CVE-2025-39980
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:
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
AI Insight
LLM-synthesized narrative grounded in this CVE's description and references.
Linux kernel nexthop allows changing FDB status while in a group, causing NULL pointer dereference.
Vulnerability
Description
In the Linux kernel, the nexthop subsystem had a flaw where the kernel did not prevent changing the FDB (Forwarding Database) status of a nexthop while it was part of a group. The kernel already forbade creating groups with mixed FDB and non-FDB nexthops, but it allowed changing the type of an individual nexthop from FDB to non-FDB or vice versa even if that nexthop was already in a group. This could lead to an invalid configuration.
Exploitation
The attack requires local access and the ability to execute ip commands. An attacker can create a non-FDB nexthop group, then replace one of its nexthops with an FDB nexthop using the ip nexthop replace command. This bypasses the consistency checks that apply to group creation. As long as no routes point to the group, the change is allowed. Subsequently, when a route is added using the group and traffic is sent, the kernel attempts to use the FDB nexthop, which is not associated with a device, leading to a NULL pointer dereference.
Impact
Exploitation results in a kernel NULL pointer dereference, causing a system crash (oops) and denial of service. The vulnerability is triggered by sending network traffic (e.g., ping) to an IP address routed through the malformed nexthop group. The crash trace shows the failure occurs in fib_lookup_good_nhc during route lookup.
Mitigation
The fix prevents changing the FDB status of a nexthop while it is in a group. The kernel now returns an error for such attempts. The patch has been applied to the stable kernel trees [1][2][3]. Users should update their kernels to include the fix.
AI Insight generated on May 19, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.
Affected products
1Patches
7e1e87ac0daac0e7bfe7a268cec428fff792b24046d31f6f9f0e49fd13afe8dd4aa012288390b3a300d78Vulnerability mechanics
Generated on May 9, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.
References
7- git.kernel.org/stable/c/0e7bfe7a268ccbd7859730c529161cafbf44637cnvd
- git.kernel.org/stable/c/24046d31f6f92220852d393d510b6062843e3fbdnvd
- git.kernel.org/stable/c/390b3a300d7872cef9588f003b204398be69ce08nvd
- git.kernel.org/stable/c/8dd4aa0122885f710930de135af2adc4ccc3238fnvd
- git.kernel.org/stable/c/e1e87ac0daacd51f522ecd1645cd76b5809303ednvd
- git.kernel.org/stable/c/ec428fff792b7bd15b248dafca2e654b666b1304nvd
- git.kernel.org/stable/c/f0e49fd13afe9dea7a09a1c9537fd00cea22badbnvd
News mentions
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