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CVE-2026-63979
net/handshake: hand off the pinned file reference to accept_doit
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem net/handshake. Debian urgency: unimportant.
What to do, per Debian release
Debian's own status for CVE-2026-63979 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | not affected | 6.1.190-1 | Nothing to do. bookworm never shipped the vulnerable code. |
| Debian 13trixie | vulnerable | 6.12.113-1 | No fix published anywhere yet, neither in Debian nor upstream for the series trixie tracks. Watch the Debian tracker page for this CVE. Advisories for trixie have historically landed a median of 11 days after publication. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
handshake_req_next() removes the request from the per-net pending list and drops hn_lock before handshake_nl_accept_doit() reads req->hr_sk->sk_socket and dereferences sock->file (once in FD_PREPARE() and again in get_file()). In that window a consumer running tls_handshake_cancel() followed by sockfd_put() (svc_sock_free) or __fput_sync() (xs_reset_transport) releases sock->file. sock_release() then runs sock_orphan(), zeroing sk_socket, and frees the struct socket. The accept-side code either reads NULL through sk_socket or chases freed memory. The submit-side sock_hold() does not prevent this. sk_refcnt protects struct sock, but struct socket and sock->file are independently refcounted via the file descriptor the consumer owns. Pinning sk leaves sock and sock->file unprotected. Retarget the accept-side dereferences at req->hr_file, which was pinned at submit time, instead of req->hr_sk->sk_socket->file. Pinning on its own is not sufficient: a consumer that cancels between handshake_req_next() returning and accept_doit reaching FD_PREPARE() takes the !remove_pending() branch in handshake_req_cancel() and drops hr_file before the accept side takes its own reference. Hand off an additional file reference inside handshake_req_next(), under hn_lock, so the accept side operates on a reference that no concurrent handshake_req_cancel() can revoke. FD_PREPARE() consumes that handed-off reference, either by transferring it to the new fd in fd_publish() or by dropping it in the cleanup destructor on error; the explicit get_file() that previously balanced FD_PREPARE() is therefore redundant and goes away. Update handshake_req_cancel_test2 and _test3 to simulate the FD_PREPARE() consumption with an fput() so the kunit file-count assertions stay balanced.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
net/handshake/handshake-test.c
net/handshake/netlink.c
net/handshake/request.c
- What this is
- HANDSHAKE UPCALL FOR TRANSPORT LAYER SECURITYMAINTAINERS section matched on net/handshake/ · kernel-tls-handshake@lists.linux.dev · Maintained
Check it on the machine
grep -wE 'CONFIG_NET_HANDSHAKE|CONFIG_NET_HANDSHAKE_KUNIT_TEST' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian does not build this code in 3 releases. The build rule could not be established for 1 release, so nothing is claimed about those.
- bookworm?
- trixie, forky, sidn
- ?
- not established - no Makefile entry for the affected file
- n
- not enabled on any flavour - this release does not build the code
Where a release above reads "not established", no build rule for the affected file was found in that release's kernel tree. That is a gap in this lookup, not a finding that the code is absent.
- Build switches
- CONFIG_NET_HANDSHAKE, CONFIG_NET_HANDSHAKE_KUNIT_TEST
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-07-19 (3 months ago)source
- CVSS
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.61%, higher than 48.4% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- unimportantsource
- Subsystem
- net/handshake
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.4 | 6.18.44 | 68eba6519cbd |
| 6.4 | 7.0.12 | c06876d4fac3 |
| 6.4 | 7.1 | f4251190e58b |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:NAttack vector, network
- The race is triggered by remote TLS handshake activity on network-facing kernel consumers (nfsd RPC-over-TLS STARTTLS probe, NVMe-oTCP TLS13 connections). A remote peer can initiate handshake_req_submit() and force cancellation via disconnect, while tlshd automatically issues HANDSHAKE_CMD_ACCEPT in response to kernel notifications.
- AC:LAttack complexity, low
- The attacker controls both sides of the race by opening TLS connections and timing disconnects or aborts against tlshd accept processing. Repeated parallel connection attempts make winning the window between handshake_req_next() and FD_PREPARE() practical without conditions outside attacker control.
- PR:NPrivileges required, none
- Exploitation requires no credentials on the target server. NFS RPC_AUTH_TLS uses a NULL-procedure STARTTLS probe before any RPC authentication, and NVMe-oTCP TLS handshakes run at initial connection setup prior to namespace authentication.
- UI:NUser interaction, none
- No victim interaction is required. The tlshd daemon automatically accepts pending handshake requests upon receiving HANDSHAKE_CMD_READY multicast notifications; the attacker only needs to manipulate connection timing over the network.
- S:UScope, unchanged
- The vulnerability corrupts kernel heap memory (struct socket/file UAF or NULL dereference through sk_socket) within the kernel's security authority. It does not cross a VM, container, or IOMMU security boundary.
- C:HConfidentiality impact, high
- The accept path dereferences sock->file after sock_release() may free the struct socket and sock_orphan() zeroes sk_socket, constituting a use-after-free. UAF of socket/file structures can be leveraged for arbitrary kernel memory disclosure via heap grooming.
- I:HIntegrity impact, high
- Use-after-free of the pinned socket file object provides a foundation for heap spraying and arbitrary write primitives to hijack kernel control flow, not merely a bounded or transient modification.
- A:HAvailability impact, high
- The bug manifests as either a NULL pointer dereference through zeroed sk_socket or chasing freed socket memory in FD_PREPARE()/get_file(), both of which cause kernel oops or panic and deny service to the affected host.