Medium [CVE-2026-64538] Fix null-ptr-deref in fib6_nh_mtu_change
This medium-severity Red Hat Linux advisory covers CVE-2026-64538 affecting Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 9.
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Summary
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 A flaw was found in the Linux kernel's IPv6 networking component.
A null pointer dereference vulnerability exists in the `fib6_nh_mtu_change()` function. An attacker could potentially trigger this condition, leading to a kernel crash and a Denial of Service (DoS).
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · high-confidence parse· fetched 12 days ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 12 days ago·verify at source
Mitigation checklist
- There is no mitigation for this issue other than applying the fix. Exploitation requires an attacker on the same local network segment as the target, and requires a network interface carrying nexthop-backed IPv6 routes to be undergoing teardown at the same time an ICMPv6 Router Advertisement is processed against it, a race window that is not reliably controllable by an attacker.
Official advisory · high-confidence parse· fetched 12 days ago·verify at source
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