Bug 2507809 (CVE-2026-64542)

Summary: CVE-2026-64542 kernel: ipv6: ndisc: fix NULL deref in accept_untracked_na()
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's IPv6 Neighbor Discovery (NDISC) component. An unprivileged user, operating within a network namespace, can trigger a null pointer dereference in the `accept_untracked_na()` function. This occurs due to a race condition where the `inet6_dev` structure is re-fetched without a null check, while another process concurrently clears the device's IPv6 pointer. Successful exploitation of this vulnerability leads to a kernel crash, resulting in a Denial of Service (DoS).
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Description OSIDB Bzimport 2026-07-27 21:05:56 UTC 
In the Linux kernel, the following vulnerability has been resolved:

ipv6: ndisc: fix NULL deref in accept_untracked_na()

accept_untracked_na() re-fetches the inet6_dev with __in6_dev_get(dev)
and dereferences idev->cnf.accept_untracked_na without a NULL check,
even though its only caller ndisc_recv_na() already fetched and
NULL-checked idev for the same device.

Both reads of dev->ip6_ptr run in the same RCU read-side critical
section, but a concurrent addrconf_ifdown() can clear dev->ip6_ptr
between them: lowering the MTU below IPV6_MIN_MTU calls addrconf_ifdown()
without the synchronize_net() that orders the unregister path, so the
re-fetch returns NULL and oopses:

 BUG: KASAN: null-ptr-deref in ndisc_recv_na (net/ipv6/ndisc.c:974)
 Read of size 4 at addr 0000000000000364
 Call Trace:
  <IRQ>
  ndisc_recv_na (net/ipv6/ndisc.c:974)
  icmpv6_rcv (net/ipv6/icmp.c:1193)
  ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:479)
  ip6_input_finish (net/ipv6/ip6_input.c:534)
  ip6_input (net/ipv6/ip6_input.c:545)
  ip6_mc_input (net/ipv6/ip6_input.c:635)
  ipv6_rcv (net/ipv6/ip6_input.c:351)
  </IRQ>

It is reachable by an unprivileged user via a network namespace.

Pass the caller's already validated idev instead of re-fetching it; the
idev stays alive for the whole RCU critical section, so it is safe even
after dev->ip6_ptr has been cleared.
Comment 1 TEJ RATHI 2026-07-29 04:59:06 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026072736-CVE-2026-64542-fe95@gregkh/T