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CVE-2026-63867
mptcp: close TOCTOU race while computing rcv_wnd
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem net/mptcp. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-63867 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 | fixed | 6.1.176-1 | Update the kernel and reboot. Fixed in 6.1.176-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.94-1 | Update the kernel and reboot. Fixed in 6.12.94-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.13-1 | Update the kernel and reboot. Fixed in 7.0.13-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.13-1 | Update the kernel and reboot. Fixed in 7.0.13-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
The MPTCP output path access locklessly the MPTCP-level ack_seq in multiple times, using possibly different values for the data_ack in the DSS option and to compute the announced rcv wnd for the same packet. Refactor the cote to avoid inconsistencies which may confuse the peer. Also ensure that the MPTCP level rcv wnd is updated only when the egress packet actually contains a DSS ack.
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/mptcp/options.c
- What this is
- NETWORKING [MPTCP]MAINTAINERS section matched on net/mptcp/ · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_MPTCP /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 builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sidy
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switch
- CONFIG_MPTCP
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
- 8.2 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.34%, higher than 25.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/mptcp
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.11 | 5.15.210 | 301a33fd590c |
| 5.11 | 6.1.176 | 68364963e5ba |
| 5.11 | 6.6.143 | 907ac6b1658e |
| 5.11 | 6.12.94 | c4f4cf607979 |
| 5.11 | 6.18.36 | 3b8cbba7c0ed |
| 5.11 | 7.0.13 | 8f4f0a157e84 |
| 5.11 | 7.1 | 8ab24fdebc36 |
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 flaw is in the MPTCP TCP output path (`__tcp_transmit_skb()` → `mptcp_established_options_dss()` / `mptcp_write_options()`), reachable by any remote peer that can exchange MPTCP packets over the network.
- AC:LAttack complexity, low
- The attacker controls both sides of the race by timing concurrent ingress traffic (advancing `msk->ack_seq` on receive) against egress transmissions on the same or another subflow; no special memory layout or victim-specific state is required.
- PR:NPrivileges required, none
- Exploitation requires only the ability to complete an MPTCP/TCP connection as a remote peer; no local privileges, capabilities, or authenticated access to the victim system is needed.
- UI:NUser interaction, none
- Exploitation is fully automated over the network once an MPTCP connection is established; no victim user action is required beyond normal service connectivity.
- S:UScope, unchanged
- The impact is confined to the kernel networking stack and the affected MPTCP connection state; it does not cross a VM, sandbox, or IOMMU security boundary.
- C:NConfidentiality impact, none
- This is a protocol-state TOCTOU inconsistency, not a memory-safety bug; it does not provide arbitrary or bounded kernel memory disclosure.
- I:LIntegrity impact, low
- Inconsistent DSS ACK versus receive-window tracking can corrupt MPTCP-level window state (`rcv_wnd_sent`), allowing acceptance or handling of data outside the window actually announced to the peer.
- A:HAvailability impact, high
- A remote attacker can repeatedly trigger the race to cause connection disruption (reset/fallback), stall MPTCP sessions, and inflate out-of-order receive buffering via incorrect `rcv_wnd_sent` values, denying service to applications using the connection.