{"id":"AZL-67536","summary":"CVE-2025-39851 affecting package kernel 6.6.126.1-1","details":"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 \u003cTASK\u003e\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","modified":"2026-04-01T05:21:14.361189Z","published":"2025-09-19T16:15:43Z","upstream":["CVE-2025-39851"],"references":[{"type":"WEB","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-39851"}],"affected":[{"package":{"name":"kernel","ecosystem":"Azure Linux:3","purl":"pkg:rpm/azure-linux/kernel"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"6.6.126.1-1"}]}],"database_specific":{"source":"https://github.com/microsoft/AzureLinuxVulnerabilityData/blob/main/osv/AZL-67536.json"}}],"schema_version":"1.7.5"}