Unprivileged User  ──►  AF_ALG + splice()  ──►  4-byte Page Cache Write  ──►  ROOT

AF_ALG (userspace crypto API)
└── algif_aead module
    └── authencesn template  ◄── VULNERABLE
        └── in-place optimization (2017)
            └── req->src == req->dst  ◄── pages from splice() chained into writable dst scatterlist

Step 1: Open AF_ALG AEAD socket
        socket(AF_ALG, SOCK_SEQPACKET, 0)

Step 2: Send splice() pages referencing target file
        (page cache pages of a privileged binary, e.g. /usr/bin/sudo)

Step 3: Trigger authencesn scratch write
        authencesn uses dst buffer as scratch pad →
        writes 4 controlled bytes PAST the legitimate output region

Step 4: Page cache entry for the target file is now corrupted
        (disk file untouched — only in-memory copy modified)

Step 5: Execute the modified binary → ROOT

# Fixed in commit: a664bf3d603d
git show a664bf3d603d

CVE-2026-31431-CopyFail/
│
├── 📄 README.md                          ← You are here
├── 📄 LICENSE                            ← Research/Educational license
├── 📄 DISCLAIMER.md                      ← Legal & ethical use policy
├── 📄 CHANGELOG.md                       ← Version history
│
├── 📂 docs/
│   ├── vulnerability-analysis.md         ← Deep-dive: root cause & mechanics
│   ├── exploitation-walkthrough.md       ← Step-by-step methodology
│   ├── affected-kernels.md               ← Full kernel/distro version matrix
│   ├── detection.md                      ← Defender, Tenable, Wazuh coverage
│   └── references.md                     ← All CVEs, blogs, advisories
│
├── 📂 exploit/
│   ├── README.md                         ← Usage, prerequisites, tested distros
│   ├── copyfail.py                       ← Reference PoC (educational)
│   ├── trigger.c                         ← AF_ALG + splice() C trigger
│   └── variants/
│       ├── ubuntu.py                     ← Ubuntu-specific variant
│       ├── rhel.py                       ← RHEL/CentOS variant
│       └── amazon_linux.py              ← Amazon Linux variant
│
├── 📂 detection/
│   ├── yara/
│   │   └── copyfail.yar                  ← YARA rule for exploit artefacts
│   ├── sigma/
│   │   └── copyfail_lpe.yml              ← Sigma rule for SIEM/SOC
│   └── scripts/
│       ├── check_vulnerable.sh           ← Quick kernel vulnerability check
│       └── detect_algif_aead.sh         ← Check if module is loaded/active
│
├── 📂 mitigation/
│   ├── README.md                         ← Mitigation overview
│   ├── disable_algif_aead.sh             ← Disable affected kernel module
│   ├── patch-notes.md                    ← Upstream patch details
│   └── kubernetes-hardening.md          ← K8s / container hardening
│
├── 📂 lab/
│   ├── Vagrantfile                       ← Reproducible vulnerable VM
│   ├── setup.sh                          ← Lab bootstrap script
│   └── docker/
│       └── Dockerfile                    ← Vulnerable Ubuntu container
│
├── 📂 reports/
│   ├── pentest-report-template.md        ← Client-ready report template
│   └── sample-finding.md                ← Sample finding write-up
│
└── 📂 assets/
    ├── demo.gif                          ← (Optional) terminal demo
    └── diagrams/
        └── page-cache-write.png         ← Attack flow diagram

# Clone the repository
git clone https://github.com/0xFuffM3/CVE-2026-31431-CopyFail.git
cd CVE-2026-31431-CopyFail

# Start the vulnerable lab VM
cd lab/
vagrant up

# SSH into the lab
vagrant ssh

# Verify kernel version (should be vulnerable)
uname -r

cd lab/docker/

# Build vulnerable container image
docker build -t copyfail-lab .

# Run with required privileges for kernel interaction
docker run --rm -it --privileged copyfail-lab /bin/bash

# Run the quick check script
chmod +x detection/scripts/check_vulnerable.sh
./detection/scripts/check_vulnerable.sh

[!] Kernel version: 5.15.0-91-generic
[!] algif_aead module: LOADED
[✗] System appears VULNERABLE to CVE-2026-31431 (Copy Fail)
[*] Recommended action: Apply kernel update or disable algif_aead

[✓] Kernel version: 6.1.132
[✓] System appears PATCHED against CVE-2026-31431 (Copy Fail)

# Python 3.6+
python3 --version

# Required kernel modules present
lsmod | grep algif_aead

# Verify the target binary is readable
ls -la /usr/bin/sudo

cd exploit/

# Check current privilege level
id
# uid=1000(user) gid=1000(user) groups=1000(user)

# Run Copy Fail PoC
python3 copyfail.py

# Verify privilege escalation
id
# uid=0(root) gid=0(root) groups=0(root)

1. Enumerate kernel version
   └─► uname -r / cat /proc/version

2. Check if algif_aead is loaded
   └─► lsmod | grep algif_aead

3. Confirm low-privilege foothold
   └─► id / whoami

4. Execute Copy Fail PoC
   └─► python3 exploit/copyfail.py

5. Verify root access
   └─► id && cat /etc/shadow

6. Document evidence
   └─► Screenshot + log kernel version, distro, exploit hash

7. Apply mitigation (post-test)
   └─► sudo bash mitigation/disable_algif_aead.sh

8. Include in pentest report
   └─► Use reports/pentest-report-template.md

// detection/yara/copyfail.yar
rule CopyFail_CVE_2026_31431 {
    meta:
        description = "Detects Copy Fail (CVE-2026-31431) exploit artefacts"
        author      = "0xFuffM3"
        date        = "2026-04-30"
        reference   = "https://copy.fail"
    strings:
        $py1 = "algif_aead" ascii
        $py2 = "AF_ALG" ascii
        $py3 = "splice" ascii
        $py4 = "page_cache" ascii
        $py5 = "authencesn" ascii
    condition:
        3 of them
}

yara detection/yara/copyfail.yar /tmp/ -r

# detection/sigma/copyfail_lpe.yml
title: Copy Fail LPE Exploit Execution (CVE-2026-31431)
status: stable
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        CommandLine|contains:
            - 'AF_ALG'
            - 'algif_aead'
            - 'authencesn'
    condition: selection
falsepositives:
    - Legitimate crypto API testing
level: high

chmod +x detection/scripts/detect_algif_aead.sh
./detection/scripts/detect_algif_aead.sh

# Ubuntu / Debian
sudo apt-get update && sudo apt-get upgrade linux-image-generic

# RHEL / CentOS
sudo yum update kernel

# Amazon Linux
sudo yum update kernel

# After update, reboot
sudo reboot

# Run the mitigation script
chmod +x mitigation/disable_algif_aead.sh
sudo bash mitigation/disable_algif_aead.sh

echo "install algif_aead /bin/false" > /etc/modprobe.d/disable-algif.conf
rmmod algif_aead 2>/dev/null || echo "[*] Module not currently loaded"
echo "[✓] algif_aead disabled. Reboot to confirm persistence."

Finding:    Local Privilege Escalation via Copy Fail (CVE-2026-31431)
Severity:   HIGH (CVSS 7.8)
Host:       10.10.10.55 (ubuntu-prod-01)
Kernel:     5.15.0-91-generic

Evidence:
  - Unprivileged shell (uid=1000) escalated to root (uid=0)
  - Kernel module algif_aead confirmed loaded
  - No disk artefacts — page cache only

Recommendation:
  1. Apply vendor kernel patch immediately
  2. Interim: disable algif_aead module
  3. Review Kubernetes nodes and CI/CD runners