CVE-2025-21765 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
ipv6: use RCU protection in ip6_default_advmss()
ip6_default_advmss() needs 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-2025-21765 resides within the Linux kernel's IPv6 implementation and represents a classic race condition scenario that could lead to system instability or potential privilege escalation. This issue specifically affects the ip6_default_advmss() function which is responsible for determining the default advertised maximum segment size for IPv6 packets. The function operates within the network subsystem where it accesses network structure data that may be subject to concurrent modification or deallocation. The vulnerability manifests when the function reads from a network structure without proper RCU (Read-Copy-Update) protection mechanisms, creating a window where the referenced data could be freed or modified by another thread while the function is executing.
The technical flaw stems from insufficient synchronization primitives within the ip6_default_advmss() implementation, which operates under the assumption that the network structure it references remains stable throughout execution. This assumption fails in multi-threaded environments where network configuration changes or interface removals can occur concurrently with packet processing operations. The RCU mechanism provides a scalable synchronization method that allows readers to access data without blocking writers, but requires explicit protection when accessing shared data structures that may be modified. Without proper RCU protection, the function can dereference a pointer to a freed memory location, resulting in memory corruption or system crashes. This type of vulnerability falls under CWE-362, which describes a race condition where multiple threads access shared resources without proper synchronization, and aligns with ATT&CK technique T1068 which covers local privilege escalation through kernel vulnerabilities.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable more sophisticated attack vectors. An attacker who can trigger the specific race condition scenario could cause denial of service by crashing the network stack or potentially escalate privileges by corrupting kernel memory structures. The vulnerability is particularly concerning in high-traffic network environments where IPv6 packet processing occurs frequently, as the race condition window increases with system load. Systems running kernel versions prior to the patch would be susceptible to exploitation, with the risk being highest on servers handling substantial network traffic or systems with dynamic network configuration changes. The vulnerability demonstrates a critical gap in kernel security practices where even seemingly simple functions can become attack vectors when proper concurrency control mechanisms are omitted. The fix implemented involves adding appropriate RCU read lock protection around the network structure access, ensuring that the function can safely read from potentially shared data structures while modifications are in progress.
Mitigation strategies should focus on immediate kernel updates to versions containing the patched ip6_default_advmss() implementation, as this represents the most effective protection against exploitation. System administrators should prioritize patching in production environments, particularly for network servers and infrastructure components where the vulnerability could be exploited to cause significant service disruption. Monitoring for unusual network stack behavior or kernel oops messages should be implemented as additional defensive measures. The vulnerability highlights the importance of comprehensive security testing for kernel components, particularly those handling network packet processing and memory management operations. Organizations should also consider implementing network segmentation and access controls to limit the potential impact if exploitation were to occur, while maintaining regular kernel update schedules to address similar vulnerabilities proactively. The fix demonstrates the critical role of proper synchronization in kernel development and serves as a reminder that even well-established kernel functions require careful attention to concurrency safety mechanisms.