# CVE-2026-64538: ipv6: Fix null-ptr-deref in fib6_nh_mtu_change().

Status: published. Published: 2026-07-27. Updated: 2026-11-10.

In the Linux kernel, the following vulnerability has been resolved:

ipv6: Fix null-ptr-deref in fib6_nh_mtu_change().

fib6_nh_mtu_change() re-fetches idev via __in6_dev_get(arg->dev) and
dereferences idev->cnf.mtu6 without a NULL check. addrconf_ifdown()
clears dev->ip6_ptr with RCU_INIT_POINTER() after rt6_disable_ip() has
released tb6_lock, so the RA-driven MTU walk can observe a NULL idev and
oops. The caller rt6_mtu_change_route() guards its own __in6_dev_get(),
but this re-fetch is unguarded; nexthop-backed routes survive
addrconf_ifdown()'s flush, so the walk still reaches it after ip6_ptr is
nulled.

Return 0 when idev is NULL, matching rt6_mtu_change_route() and the
fib6_mtu() fix in commit 5ad509c1fdad ("ipv6: Fix null-ptr-deref in
fib6_mtu().").

  Oops: general protection fault, ... KASAN: null-ptr-deref in range
        [0x00000000000002a8-0x00000000000002af]
  RIP: 0010:fib6_nh_mtu_change+0x203/0x990
   rt6_mtu_change_route+0x141/0x1d0
   __fib6_clean_all+0xd0/0x160
   rt6_mtu_change+0xb4/0x100
   ndisc_router_discovery+0x24b5/0x2cb0
   icmpv6_rcv+0x12e9/0x1710
   ipv6_rcv+0x39b/0x410

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Linux / Linux: ≥ c0b220cf7d80eb8a36ae9f12cae6df4577a6814e; < d08d019f2f43a6f9a71e81868bbc326b3afaf37b (affected, git); ≥ c0b220cf7d80eb8a36ae9f12cae6df4577a6814e; < 1451deca9896957159f0666520a792c1b861af4f (affected, git); ≥ c0b220cf7d80eb8a36ae9f12cae6df4577a6814e; < b2c70dd3326809429b709a9c7e9220d29923051a (affected, git); ≥ c0b220cf7d80eb8a36ae9f12cae6df4577a6814e; < 302d57ed7872838b40e56a868fb4c7da7da606e9 (affected, git); ≥ c0b220cf7d80eb8a36ae9f12cae6df4577a6814e; < 80600b5d0f3ecb9324120dc95b5e915130f516c5 (affected, git); ≥ c0b220cf7d80eb8a36ae9f12cae6df4577a6814e; < b0d0eb13a0441a8ebf4f227843deaf494f1e2c33 (affected, git); ≥ c0b220cf7d80eb8a36ae9f12cae6df4577a6814e; < 6428634f7a0b7878144b4925c37856bef3224967 (affected, git); ≥ c0b220cf7d80eb8a36ae9f12cae6df4577a6814e; < 46c3b8191aad3d032776bf3bebf03efdf5f4b905 (affected, git). Source: CNA: Linux
- Linux / Linux: 5.3 (affected); ≥ 0; < 5.3 (unaffected, semver); ≥ 5.10.261; ≤ 5.10.* (unaffected, semver); ≥ 5.15.212; ≤ 5.15.* (unaffected, semver); ≥ 6.1.178; ≤ 6.1.* (unaffected, semver); ≥ 6.6.145; ≤ 6.6.* (unaffected, semver); ≥ 6.12.97; ≤ 6.12.* (unaffected, semver); ≥ 6.18.40; ≤ 6.18.* (unaffected, semver); ≥ 7.1.5; ≤ 7.1.* (unaffected, semver); ≥ 7.2; ≤ * (unaffected, original_commit_for_fix). Source: CNA: Linux
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0): ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0): ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0): ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0): ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0): ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0): ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0): ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0): ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0): ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0): ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP

## Severity and prioritization

- CISA KEV: No listing in collected sources

- EPSS: 0.20% as of 2026-11-09; percentile 9.96%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: Not reported

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://git.kernel.org/stable/c/d08d019f2f43a6f9a71e81868bbc326b3afaf37b (CNA: Linux)

- https://git.kernel.org/stable/c/1451deca9896957159f0666520a792c1b861af4f (CNA: Linux)

- https://git.kernel.org/stable/c/b2c70dd3326809429b709a9c7e9220d29923051a (CNA: Linux)

- https://git.kernel.org/stable/c/302d57ed7872838b40e56a868fb4c7da7da606e9 (CNA: Linux)

- https://git.kernel.org/stable/c/80600b5d0f3ecb9324120dc95b5e915130f516c5 (CNA: Linux)

- https://git.kernel.org/stable/c/b0d0eb13a0441a8ebf4f227843deaf494f1e2c33 (CNA: Linux)

- https://git.kernel.org/stable/c/6428634f7a0b7878144b4925c37856bef3224967 (CNA: Linux)

- https://git.kernel.org/stable/c/46c3b8191aad3d032776bf3bebf03efdf5f4b905 (CNA: Linux)

- https://cert-portal.siemens.com/productcert/html/ssa-019113.html (ADP: siemens-SADP)

- https://cert-portal.siemens.com/productcert/html/ssa-457936.html (ADP: siemens-SADP)

- https://git.kernel.org/stable/c/1451deca9896957159f0666520a792c1b861af4f (416baaa9-dc9f-4396-8d5f-8c081fb06d67)

- https://git.kernel.org/stable/c/302d57ed7872838b40e56a868fb4c7da7da606e9 (416baaa9-dc9f-4396-8d5f-8c081fb06d67)

- https://git.kernel.org/stable/c/46c3b8191aad3d032776bf3bebf03efdf5f4b905 (416baaa9-dc9f-4396-8d5f-8c081fb06d67)

- https://git.kernel.org/stable/c/6428634f7a0b7878144b4925c37856bef3224967 (416baaa9-dc9f-4396-8d5f-8c081fb06d67)

- https://git.kernel.org/stable/c/80600b5d0f3ecb9324120dc95b5e915130f516c5 (416baaa9-dc9f-4396-8d5f-8c081fb06d67)

- https://git.kernel.org/stable/c/b0d0eb13a0441a8ebf4f227843deaf494f1e2c33 (416baaa9-dc9f-4396-8d5f-8c081fb06d67)

- https://git.kernel.org/stable/c/b2c70dd3326809429b709a9c7e9220d29923051a (416baaa9-dc9f-4396-8d5f-8c081fb06d67)

- https://git.kernel.org/stable/c/d08d019f2f43a6f9a71e81868bbc326b3afaf37b (416baaa9-dc9f-4396-8d5f-8c081fb06d67)

- https://cert-portal.siemens.com/productcert/html/ssa-019113.html (0b142b55-0307-4c5a-b3c9-f314f3fb7c5e)

- https://cert-portal.siemens.com/productcert/html/ssa-457936.html (0b142b55-0307-4c5a-b3c9-f314f3fb7c5e)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2026-64538; retrieved 2026-11-10; source updated 2026-11-10

- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64538; retrieved 2026-11-06; source updated 2026-11-06

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-64538; retrieved 2026-11-10; source updated Not reported

Revision: 43c64fd5d9c373f4234ac33d14df57277ed9c39708fe1a06cb3a0a2905a82254

Canonical: https://y2.dev/cve/CVE-2026-64538

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