CVE-2021-47109 in Linux
Summary
by MITRE • 03/15/2024
In the Linux kernel, the following vulnerability has been resolved:
neighbour: allow NUD_NOARP entries to be forced GCed
IFF_POINTOPOINT interfaces use NUD_NOARP entries for IPv6. It's possible to fill up the neighbour table with enough entries that it will overflow for valid connections after that.
This behaviour is more prevalent after commit 58956317c8de ("neighbor: Improve garbage collection") is applied, as it prevents removal from entries that are not NUD_FAILED, unless they are more than 5s old.
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Analysis
by VulDB Data Team • 11/23/2025
The vulnerability described in CVE-2021-47109 represents a significant denial of service weakness within the Linux kernel's network stack implementation. This issue specifically affects the neighbor table management system that handles IPv6 addressing for point-to-point network interfaces. The problem manifests when IFF_POINTOPOINT interfaces utilize NUD_NOARP entries, which are designed to maintain static neighbor cache entries without requiring dynamic address resolution. However, the kernel's garbage collection mechanism fails to properly handle these specific entry types, creating a potential avenue for resource exhaustion attacks.
The technical flaw stems from the kernel's neighbor table garbage collection algorithm introduced in commit 58956317c8de, which implements stricter policies for removing stale neighbor entries. While this enhancement was intended to improve stability and prevent premature removal of valid entries, it inadvertently creates a vulnerability where NUD_NOARP entries remain in the table indefinitely unless they exceed the 5-second age threshold. This behavior becomes particularly problematic on systems handling high volumes of point-to-point connections, as each connection may create a persistent entry that cannot be reclaimed by the garbage collector.
The operational impact of this vulnerability extends beyond simple resource consumption, potentially enabling sophisticated denial of service attacks against network services. When the neighbor table becomes saturated with NUD_NOARP entries, legitimate network traffic may be disrupted as the system cannot accommodate new valid connections. This affects not only basic network functionality but also critical services that depend on stable network connectivity, potentially causing cascading failures in network infrastructure. The vulnerability affects systems running Linux kernel versions where the problematic garbage collection logic was introduced, making it a widespread concern across various deployment environments.
This vulnerability maps directly to CWE-400, which addresses unspecified resource exhaustion issues, and aligns with ATT&CK technique T1499.004 for network denial of service. The issue demonstrates how seemingly benign kernel optimization changes can create unexpected security implications. The root cause lies in the improper handling of neighbor table entries for point-to-point interfaces, where the garbage collection logic fails to account for special entry types that should be subject to forced removal under certain conditions. Organizations should implement immediate mitigation strategies including kernel updates to versions containing the fix, monitoring neighbor table utilization, and implementing network-level rate limiting to prevent exploitation. Additionally, system administrators should consider adjusting network configuration parameters to minimize the creation of excessive neighbor entries and regularly audit network interface configurations to identify potentially vulnerable point-to-point connections.