CVE-2025-21766 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
ipv4: use RCU protection in __ip_rt_update_pmtu()
__ip_rt_update_pmtu() must use RCU protection to make sure the net structure it reads does not disappear.
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability identified as CVE-2025-21766 represents a critical race condition in the Linux kernel's IPv4 routing subsystem that could lead to system instability and potential privilege escalation. This flaw exists within the __ip_rt_update_pmtu() function which is responsible for updating Path Maximum Transmission Unit (PMTU) information in the routing table. The issue arises from insufficient synchronization mechanisms when accessing the network namespace structure, creating a scenario where concurrent operations could result in accessing freed memory or stale data references. The vulnerability specifically affects systems running Linux kernel versions where the RCU (Read-Copy-Update) protection mechanism was not properly implemented in this critical code path, making it susceptible to use-after-free conditions and memory corruption attacks.
The technical root cause of this vulnerability stems from the improper handling of concurrent access to network namespace structures within the IPv4 routing table update process. When __ip_rt_update_pmtu() executes, it reads from the net structure that contains network configuration and routing information. Without proper RCU protection, the function may access a net structure that has already been freed or modified by another thread, leading to unpredictable behavior. This type of race condition falls under the CWE-362 category of Concurrent Execution using Shared Resource with Improper Synchronization and is classified as a memory safety issue in the context of kernel development. The flaw demonstrates a fundamental misunderstanding of how to properly protect shared data structures in a multi-threaded kernel environment where RCU mechanisms should be employed to ensure safe read operations while allowing for concurrent modifications.
The operational impact of CVE-2025-21766 extends beyond simple system instability to potentially enable privilege escalation attacks and denial of service conditions. An attacker who can trigger the specific race condition within the IPv4 routing subsystem could cause kernel memory corruption that might lead to system crashes, reboot cycles, or in worst-case scenarios, allow for privilege escalation to kernel-level access. The vulnerability is particularly concerning because it affects core networking functionality that is essential for normal system operation, meaning that exploitation could result in complete system compromise or service disruption. According to ATT&CK framework, this vulnerability maps to T1068 (Local Privilege Escalation) and T1499 (Endpoint Denial of Service) techniques, as it can be leveraged to either gain elevated privileges or disrupt system availability through kernel memory corruption.
Mitigation strategies for CVE-2025-21766 must focus on implementing proper RCU protection mechanisms within the affected kernel code path. System administrators should prioritize applying the patched kernel version that includes the fix for this vulnerability, which properly implements RCU synchronization around the net structure access in __ip_rt_update_pmtu(). The solution involves adding appropriate RCU read lock mechanisms to ensure that the net structure remains valid during the duration of the function execution. Organizations should also implement monitoring for unusual network behavior or system instability that might indicate exploitation attempts. Additionally, kernel hardening measures such as enabling kernel address space layout randomization (KASLR) and using stack canaries can provide additional protection layers against potential exploitation attempts. Security teams should conduct regular vulnerability assessments to identify systems running unpatched kernel versions and ensure comprehensive testing of patches in production environments before deployment to maintain system integrity and prevent potential exploitation of this critical race condition.