. The write goes to the page cache, not the filesystem.**
Any detection tool that monitors file system events — `inotify`, `fanotify`, AIDE, Tripwire, auditd path watches — will **not** observe the modification. The on-disk file is never written. This means the `-p w` (write) flags in auditd path watches for `/usr/bin/su` or `/etc/passwd` will not catch the actual exploitation write.

**2. The mechanism uses legitimate kernel interfaces.**
`AF_ALG` sockets, `splice()`, and `authencesn` all have legitimate uses (IPsec, kernel self-tests, sendfile-style I/O). Detection must focus on the *combination* of these primitives rather than any one in isolation, and false positives should be expected on systems running IPsec or doing kernel crypto testing.

**What detection CAN catch:**
- The `socket(AF_ALG, SOCK_SEQPACKET, 0)` syscall
- The `splice()` syscall correlated with the above, especially near setuid binary access
- The PoC script itself (via YARA)
- The specific `authencesn(hmac(sha256),cbc(aes))` algorithm string in process memory or script files

**What detection CANNOT catch:**
- The actual page-cache write (in-memory, no filesystem event)
- Post-exploitation use of the modified page-cache entry (looks like a normal `su` or `passwd` call)
- Variants that avoid Python or the specific algorithm string

---

## Immediate Mitigation

Before deploying detection rules, apply this mitigation on any unpatched host:```bash
# Disable algif_aead kernel module — blocks the exploit primitive entirely
echo "install algif_aead /bin/false" | sudo tee /etc/modprobe.d/disable-algif-aead.conf
sudo rmmod algif_aead 2>/dev/null || true

> **注記:** `rmmod` コマンドは、モジュールが現在ロードされていない場合に失敗しますが、これは許容されます。`modprobe.d` 設定により、今後のロードが防止されます。この緩和策は、標準的なTLS、SSH、ファイルシステム暗号化のワークロードには影響しません — `authencesn` テンプレートを使用する拡張シーケンス番号付きIPsecにのみ影響します。これは、専用VPNゲートウェイ以外では一般的ではありません。

---

## YARAルール

`cve_2026_31431.yar` として保存

> **スキャン範囲:** このルールは、ディスク上またはメモリダンプから取得したPythonスクリプトファイルをスキャンするように設計されています。既知のPoCおよび類似の変種に一致します。syscallレベルでの悪用活動は検出しません — それにはauditd/Wazuhルールを使用してください。```yara
rule CVE_2026_31431_CopyFail_PoC_HighConfidence {
    meta:
        description     = "High-confidence match: CVE-2026-31431 Copy Fail PoC or close variant"
        author          = "Detection Engineering"
        reference       = "https://xint.io/blog/copy-fail-linux-distributions"
        cve             = "CVE-2026-31431"
        date            = "2026-04-30"
        severity        = "High"
        cvss            = "7.8"

    strings:
        // Algorithm string unique to this exploit path — very high fidelity
        $alg_full      = "authencesn(hmac(sha256),cbc(aes))" ascii

        // Specific socket call signature from PoC: AF_ALG=38, SOCK_SEQPACKET=5
        $socket_call   = "socket(38,5,0)" ascii

        // SOL_ALG socket option (decimal 279)
        $solalg        = "setsockopt(279" ascii

        // Hex key/iv payload written via setsockopt in PoC
        $key_payload   = "0800010000000010" ascii

        // splice() usage in context of AEAD operations
        $splice        = "splice(" ascii

        // Target indicators from PoC (page-cache corruption targets)
        $target_passwd = "/etc/passwd" ascii
        $target_su     = "/usr/bin/su" ascii

        // AF_ALG aead bind strings
        $aead_bind     = "\"aead\"" ascii

    condition:
        // High-confidence: unique algorithm string alone is sufficient
        $alg_full
        or
        // Medium-confidence: socket primitive + option number
        ($socket_call and $solalg)
        or
        // Medium-confidence: splice into AEAD socket targeting a setuid path
        ($aead_bind and $splice and ($target_passwd or $target_su))
        or
        // PoC hex payload present alongside splice
        ($key_payload and $splice)
}

rule CVE_2026_31431_CopyFail_Mechanism {
    meta:
        description     = "Behavioral: AF_ALG AEAD + splice combination suggestive of CVE-2026-31431 technique"
        author          = "Detection Engineering"
        reference       = "https://xint.io/blog/copy-fail-linux-distributions"
        cve             = "CVE-2026-31431"
        date            = "2026-04-30"
        severity        = "Medium"
        note            = "Higher false positive rate than HighConfidence rule — review matches in context"

    strings:
        $authencesn    = "authencesn" ascii nocase
        $af_alg_num    = "socket(38" ascii
        $sol_alg_num   = "279" ascii
        $splice        = "splice(" ascii

    condition:
        ($authencesn and $splice)
        or
        ($af_alg_num and $sol_alg_num and $splice)
}

