CVE-2025-39851
Full vulnerability record from the local CVENow database.
Description
In the Linux kernel, the following vulnerability has been resolved: vxlan: Fix NPD when refreshing an FDB entry with a nexthop object VXLAN FDB entries can point to either a remote destination or an FDB nexthop group. The latter is usually used in EVPN deployments where learning is disabled. However, when learning is enabled, an incoming packet might try to refresh an FDB entry that points to an FDB nexthop group and therefore does not have a remote. Such packets should be dropped, but they are only dropped after dereferencing the non-existent remote, resulting in a NPD [1] which can be reproduced using [2]. Fix by dropping such packets earlier. Remove the misleading comment from first_remote_rcu(). [1] BUG: kernel NULL pointer dereference, address: 0000000000000000 [...] CPU: 13 UID: 0 PID: 361 Comm: mausezahn Not tainted 6.17.0-rc1-virtme-g9f6b606b6b37 #1 PREEMPT(voluntary) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014 RIP: 0010:vxlan_snoop+0x98/0x1e0 [...] Call Trace: <TASK> vxlan_encap_bypass+0x209/0x240 encap_bypass_if_local+0xb1/0x100 vxlan_xmit_one+0x1375/0x17e0 vxlan_xmit+0x6b4/0x15f0 dev_hard_start_xmit+0x5d/0x1c0 __dev_queue_xmit+0x246/0xfd0 packet_sendmsg+0x113a/0x1850 __sock_sendmsg+0x38/0x70 __sys_sendto+0x126/0x180 __x64_sys_sendto+0x24/0x30 do_syscall_64+0xa4/0x260 entry_SYSCALL_64_after_hwframe+0x4b/0x53 [2] #!/bin/bash ip address add 192.0.2.1/32 dev lo ip address add 192.0.2.2/32 dev lo ip nexthop add id 1 via 192.0.2.3 fdb ip nexthop add id 10 group 1 fdb ip link add name vx0 up type vxlan id 10010 local 192.0.2.1 dstport 12345 localbypass ip link add name vx1 up type vxlan id 10020 local 192.0.2.2 dstport 54321 learning bridge fdb add 00:11:22:33:44:55 dev vx0 self static dst 192.0.2.2 port 54321 vni 10020 bridge fdb add 00:aa:bb:cc:dd:ee dev vx1 self static nhid 10 mausezahn vx0 -a 00:aa:bb:cc:dd:ee -b 00:11:22:33:44:55 -c 1 -q
Severity
MEDIUM
CVSS score
5.5
(v3.1)
Vulnerability status
Modified
Source identifier
416baaa9-dc9f-4396-8d5f-8c081fb06d67
Published
2025-09-19 16:15:43.983000
Last modified
2026-10-30 13:18:10.547000
View original CVE JSON
{
"id": "CVE-2025-39851",
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"published": "2025-09-19T16:15:43.983",
"lastModified": "2026-10-30T13:18:10.547",
"vulnStatus": "Modified",
"cveTags": [],
"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nvxlan: Fix NPD when refreshing an FDB entry with a nexthop object\n\nVXLAN FDB entries can point to either a remote destination or an FDB\nnexthop group. The latter is usually used in EVPN deployments where\nlearning is disabled.\n\nHowever, when learning is enabled, an incoming packet might try to\nrefresh an FDB entry that points to an FDB nexthop group and therefore\ndoes not have a remote. Such packets should be dropped, but they are\nonly dropped after dereferencing the non-existent remote, resulting in a\nNPD [1] which can be reproduced using [2].\n\nFix by dropping such packets earlier. Remove the misleading comment from\nfirst_remote_rcu().\n\n[1]\nBUG: kernel NULL pointer dereference, address: 0000000000000000\n[...]\nCPU: 13 UID: 0 PID: 361 Comm: mausezahn Not tainted 6.17.0-rc1-virtme-g9f6b606b6b37 #1 PREEMPT(voluntary)\nHardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014\nRIP: 0010:vxlan_snoop+0x98/0x1e0\n[...]