CVE-2026-98377 vlan: require the MAC header to be present in __vlan_insert_inner_tag()
A local attacker can leak uninitialized kernel memory by sending crafted packets through VLAN interfaces, exposing sensitive slab data.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-98377?
CVE-2026-98377 is a vulnerability in Linux, published on 9 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
vlan: require the MAC header to be present in __vlan_insert_inner_tag()
__vlan_insert_inner_tag() only guarantees head room via skb_cow_head(), never that mac_len bytes of MAC header are present. Its ETH_HLEN wrappers - __vlan_insert_tag() under skb_vlan_push(), and vlan_insert_tag() under validate_xmit_vlan() on the generic transmit path - therefore rewrite the first 16 bytes at skb->data: a 12-byte memmove plus two 2-byte stores at +12 and +14. No caller supplies the bound, while the pop helpers use skb_ensure_writable()/pskb_may_pull().
An IFF_TUN device has hard_header_len == 0, so packet_snd() accepts a one-byte AF_PACKET/SOCK_RAW frame. The first vlan push only sets a hwaccel tag; the next - clsact "action vlan push" or bpf_skb_vlan_push() - enters the helper with skb->len still 1. The head comes from skbuff_small_head without __GFP_ZERO, so each push drags bytes from beyond skb->tail into the frame. After three the one-byte send leaves as 13 bytes carrying 11 bytes of uninitialised slab:
0000: 5a b3 62 12 80 88 ff ff 00 b3 62 12 81 `------------------------------' only 0x5a was sent; the rest is slab, here the top 56 bits of a linear-map address
Require the MAC header the helper rewrites to be present, so such a frame is dropped rather than transmitted.
Which versions are affected by CVE-2026-98377?
How do I fix CVE-2026-98377?
Apply the vendor's patch.
Is CVE-2026-98377 being exploited?
There is no known exploitation of CVE-2026-98377 in the wild and no public exploit code.
Sources
Also known as AZL-106896, BELL-CVE-2026-98377, DEBIAN-CVE-2026-98377, GHSA-jjqx-7ghh-p62x, UBUNTU-CVE-2026-98377.