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CVE-2026-98023
vxlan: reject dynamic fdb entries that reference a nexthop id
Published 2026-09-25 (24 days ago) by the Linux kernel CNA. Subsystem drivers/net. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-98023 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-4841-1 · 2026-10-17 · text | 6.1.190-1 | Update the kernel and reboot. DLA-4841-1 shipped the fix on 2026-10-17, in 6.1.190-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6528-1 · 2026-09-29 · text | 6.12.111-1 | Update the kernel and reboot. DSA-6528-1 shipped the fix on 2026-09-29, in 6.12.111-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.2.7-1 | Update the kernel and reboot. Fixed in 7.2.7-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.2.7-1 | Update the kernel and reboot. Fixed in 7.2.7-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4817-1 · 2026-10-04 · text | 6.12.111-1~deb12u1 | Update the kernel and reboot. DLA-4817-1 shipped the fix on 2026-10-04, in 6.12.111-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
The commit cited in the Fixes tag allowed VXLAN FDB entries to point to FDB nexthops so that overlay traffic could be load balanced across multiple VTEPs. Such entries can only be configured from user space, cannot be learned and cannot roam. They only make sense with a user space control plane such as E-VPN where data plane learning is disabled. Despite that, the VXLAN driver does not currently prevent such entries from being configured with the "dynamic" flag. The per-nexthop FDB list is only protected by the per-device hash lock, which is not sufficient when two VXLAN devices point to the same FDB nexthop and therefore share the list. Aging runs in softirq context without RTNL, so an entry deleted by one device can race with an addition or deletion from the other, leading to list corruption: list_del corruption. next->prev should be ffff8881069d9548, but was dead000000000122. (next=ffff8881069d9448) WARNING: CPU: 0 PID: 90 at lib/list_debug.c:65 __list_del_entry_valid_or_report+0x1aa/0x210 ... vxlan_fdb_destroy+0x5b8/0xad0 vxlan_cleanup+0x328/0x450 call_timer_fn+0x2a/0x1c0 run_timer_softirq+0x18c/0x210 BUG: KASAN: slab-use-after-free in vxlan_fdb_destroy Fix this by rejecting the bogus configuration of dynamic FDB entries that point to FDB nexthops, both when created and when an existing entry is updated. As such, the per-nexthop FDB list is only ever mutated under the RTNL lock. Add test cases to make sure that this does not regress in the future.
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.
drivers/net/vxlan/vxlan_core.c
tools/testing/selftests/net/fib_nexthops.sh
- What this is
- NETWORKING DRIVERSMAINTAINERS section matched on drivers/net/ · netdev@vger.kernel.org · Maintained
- Kernel module
- vxlan
Check it on the machine
grep -w CONFIG_VXLAN /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.
lsmod | grep -w vxlanPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sid, bookworm · linux-6.12m
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switch
- CONFIG_VXLAN
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
- 2026-09-25 (24 days ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.12%, higher than 2.0% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- drivers/net
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.8 | 5.10.271 | 00c7f4b8144e |
| 5.8 | 5.15.222 | 55a33882808f |
| 5.8 | 6.1.189 | c8f29af91e08 |
| 5.8 | 6.6.158 | b2a8e165ddb6 |
| 5.8 | 6.12.111 | 28f86eec851b |
| 5.8 | 6.18.53 | 53359916ceb6 |
| 5.8 | 7.2.7 | 3fa64d86fc64 |
| 5.8 | 7.3 | 98fc57d16744 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4841-1 | 2026-10-17 | bookworm 6.1.190-1 | tracker |
| DLA-4817-1 | 2026-10-04 | bookworm 6.12.111-1~deb12u1 | tracker |
| DSA-6528-1 | 2026-09-29 | trixie 6.12.111-1 | 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:LAttack vector, local
- The bad state is a dynamic VXLAN FDB entry referencing an FDB nexthop id, installed via local rtnetlink RTM_NEWNEIGH -> rtnl_fdb_add -> vxlan_fdb_add (with RTM_NEWNEXTHOP/RTM_NEWLINK setup). Nexthop FDB entries are never learned from received packets, so no remote peer supplies the input.
- AC:LAttack complexity, low
- The attacker creates both vxlan devices sharing one nexthop group, sets a short ageing interval, and races vxlan_cleanup() timer-driven vxlan_fdb_destroy() on one device against fdb add/delete on the other; they control both sides and can retry in a loop.
- PR:LPrivileges required, low
- rtnetlink_rcv_msg only requires netlink_net_capable(CAP_NET_ADMIN) in the netns's user_ns, so an unprivileged user can create vxlan devices, nexthops and FDB entries inside an unshare -Urn namespace.
- UI:NUser interaction, none
- No victim action is needed; the attacker drives every netlink request and the ageing timer fires by itself.
- S:UScope, unchanged
- Corruption stays inside the kernel's own memory; this is a standard kernel privilege escalation or crash with no crossing into a separate security authority.
- C:HConfidentiality impact, high
- Unsynchronized list_del_rcu/list_add_tail_rcu on the shared nh->fdb_list corrupts the list and leaves pointers to vxlan_fdb objects freed by call_rcu (KASAN slab-use-after-free in vxlan_fdb_destroy), which can be groomed into a kernel read primitive.
- I:HIntegrity impact, high
- List unlink/link through dangling nh_list pointers writes to freed and reallocated slab objects, giving a heap write primitive that can be used for control-flow hijack.
- A:HAvailability impact, high
- The race produces list_del corruption and use-after-free in vxlan_fdb_destroy from timer softirq, which crashes the kernel (BUG/oops/panic on panic_on_warn).