CVE-2026-74478

What the bug is

When vector_mmsg_rx() discards a packet whose overlay header fails verify_header(), it frees the skb and continues the loop: if (header_check < 0) { dev_kfree_skb_irq(skb); vp->estats.rx_encaps_errors++; continue; } The normal and short-packet paths fall through to the bottom of the loop body, which clears the consumed slot and advances the cursors: (*skbuff_vector) = NULL; mmsg_vector++; skbuff_vector++; The verify_header() < 0 path skips that via continue, so the freed skb is left in skbuff_vector[] and the cursors do not advance. The next iteration reads the same slot, gets the freed skb, and frees it again, producing a refcount underflow / use-after-free in the RX path. Discard the slot the same way the other paths do before continuing. Only transports whose verify_header() can return negative are affected: GRE and L2TPv3 do so on a cookie/session-id mismatch (raw/tap do not), so any peer on such a transport can trigger it without authentication.

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.

arch/um/drivers/vector_kern.c

What this is

USER-MODE LINUX (UML)MAINTAINERS section matched on arch/um/ · linux-um@lists.infradead.org · Maintained

Kernel module

none to look for. The file would have gone into vector, and Debian does not compile it into that module, so finding vector in lsmod would say nothing about this CVE.

Check it on the machine

grep -w CONFIG_UML_NET_VECTOR /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 5 releases below, every kernel flavour Debian publishes has the switch off, so the vulnerable file is compiled into no kernel binary Debian ships.

• bookworm, trixie, forky, sid, bookworm · linux-6.12n

n

not enabled on any flavour - this release does not build the code

Build switch

CONFIG_UML_NET_VECTOR

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

2026-08-15 (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

0.86%, higher than 57.9% of all CVEssource

CISA KEV

not listedthe KEV catalogue

Debian urgency

not yet assignedsource

Subsystem

arch/um

Upstream fixes

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

Introduced	Fixed in	Commit
4.17	5.10.265	967c779c9853
4.17	5.15.216	a7bc015bb798
4.17	6.1.183	7dc9781e320d
4.17	6.6.151	4b9601595e8b
4.17	6.12.103	67d58ab4f2cc
4.17	6.18.44	180ff4c81faf
4.17	7.1.8	804b681002ea
4.17	7.2	af421e9aed39

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