CVE-2026-64542 in Linux
Summary
by MITRE • 07/28/2026
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.
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Analysis
by VulDB Data Team • 07/28/2026
The vulnerability described represents a critical null pointer dereference in the Linux kernel's IPv6 neighbor discovery implementation, specifically within the accept_untracked_na() function. This flaw exists in the network stack's handling of neighbor advertisement messages and stems from improper NULL pointer validation during concurrent execution scenarios. The issue manifests when the kernel processes incoming IPv6 neighbor advertisements through the ndisc_recv_na() function, which internally calls accept_untracked_na() without proper safeguards against race conditions. The vulnerability demonstrates a classic race condition pattern where the kernel re-fetches network device configuration data using __in6_dev_get(dev) after an initial validation has already occurred, creating a window where concurrent operations can invalidate the retrieved pointer.
The technical root cause lies in the improper handling of RCU (Read-Copy-Update) critical sections within the IPv6 networking subsystem. When ndisc_recv_na() processes incoming neighbor advertisements, it first retrieves the inet6_dev structure through __in6_dev_get(dev), validates it for NULL, and then later calls accept_untracked_na() which performs the same re-fetch operation without additional NULL checking. This design flaw becomes exploitable when concurrent operations such as addrconf_ifdown() execute between these two fetch operations. The addrconf_ifdown() function can clear the dev->ip6_ptr pointer when lowering MTU below IPV6_MIN_MTU, creating a scenario where the second fetch returns NULL while the first validation has already passed. This race condition creates a direct path to kernel oops and potential system crashes through KASAN (Kernel Address Sanitizer) detection.
The operational impact of this vulnerability extends beyond simple system stability concerns into potential denial-of-service scenarios and privilege escalation risks. An unprivileged user with network namespace access can trigger this condition by sending crafted neighbor advertisement messages, leading to kernel memory corruption and system instability. The vulnerability affects systems running Linux kernels with IPv6 support, particularly those handling dynamic network configuration changes where multiple threads or processes might concurrently modify network device states. This presents a significant risk in environments where network configuration changes occur frequently or where untrusted users have access to network namespaces, as the attack surface can be exploited without requiring elevated privileges.
The fix for this vulnerability involves modifying the accept_untracked_na() function to utilize the already validated inet6_dev pointer passed from its caller rather than performing a redundant re-fetch operation. This approach eliminates the race condition by ensuring that the same validated pointer reference is used throughout the function execution, preventing the scenario where dev->ip6_ptr gets cleared between fetch operations. The solution aligns with established best practices for concurrent programming in kernel space, specifically addressing the RCU read-side critical section management patterns. This remediation approach directly addresses the underlying CWE-476 (NULL Pointer Dereference) vulnerability while maintaining the intended functionality of the neighbor discovery protocol. The fix also reflects proper adherence to ATT&CK framework concepts related to privilege escalation and defense evasion through kernel-level vulnerabilities, as it prevents exploitation paths that could lead to system compromise or denial-of-service conditions in networked environments.
This vulnerability demonstrates the complexity inherent in kernel-level race condition management and highlights the importance of careful pointer validation in concurrent environments. The fix serves as a model for similar issues in other kernel subsystems where re-fetching validated data structures without proper synchronization can create exploitable race windows. Security practitioners should monitor for similar patterns in other network stack implementations and ensure that all RCU critical sections properly account for potential concurrent modifications to shared data structures, particularly in high-frequency network processing paths where multiple threads may access the same device configuration data simultaneously.