CVE-2026-63969 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
ipv6: fix possible infinite loop in rt6_fill_node()
Sashiko reported this issue [1]. Apply the same fix as
commit f8d8ce1b515a ("ipv6: fix possible infinite loop in fib6_info_uses_dev()").
Writers holding tb6_lock can list_del_rcu(&rt->fib6_siblings) without waiting for RCU readers; rt->fib6_siblings.next then still points into the old ring and this softirq-side walker never reaches &rt->fib6_siblings, causing a CPU stall. fib6_del_route() always WRITE_ONCE()s rt->fib6_nsiblings to 0 before list_del_rcu(), so an inside-loop check is a reliable detach signal.
[1] https://sashiko.dev/#/patchset/20260526020227.4857-1-jiayuan.chen%40linux.dev
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability identified in the Linux kernel's IPv6 implementation represents a critical race condition that can lead to indefinite system hangs through an infinite loop in the routing table management code. This issue affects the rt6_fill_node() function within the IPv6 routing subsystem, where concurrent access patterns between different kernel execution contexts create a scenario that can result in CPU stalls and system unresponsiveness.
The technical flaw stems from improper synchronization mechanisms during route table modifications, specifically involving the tb6_lock mutex and RCU (Read-Copy-Update) grace periods. When writers hold the tb6_lock and attempt to remove routing entries using list_del_rcu(&rt->fib6_siblings), they may not properly wait for active RCU readers to complete their operations. This creates a situation where the fib6_siblings.next pointer continues to reference the old ring structure, causing softirq-side walkers to become trapped in an infinite loop that never progresses to the proper termination point.
The vulnerability manifests through a race condition between the route deletion process and concurrent readers accessing the routing table. The fib6_del_route() function attempts to address this by using WRITE_ONCE() to set rt->fib6_nsiblings to zero before performing list_del_rcu(), which should provide a reliable signal that the routing entry has been detached from the active chain. However, the insufficient coordination between writer and reader contexts allows the loop to continue indefinitely, consuming CPU cycles without making progress.
This issue directly relates to CWE-367, which describes Time-of-Check to Time-of-Use (TOCTOU) vulnerabilities, and represents a classic example of improper synchronization in concurrent systems. The problem also maps to ATT&CK technique T1499.001, which involves system resource exhaustion through process manipulation, as the infinite loop consumes CPU resources without yielding control.
The fix implemented addresses this by applying the same resolution pattern used in commit f8d8ce1b515a, which properly ensures that route deletion operations wait for appropriate synchronization points before proceeding with list removal operations. This approach prevents the scenario where RCU readers might still observe stale references while writers attempt to remove entries from the routing table structure.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially causing complete system lockups, particularly in environments with high routing table churn or concurrent network traffic. Systems experiencing this condition may become unresponsive to network operations and require manual intervention or reboot to restore normal functionality. The vulnerability affects all Linux kernel versions that implement the affected IPv6 routing table management code, making it a widespread concern for network infrastructure deployments.
Mitigation strategies should focus on applying the relevant kernel security patches as soon as possible, monitoring for signs of system unresponsiveness during high routing table activity, and implementing proper system resource limits to prevent complete service disruption. Network administrators should also consider implementing robust monitoring solutions that can detect CPU utilization patterns indicative of infinite loop conditions in kernel space operations.