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Linux kernel amt driver: Use-after-free in relay General Query transmission

CVE-2026-98968High 8.1

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Summary

The amt network driver of the Linux kernel can access a tunnel structure after it has been freed. A General Query of an AMT relay can remain queued in a qdisc, for example behind a long netem delay, while its tunnel expires; on dequeue, amt_dev_xmit() then uses the freed memory. The description states that this can lead to a kernel crash or memory corruption; the conditions are a relay with amt devices and a long-delay qdisc on the amt device. The vendor fix sends relay General Queries directly from the receive path instead of through the qdisc.

Affected

  • Linux kernel from 5.16 up to the fixed stable releases (per CNA record, commit range starting at cbc21dc1cfe949e37b2a54c71511579f1899e8d4)
  • Stable branches 6.1, 6.6, 6.12, 6.18 and 7.2 before the fixed versions listed below
  • Distribution kernels that include the affected commits: Unknown.

Fixed in

  • Linux stable 6.1.191 and later 6.1 releases
  • Linux stable 6.6.160 and later 6.6 releases
  • Linux stable 6.12.113 and later 6.12 releases
  • Linux stable 6.18.57 and later 6.18 releases
  • Linux stable 7.2.11 and later 7.2 releases
  • Distribution-specific fixed package versions: Unknown.

Workaround

  • Unknown.
  • If no amt tunnels are used, the amt module can be unloaded or blacklisted after verifying that no amt devices are configured.

Exploitation

  • Not known to be exploited (OffSeq).
  • Not listed as KEV (Strix).
  • No public proof of concept found in the research.

Our recommendation

  • Check whether amt devices (amt module) are in use on your systems; if not, no action is needed beyond normal kernel updates.
  • Apply the kernel update from your Linux distribution as soon as it is available, or update to a fixed stable kernel.
  • Until patched, consider unloading or blacklisting the amt module where it is not needed.
  • Note that relay General Queries no longer pass through qdisc, tc, and netfilter egress rules on amt devices after the fix; adjust monitoring accordingly.

Public exploits

PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.

Research sources

Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 17:56.

Original description (NVD)

In the Linux kernel, the following vulnerability has been resolved: amt: send the relay's General Query directly from the receive path An skb queued in a qdisc can outlive the tunnel it references through a raw pointer in skb->cb. For example, with igmp_qrv set to 1 on the relay a tunnel lives for 135s, so a netem delay of 180s on the amt device outlives it; when the tunnel expires and is freed, the subsequent dequeue triggers a use-after-free in amt_dev_xmit(). BUG: KASAN: slab-use-after-free in amt_dev_xmit+0x2763/0x2e20 Call Trace: amt_dev_xmit+0x2763/0x2e20 [drivers/net/amt.c:1262] dev_hard_start_xmit+0x22f/0x620 sch_direct_xmit+0x12e/0xac0 netem_dequeue+0x333/0xc50 net_tx_action+0x35c/0xa60 amt_send_igmp_gq() and amt_send_mld_gq() are only called from amt_request_handler(), inside the rcu_read_lock_bh() section of amt_rcv(). amt_request_handler() already has the tunnel the query is for: it found or created it inside that section. Queuing the query with dev_queue_xmit() only leads back into amt_dev_xmit(), which strips the Ethernet header and calls amt_send_membership_query() for that tunnel. Make that call directly from the two senders instead, the same way amt_send_advertisement() transmits from the receive path. The query never waits in a qdisc, the tunnel is only dereferenced inside the RCU section that found or created it, and nothing is stored in skb->cb, so no lookup or refcount is needed. Remove the query branch of amt_dev_xmit(), amt_skb_cb() and struct amt_skb_cb, which have no users left. Behaviour changes: - The relay's own General Queries no longer pass through the amt device's egress path: its qdisc, tc egress (clsact/tcx), the netfilter egress hook and packet taps. They are still visible as UDP on the underlay. - A query that is sent successfully is no longer counted as tx_dropped. The old query branch left through the unlock label, which counted every sent query as dropped. - A query that reaches amt_dev_xmit() on a relay from elsewhere, such as a userspace querier, is now dropped at the IGMP/MLD type switch. Before, it trusted whatever skb->cb held, and a NULL tunnel hit the WARN_ON(1).

NVD references (6)

Sources

The tracker combines official, freely usable data sources – updated continuously:

This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.

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