该PoC针对`/etc/passwd`：它找到当前用户UID字段的偏移量，并将其覆盖为`0000`，然后调用`su`获取root shell。

---

## 检测限制

> **在部署以下规则之前，请先阅读本节内容。**

该漏洞利用有两个特性，极大地限制了检测覆盖范围：

**1. 写入操作针对的是页缓存，而非文件系统。**
任何监控文件系统事件的检测工具——`inotify`、`fanotify`、AIDE、Tripwire、auditd路径监控——都**不会**观察到该修改。磁盘上的文件从未被写入。这意味着auditd路径监控中针对`/usr/bin/su`或`/etc/passwd`的`-p w`（写入）标志将不会捕获实际的漏洞利用写入。

**2. 该机制使用了合法的内核接口。**
`AF_ALG`套接字、`splice()`和`authencesn`都有合法用途（IPsec、内核自测试、sendfile风格的I/O）。检测必须关注这些原始操作的**组合**，而非孤立地看待任何一个，并且在运行IPsec或进行内核加密测试的系统上，预期会出现误报。

**检测能够捕捉到的内容：**
- `socket(AF_ALG, SOCK_SEQPACKET, 0)`系统调用
- 与上述相关的`splice()`系统调用，尤其是在访问setuid二进制文件附近
- PoC脚本本身（通过YARA）
- 进程内存或脚本文件中特定的`authencesn(hmac(sha256),cbc(aes))`算法字符串

**检测无法捕捉到的内容：**
- 实际的页缓存写入（内存中，无文件系统事件）
- 利用后对修改后的页缓存条目的使用（看起来像正常的`su`或`passwd`调用）
- 避开Python或特定算法字符串的变种

---

## 即时缓解措施

在部署检测规则之前，请在未打补丁的主机上应用此缓解措施：```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

> **注意：** `rmmod` 命令在模块未加载时会失败；这是可接受的。`modprobe.d` 配置可防止未来加载。此缓解措施对标准 TLS、SSH 或文件系统加密工作负载没有影响——它仅影响使用 `authencesn` 模板的具有扩展序列号的 IPsec，这在不属于专用 VPN 网关的情况外并不常见。

---

## YARA 规则

保存为 `cve_2026_31431.yar`

> **扫描范围：** 此规则旨在扫描磁盘上的或从内存转储中提取的 Python 脚本文件。它将匹配已知的 PoC 和相近变体。它不会检测系统调用级别的利用活动——请使用 auditd/Wazuh 规则进行检测。```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)
}

（空）```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 规则

保存为本地规则文件（通常为 `/var/ossec/etc/rules/local_rules.xml`）。

> **先决条件：** 这些规则依赖于已按上述规则配置的 auditd 以及生效的 Wazuh auditd 解码器。它们匹配由 auditd 填充的 `audit.key` 字段，这是连接两个系统的正确且可靠的方式。规则使用 `<if_group>auditd</if_group>` 而非特定的 `<if_sid>`，以保持与不同 Wazuh 版本的兼容性。```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>

---

## 补丁与修复

### 内核补丁

| 分支 | 修复版本 | 修复提交 |
|--------|--------------|------------|
| 稳定版 6.18.x | 6.18.22 | `fafe0fa2995a0f7073c1c358d7d3145bcc9aedd8` |
| 稳定版 6.19.x | 6.19.12 | `ce42ee423e58dffa5ec03524054c9d8bfd4f6237` |
| 主线版 | 7.0 | `a664bf3d603dc3bdcf9ae47cc21e0daec706d7a5` |

该修复将 2017 年在 `algif_aead.c` 中引入的原地 AEAD 优化还原为异地操作，确保页面缓存页面永远不会被放入可写的散列表中。

### 特定发行版指南

| 发行版 | 操作 |
|---|---|
| Ubuntu | `apt-get update && apt-get upgrade linux-image-generic`；查看 USN 公告 |
| RHEL / Rocky / Alma | `dnf update kernel`；查看 RHSB 公告 |
| Amazon Linux 2023 | `dnf update kernel`；查看 ALAS 公告 |
| SUSE / openSUSE | `zypper update kernel-default`；查看 SUSE SA 公告 |
| Debian | 检查安全追踪器；回溯补丁可能早于内核更新到达 |
| Arch | `pacman -Syu`（滚动更新；上游修复发布后即可获取） |

### 暴露后的完整性验证

如果您怀疑主机在打补丁之前已被利用：```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"