CVE-2025-39851
Unknown Unknown - Not Provided
BaseFortify

Publication date: 2025-09-19

Last updated on: 2025-12-12

Assigner: kernel.org

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
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Meta Information
Published
2025-09-19
Last Modified
2025-12-12
Generated
2026-05-07
AI Q&A
2025-09-19
EPSS Evaluated
2026-05-05
NVD
CVE-2025-39851
Affected Vendors & Products
Showing 6 associated CPEs
Vendor Product Version / Range
linux linux_kernel From 5.8 (inc) to 6.12.46 (exc)
linux linux_kernel From 6.13 (inc) to 6.16.6 (exc)
linux linux_kernel 6.17
linux linux_kernel 6.17
linux linux_kernel 6.17
linux linux_kernel 6.17
Helpful Resources
Exploitability
CWE
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KEV
KEV Icon
CWE ID Description
CWE-476 The product dereferences a pointer that it expects to be valid but is NULL.
Attack-Flow Graph
AI Powered Q&A
Can you explain this vulnerability to me?

This vulnerability is in the Linux kernel's VXLAN implementation. VXLAN FDB (Forwarding Database) entries can point to either a remote destination or an FDB nexthop group, the latter used in EVPN deployments where learning is disabled. When learning is enabled, an incoming packet might try to refresh an FDB entry that points to an FDB nexthop group without a remote destination. Such packets should be dropped immediately, but due to the vulnerability, they are only dropped after the kernel dereferences a non-existent remote pointer, causing a NULL pointer dereference (NPD) and a kernel crash.


How can this vulnerability impact me? :

This vulnerability can cause a kernel NULL pointer dereference leading to a system crash or denial of service (DoS) on affected Linux systems using VXLAN with learning enabled. An attacker could potentially exploit this by sending crafted packets that refresh FDB entries pointing to nexthop groups, causing the kernel to crash and disrupt network operations.


How can this vulnerability be detected on my network or system? Can you suggest some commands?

This vulnerability can be detected by reproducing the NULL pointer dereference (NPD) condition in the Linux kernel related to VXLAN FDB entries. The provided script in the description can be used to test for the vulnerability by setting up VXLAN interfaces and FDB entries, then sending crafted packets using the 'mausezahn' tool. The commands include adding IP addresses, creating nexthop groups, adding VXLAN interfaces, adding bridge FDB entries, and finally running the mausezahn command to send packets that trigger the issue.


What immediate steps should I take to mitigate this vulnerability?

Immediate mitigation involves updating the Linux kernel to a version where this vulnerability is fixed, as the issue is resolved by dropping packets earlier to prevent NULL pointer dereference. Until an update is applied, avoid enabling learning on VXLAN interfaces that use FDB nexthop groups or disable such configurations to prevent triggering the vulnerability.


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