CVE-2026-31431 ("Copy Fail") Toolkit

Detector and proof-of-concept LPE for the Linux algif_aead / authencesn page-cache scratch-write bug disclosed 2026-04-29.

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

Authorization

Use only on hosts you own or are explicitly engaged to assess. The LPE modifies in-memory state (page cache) but the technique is real privilege escalation — running it on systems without authorization is illegal in most jurisdictions.

Vulnerability summary

algif_aead runs AEAD operations in-place (req->src == req->dst). When the source data is fed in via splice() from a regular file, the destination scatterlist contains references to the file's page-cache pages — i.e. the kernel will write into them. The authencesn(hmac(sha256), cbc(aes)) algorithm then performs a 4-byte "scratch" write of the AAD's seqno_lo field (bytes 4–7 of the sendmsg-supplied AAD) into that destination, corrupting the page-cache copy of the file.

Because the on-disk file is never modified, there is no on-disk signature; the corruption is observed only by readers that share the page cache. /etc/passwd and /usr/bin/su are both world-readable, so an unprivileged local user can corrupt the running kernel's view of either.

Affected: kernels carrying commit 72548b093ee3 (in-place AEAD, 2017) without the upstream revert. The disclosure confirmed Ubuntu 24.04 LTS, Amazon Linux 2023, RHEL 14.3, and SUSE 16, but the underlying primitive predates that range.

Files

Both scripts are pure Python 3.10+ stdlib.

Quick start

# 1. Detect
python3 test_cve_2026_31431.py
#   exit 0 = not vulnerable, 2 = vulnerable, 1 = test error

# 2. Exploit (interactive — su will prompt for your own password)
python3 exploit_cve_2026_31431.py --shell

Detector usage

python3 test_cve_2026_31431.py

What it does:

1. Confirms AF_ALG and the authencesn(hmac(sha256),cbc(aes)) algorithm are reachable from an unprivileged process.
2. Creates a 4 KiB sentinel file in a temp directory, populates the page cache.
3. Sends 8 bytes of AAD inline via sendmsg+cmsg with seqno_lo set to the marker PWND, then os.splice()s 32 bytes of the sentinel's page-cache page into the AF_ALG op socket.
4. Calls recv() to drive decryption. The auth check fails with EBADMSG; the scratch write fires regardless.
5. Re-reads the file (page cache, not disk) and looks for the marker.

Output classes:

• Precondition not met — AF_ALG or authencesn unavailable. Exit 0.
• VULNERABLE to CVE-2026-31431 — marker PWND landed in the spliced page. Exit 2.
• Page cache MODIFIED via in-place AEAD splice path — the page was written to but the marker did not land at the expected position. Treat as vulnerable. Exit 2.
• Page cache intact — patched. Exit 0.

The detector never touches /usr/bin/su, /etc/passwd, or any other file outside the temp directory it creates, and that file is removed on exit.

LPE usage

python3 exploit_cve_2026_31431.py            # patch only, print next steps
python3 exploit_cve_2026_31431.py --shell    # patch and exec `su <user>`

What it does:

1. Looks up the running user's UID line in /etc/passwd and finds the byte offset of the 4-character UID field.
2. Issues one write4 against that offset, replacing the UID with 0000.
3. Calls pwd.getpwnam(user) to confirm libc now reports UID 0.
4. With --shell, execvp("su", ["su", user]). Enter your own password. PAM validates against /etc/shadow (untouched), then setuid(getpwnam(user).pw_uid) lands at 0.

Requirements

• Running user has a 4-digit UID (1000–9999). 1- to 3-digit UIDs require multi-shot writes — extend write4 accordingly.
• No NSS caching daemon (nscd, sssd, systemd-userdbd) is masking /etc/passwd reads. If getpwnam still returns the real UID after the patch, restart or bypass the cache, or pick a different user.
• /etc/passwd page must remain in cache between the patch and the su exec. In practice this is reliable on any system with normal memory pressure.

Reverting

The on-disk /etc/passwd is unchanged.

Dry-run (exploit_cve_2026_31431.py without --shell) auto-evicts the corrupted page on exit via POSIX_FADV_DONTNEED, so UID→name lookups go back to normal immediately.

After --shell, the page is left corrupted until you clear it. While it is corrupted, anything resolving UID 1000 → name (e.g. ls, file managers, scp/sftp ownership checks) will fail or show numeric IDs. To clear:

# unprivileged - request page-cache eviction for /etc/passwd:
python3 -c "import os; fd=os.open('/etc/passwd', os.O_RDONLY); \
            os.posix_fadvise(fd, 0, 0, os.POSIX_FADV_DONTNEED); os.close(fd)"

# from the root shell:
echo 3 > /proc/sys/vm/drop_caches

A reboot also clears it.

How write4 works

sendmsg([8-byte AAD], cmsg=[ALG_SET_OP=DECRYPT, ALG_SET_IV, ALG_SET_AEAD_ASSOCLEN=8],
        flags=MSG_MORE)
splice(target_fd, pipe_w, 32, offset_src=file_offset)
splice(pipe_r, op_fd, 32)
recv(op_fd)   # EBADMSG; scratch write has already landed

The 4 bytes from AAD positions 4–7 (seqno_lo) are written by authencesn into the destination scatterlist, which on this code path is the page-cache page we spliced from target_fd. The landing offset within the page corresponds to the offset_src we passed to splice().

Mitigation

Until the patched kernel reaches your distro:

sudo tee /etc/modprobe.d/disable-algif-aead.conf <<<'install algif_aead /bin/false'
sudo rmmod algif_aead 2>/dev/null

After applying, test_cve_2026_31431.py should report Precondition not met and exit 0.

The upstream fix reverts in-place AEAD operations to out-of-place, keeping page-cache pages out of writable scatterlists.

References

• Xint disclosure writeup: https://xint.io/blog/copy-fail-linux-distributions
• CVE-2026-31431