CVE-2026-80847

What the bug is

tcp_select_initial_window() assumes that callers never pass an MSS smaller than 1, but route-derived advmss values can violate that assumption. A too-small explicit RTAX_ADVMSS is one way to get there, but it is not the only one. The same divide-by-zero can also be reached through the "default advmss" path when RTAX_ADVMSS is left at 0 and the effective advmss is later driven down by route MTU and min_adv_mss. Introduce a tcp_dst_advmss() helper that clamps route advmss to TCP_MIN_MSS before TCP consumes it, and use it in the TCP paths that derive advmss from dst metrics. This keeps the effective MSS from dropping to zero before tcp_select_initial_window() rounds the receive window.

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.

include/net/tcp.h
net/ipv4/tcp_ipv4.c
net/ipv4/tcp_minisocks.c
net/ipv4/tcp_output.c
net/ipv6/tcp_ipv6.c

What this is

NETWORKING [TCP]MAINTAINERS section matched on include/net/tcp.h · netdev@vger.kernel.org · Maintained

Check it on the machine

grep -w CONFIG_INET /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_INET

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

2026-09-04 (44 days ago)source

CVSS

no vector published, unrated

Attack vector

not published

EPSS

0.17%, higher than 5.7% of all CVEssource

CISA KEV

not listedthe KEV catalogue

Debian urgency

not yet assignedsource

Subsystem

include/net

Upstream fixes

The versions the CNA records as introducing and fixing this, and the commit for each. source

Introduced	Fixed in	Commit
2.6.12	6.12.111	eaf4af9bd697
2.6.12	6.18.51	cfb44c6028e6
2.6.12	7.1.13	6b8c20bf6192
2.6.12	7.2.3	31cf23493619
2.6.12	7.3	870a9e42ecc6

Debian advisories