入力が空のため、翻訳するテキストがありません。```bash
## ============================================================
## CVE-2026-31431 "Copy Fail" — Auditd Detection Rules
## ============================================================
## These rules capture the MECHANISM of the exploit (socket +
## splice syscalls) and correlated /etc/passwd access patterns.
##
## IMPORTANT: These rules will NOT detect the page-cache write
## itself — it is an in-memory operation with no filesystem
## event. File path watches (-w) on setuid binaries or
## /etc/passwd will not fire on the exploit write.
##
## Correlate rule hits across audit.key values to build signal:
## A hit on afalg_socket followed closely by a hit on
## splice_syscall from the same process is a strong indicator.
## ============================================================

## --- Core exploit primitive: AF_ALG socket creation ---
## Monitors socket(2) syscall where a0 = 0x26 (38 decimal = AF_ALG)
## This is the first step of the exploit chain.
-a always,exit -F arch=b64 -S socket -F a0=0x26 -k cve_2026_31431_afalg_socket
-a always,exit -F arch=b32 -S socket -F a0=0x26 -k cve_2026_31431_afalg_socket

## --- splice() syscall monitoring ---
## splice() is used to feed page-cache pages into the AEAD socket.
## NOTE: splice() is commonly used for sendfile-like operations.
## Correlate with cve_2026_31431_afalg_socket hits from the same PID.
-a always,exit -F arch=b64 -S splice -k cve_2026_31431_splice
-a always,exit -F arch=b32 -S splice -k cve_2026_31431_splice

## --- /etc/passwd access monitoring ---
## The PoC reads /etc/passwd to locate the UID field offset.
## Read access (-p r) is retained here because the intent is
## to correlate this read with the AF_ALG socket key above,
## not to use the watch as a standalone alert.
-w /etc/passwd -p rwa -k cve_2026_31431_passwd_access

## --- setuid binary execution monitoring ---
## Detects execution of su after page-cache modification.
## The page-cache write makes su execute as root; this catches
## the exploitation outcome, not the write itself.
-w /usr/bin/su   -p xa -k cve_2026_31431_su_exec
-w /usr/bin/sudo -p xa -k cve_2026_31431_sudo_exec

## --- algif_aead module state monitoring ---
## The exploit requires algif_aead to be loaded.
## Monitoring modprobe helps detect attempts to load the module
## on systems where it was previously disabled as a mitigation,
## and confirms whether the mitigation is being bypassed.
-a always,exit -F arch=b64 -S finit_module -S init_module -k cve_2026_31431_module_load
-w /etc/modprobe.d -p wa -k cve_2026_31431_modprobe_conf

---

## Wazuh ルール

ローカルルールファイルとして保存します（通常は `/var/ossec/etc/rules/local_rules.xml`）。

> **前提条件:** これらのルールは、auditd が上記のルールで設定され、Wazuh の auditd デコーダーが有効になっていることに依存します。これらのルールは、auditd によって設定される `audit.key` フィールドに一致します。これは、2 つのシステムを橋渡しするための正確で信頼性の高い方法です。ルールでは、Wazuh のバージョン間での互換性を維持するために、特定の `<if_sid>` ではなく `<if_group>auditd</if_group>` を使用します。```xml
<!-- ============================================================
     CVE-2026-31431 "Copy Fail" — Wazuh Correlation Rules
     Requires: auditd rules from cve-2026-31431.rules deployed
     ============================================================ -->

<!-- Level 10: AF_ALG socket creation detected -->
<rule id="112001" level="10">
    <if_group>auditd</if_group>
    <field name="audit.key">cve_2026_31431_afalg_socket</field>
    <description>CVE-2026-31431 Copy Fail: AF_ALG socket (family 38) created by unprivileged process</description>
    <group>cve,privilege_escalation,linux,kernel,crypto,</group>
</rule>

<!-- Level 10: splice() syscall detected -->
<rule id="112002" level="10">
    <if_group>auditd</if_group>
    <field name="audit.key">cve_2026_31431_splice</field>
    <description>CVE-2026-31431 Copy Fail: splice() syscall detected — monitor for correlation with AF_ALG socket rule</description>
    <group>cve,privilege_escalation,linux,kernel,</group>
</rule>

<!-- Level 14 CRITICAL: AF_ALG socket followed by splice() from the same source -->
<!-- This chaining is the core exploit mechanism                                 -->
<rule id="112003" level="14">
    <if_matched_sid>112001</if_matched_sid>
    <if_group>auditd</if_group>
    <field name="audit.key">cve_2026_31431_splice</field>
    <same_field>audit.pid</same_field>
    <description>CVE-2026-31431 Copy Fail CRITICAL: AF_ALG socket creation followed by splice() from same process — active exploitation likely</description>
    <group>cve,privilege_escalation,linux,kernel,crypto,high_confidence,</group>
</rule>