\nCall Trace:\n <TASK>\n vxlan_encap_bypass+0x209/0x240\n encap_bypass_if_local+0xb1/0x100\n vxlan_xmit_one+0x1375/0x17e0\n vxlan_xmit+0x6b4/0x15f0\n dev_hard_start_xmit+0x5d/0x1c0\n __dev_queue_xmit+0x246/0xfd0\n packet_sendmsg+0x113a/0x1850\n __sock_sendmsg+0x38/0x70\n __sys_sendto+0x126/0x180\n __x64_sys_sendto+0x24/0x30\n do_syscall_64+0xa4/0x260\n entry_SYSCALL_64_after_hwframe+0x4b/0x53\n\n[2]\n #!/bin/bash\n\n ip address add 192.0.2.1/32 dev lo\n ip address add 192.0.2.2/32 dev lo\n\n ip nexthop add id 1 via 192.0.2.3 fdb\n ip nexthop add id 10 group 1 fdb\n\n ip link add name vx0 up type vxlan id 10010 local 192.0.2.1 dstport 12345 localbypass\n ip link add name vx1 up type vxlan id 10020 local 192.0.2.2 dstport 54321 learning\n\n bridge fdb add 00:11:22:33:44:55 dev vx0 self static dst 192.0.2.2 port 54321 vni 10020\n bridge fdb add 00:aa:bb:cc:dd:ee dev vx1 self static nhid 10\n\n mausezahn vx0 -a 00:aa:bb:cc:dd:ee -b 00:11:22:33:44:55 -c 1 -q"
},
{
"lang": "es",
"value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nvxlan: Corrección de NPD al actualizar una entrada FDB con un objeto nexthop\n\nLas entradas FDB de VXLAN pueden apuntar a un destino remoto o a un grupo nexthop FDB. Esto último se usa normalmente en implementaciones EVPN donde el aprendizaje está deshabilitado.\n\nSin embargo, cuando el aprendizaje está habilitado, un paquete entrante podría intentar actualizar una entrada FDB que apunta a un grupo nexthop FDB y por lo tanto no tiene un remoto. Dichos paquetes deberían ser descartados, pero solo se descartan después de desreferenciar el remoto inexistente, lo que resulta en un NPD [1] que puede reproducirse usando [2].\n\nSolución al descartar dichos paquetes antes. Eliminar el comentario engañoso de first_remote_rcu().\n\n[1]\nERROR: desreferencia de puntero NULL del kernel, dirección: 0000000000000000\n[...]\nCPU: 13 UID: 0 PID: 361 Comm: mausezahn Not tainted 6.17.0-rc1-virtme-g9f6b606b6b37 #1 PREEMPT(voluntary)\nHardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-4.fc41 04/01/2014\nRIP: 0010:vxlan_snoop+0x98/0x1e0\n[...]\nRastro de Llamada:\n <TAREA>\n vxlan_encap_bypass+0x209/0x240\n encap_bypass_if_local+0xb1/0x100\n vxlan_xmit_one+0x1375/0x17e0\n vxlan_xmit+0x6b4/0x15f0\n dev_hard_start_xmit+0x5d/0x1c0\n __dev_queue_xmit+0x246/0xfd0\n packet_sendmsg+0x113a/0x1850\n __sock_sendmsg+0x38/0x70\n __sys_sendto+0x126/0x180\n __x64_sys_sendto+0x24/0x30\n do_syscall_64+0xa4/0x260\n entry_SYSCALL_64_after_hwframe+0x4b/0x53\n\n[2]\n #!/bin/bash\n\n ip address add 192.0.2.1/32 dev lo\n ip address add 192.0.2.2/32 dev lo\n\n ip nexthop add id 1 via 192.0.2.3 fdb\n ip nexthop add id 10 group 1 fdb\n\n ip link add name vx0 up type vxlan id 10010 local 192.0.2.1 dstport 12345 localbypass\n ip link add name vx1 up type vxlan id 10020 local 192.0.2.2 dstport 54321 learning\n\n bridge fdb add 00:11:22:33:44:55 dev vx0 self static dst 192.0.2.2 port 54321 vni 10020\n bridge fdb add 00:aa:bb:cc:dd:ee dev vx1 self static nhid 10\n\n mausezahn vx0 -a 00:aa:bb:cc:dd:ee -b 00:11:22:33:44:55 -c 1 -q"
}
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"versionType": "original_commit_for_fix",
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}
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"privilegesRequired": "NONE",
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