CVE-2025-21763 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
neighbour: use RCU protection in __neigh_notify()
__neigh_notify() can be called without RTNL or RCU protection.
Use RCU protection to avoid potential UAF.
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability identified as CVE-2025-21763 resides within the Linux kernel's networking subsystem, specifically in the neighbor management functionality. This issue affects the __neigh_notify() function which handles notifications related to network neighbor entries. The problem manifests when this function is invoked without proper synchronization mechanisms, creating a potential race condition that could lead to dangerous memory access patterns.
The technical flaw stems from insufficient protection mechanisms within the __neigh_notify() function implementation. When this function executes without proper RTNL (Routing Netlink) or RCU (Read-Copy-Update) synchronization primitives, it creates a scenario where concurrent access to neighbor table entries can occur. This lack of protection allows for a use-after-free condition to materialize, where memory that has been deallocated is still being accessed by the notification function. The vulnerability specifically targets the neighbor table management code that maintains mappings between network addresses and link-layer addresses, which is fundamental to network communication operations.
The operational impact of this vulnerability extends beyond simple memory corruption, potentially enabling privilege escalation and system instability. An attacker who can trigger the vulnerable code path may exploit the use-after-free condition to execute arbitrary code with kernel privileges, effectively compromising the entire system. This risk is particularly severe in environments where network neighbor table modifications are frequent or where unprivileged users can influence network operations. The vulnerability affects systems running Linux kernels that include the problematic neighbor table implementation, making it a widespread concern across various deployment scenarios from servers to embedded devices.
Mitigation strategies for CVE-2025-21763 require immediate kernel updates from vendors who have patched the issue with proper RCU protection mechanisms. The fix involves implementing appropriate synchronization primitives around the __neigh_notify() function to ensure that neighbor table entries remain valid during notification processing. Organizations should prioritize patching their kernel versions to prevent exploitation attempts, while also implementing monitoring for unusual network neighbor table activity that might indicate attempted exploitation. The vulnerability aligns with CWE-416, which addresses use-after-free errors, and represents a potential ATT&CK technique under privilege escalation categories where kernel-level memory corruption can be leveraged for system compromise.
The root cause of this vulnerability demonstrates a common pattern in kernel development where synchronization mechanisms are insufficiently applied to critical data structures. The neighbor table management code requires careful consideration of concurrent access patterns, particularly when notifications are generated in response to network events. Proper implementation of RCU mechanisms ensures that readers can access data without blocking writers while preventing access to freed memory locations, which directly addresses the core issue of the use-after-free condition. System administrators should also consider implementing additional network security measures including firewall rules and network segmentation to limit potential attack vectors that could exploit this kernel vulnerability.