<!-- Level 12: /etc/passwd access correlated with AF_ALG activity -->
<rule id="112004" level="12">
    <if_matched_sid>112001</if_matched_sid>
    <if_group>auditd</if_group>
    <field name="audit.key">cve_2026_31431_passwd_access</field>
    <description>CVE-2026-31431 Copy Fail: /etc/passwd access following AF_ALG socket creation — consistent with PoC target selection</description>
    <group>cve,privilege_escalation,linux,kernel,</group>
</rule>

<!-- Level 13: su or sudo executed after AF_ALG socket was created -->
<!-- This may represent execution of the modified page-cache entry  -->
<rule id="112005" level="13">
    <if_matched_sid>112001</if_matched_sid>
    <if_group>auditd</if_group>
    <field name="audit.key">cve_2026_31431_su_exec|cve_2026_31431_sudo_exec</field>
    <description>CVE-2026-31431 Copy Fail: su/sudo execution following AF_ALG socket creation — possible post-exploitation</description>
    <group>cve,privilege_escalation,linux,kernel,</group>
</rule>

<!-- Level 12: Attempt to load algif_aead after it was disabled as a mitigation -->
<rule id="112006" level="12">
    <if_group>auditd</if_group>
    <field name="audit.key">cve_2026_31431_module_load</field>
    <field name="audit.exe" type="pcre2">^.*(python|python3|insmod|modprobe).*$</field>
    <description>CVE-2026-31431 Copy Fail: Kernel module load attempt — verify algif_aead mitigation has not been bypassed</description>
    <group>cve,privilege_escalation,linux,kernel,</group>
</rule>

<!-- Level 13: modprobe.d config modified — possible mitigation removal -->
<rule id="112007" level="13">
    <if_group>auditd</if_group>
    <field name="audit.key">cve_2026_31431_modprobe_conf</field>
    <description>CVE-2026-31431 Copy Fail: /etc/modprobe.d modified — verify algif_aead disable config has not been removed</description>
    <group>cve,privilege_escalation,linux,kernel,</group>
</rule>

---

## パッチ適用と修復

### カーネルパッチ

| ブランチ | 修正バージョン | 修正コミット |
|--------|--------------|------------|
| Stable 6.18.x | 6.18.22 | `fafe0fa2995a0f7073c1c358d7d3145bcc9aedd8` |
| Stable 6.19.x | 6.19.12 | `ce42ee423e58dffa5ec03524054c9d8bfd4f6237` |
| Mainline | 7.0 | `a664bf3d603dc3bdcf9ae47cc21e0daec706d7a5` |

この修正は、`algif_aead.c` における2017年のインプレースAEAD最適化をアウトオブプレース操作に戻し、ページキャッシュのページが書き込み可能なscatterlistに配置されないようにします。

### ディストリビューション別のガイダンス

| ディストリビューション | 対応 |
|---|---|
| Ubuntu | `apt-get update && apt-get upgrade linux-image-generic`; USNアドバイザリを確認してください |
| RHEL / Rocky / Alma | `dnf update kernel`; RHSBアドバイザリを確認してください |
| Amazon Linux 2023 | `dnf update kernel`; ALASアドバイザリを確認してください |
| SUSE / openSUSE | `zypper update kernel-default`; SUSE SAアドバイザリを確認してください |
| Debian | セキュリティトラッカーを確認してください; バックポートされたパッチがカーネル更新より先に提供される場合があります |
| Arch | `pacman -Syu` (ローリングリリース; 上流の修正が入り次第取り込みます) |

### 曝露後の整合性検証

パッチ適用前にホスト上で悪用が発生した疑いがある場合:```bash
# 1. Check if /etc/passwd UID fields have been tampered
# (compare against a known-good backup or secondary host)
awk -F: '$3 ~ /^0+$/ && $1 != "root" {print "SUSPICIOUS UID 0 ENTRY:", $0}' /etc/passwd

# 2. Drop the page cache to flush any in-memory modifications
# WARNING: This impacts performance temporarily
sync && echo 3 | sudo tee /proc/sys/vm/drop_caches

# 3. Verify setuid binaries against package manager
rpm -Va --nomtime 2>/dev/null | grep -E '^.{0,8}5.*su$|^.{0,8}5.*sudo$'   # RHEL/rpm
debsums -s 2>/dev/null | grep -E 'su|sudo'                                  # Debian/Ubuntu

# 4. Re-examine recently logged su/sudo invocations for unexpected UID transitions
journalctl -u sudo --since "48 hours ago" | grep "session opened for user root"