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CVE-2026-80586
mptcp: options: reset DSS fields in case of unexpected size
Published 2026-08-26 (2 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-2026-80586 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 | fixedDLA-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.105-1 | Update the kernel and reboot. Fixed in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.10-1 | Update the kernel and reboot. Fixed in 7.1.10-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.10-1 | Update the kernel and reboot. Fixed in 7.1.10-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4788-1 · 2026-09-19 · text | 6.12.107-1~deb12u1 | Update the kernel and reboot. DLA-4788-1 shipped the fix on 2026-09-19, in 6.12.107-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
A remote peer could send a malformed DSS with a wrong size, followed by another DSS or MPC + Data. In this case, the first suboption will be ignored, but leaving some fields written, which could lead to inconsistency or access uninitialized data. Explicitly reset the fields that could have been modified in case of unexpected size.
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, sid, bookworm · linux-6.12y
- 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
- 2026-08-26 (2 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
- 0.40%, higher than 32.6% 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 |
|---|---|---|
| 5.6 | 5.10.266 | 15e35fdad7a5 |
| 5.6 | 5.15.217 | b1256090816e |
| 5.6 | 6.1.184 | 192878df582c |
| 5.6 | 6.6.153 | 26dac5c9ffb2 |
| 5.6 | 6.12.105 | 4e80eff5c1c8 |
| 5.6 | 6.18.46 | 1fade1b2ac5b |
| 5.6 | 7.1.10 | 27ed642a4e7e |
| 5.6 | 7.2 | 35772b4981f3 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4788-1 | 2026-09-19 | bookworm 6.12.107-1~deb12u1 | tracker |
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | tracker |
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
- Remote attacker sends crafted TCP segments with malformed MPTCP options that are parsed in the softirq receive path (tcp_v4_rcv/tcp_rcv_established/tcp_data_queue/mptcp_incoming_options/mptcp_get_options), requiring only network reachability to an MPTCP-enabled peer.
- AC:LAttack complexity, low
- Attacker fully controls TCP option bytes on an established MPTCP connection and can deterministically send a size-invalid DSS (setting use_ack/use_map flags) immediately followed by MPC+Data or another DSS in the same header; no race or victim-specific state is required.
- PR:NPrivileges required, none
- Only requires the attacker to be the remote MPTCP/TCP peer of a host with CONFIG_MPTCP enabled; no local account, capabilities, or authenticated access on the victim is needed to reach the vulnerable parser.
- UI:NUser interaction, none
- Exploitation is fully automated over the network once an MPTCP session exists; the kernel processes malicious segments in softirq without any victim user action beyond ordinary connectivity.
- S:UScope, unchanged
- Impact is limited to kernel networking/MPTCP connection state on the affected host and does not cross VM, container, or IOMMU security boundaries.
- C:HConfidentiality impact, high
- Malformed DSS leaves use_ack/use_map/dsn64/data_fin set while data_ack and map fields are never read, so ack_update_msk() and DSS handling consume uninitialized stack values and corrupt metadata that can leak kernel memory contents.
- I:HIntegrity impact, high
- Stale DSS flag bits cause forged ack advancement of msk->snd_una/wnd_end/bytes_acked and bogus DATA_FIN/DSN mapping in skb extensions, corrupting rtx queues and silently altering or truncating application data delivery.
- A:HAvailability impact, high
- Bogus ack state triggers __mptcp_clean_una() WARN_ON_ONCE paths outside recovery, can abort or hang MPTCP connections, and may kernel-panic systems built with panic_on_warn.