CVE-2026-53221 in Linux
Summary
by MITRE • 06/25/2026
In the Linux kernel, the following vulnerability has been resolved:
ip6_vti: fix incorrect tunnel matching in vti6_tnl_lookup()
In vti6_tnl_lookup(), when an exact match for a tunnel fails, the code falls back to searching for wildcard tunnels:
- Tunnels matching the packet's local address, with any remote address wildcard remote).
- Tunnels matching the packet's remote address, with any local address (wildcard local).
However, vti6 stores all these different types of tunnels in the same hash table (ip6n->tnls_r_l) prone to hash collisions.
The bug is that the fallback search loops in vti6_tnl_lookup() were missing checks to ensure that the candidate tunnel actually has a wildcard address.
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Analysis
by VulDB Data Team • 06/26/2026
The vulnerability in the Linux kernel's IPv6 virtual tunnel interface implementation stems from an incorrect tunnel matching mechanism within the vti6_tnl_lookup() function. This flaw affects the kernel's ability to properly route IPv6 packets through virtual tunnel interfaces, creating potential security risks and operational instability. The issue manifests when the system attempts to match incoming packets against existing tunnel configurations stored in a shared hash table structure.
The technical implementation problem occurs in how the kernel handles fallback tunnel matching when an exact match fails during packet processing. When a packet arrives, the system first attempts to find an exact tunnel configuration that matches both local and remote addresses. However, when this fails, the code implements a fallback mechanism to search for wildcard tunnels that match either the local address with any remote address or the remote address with any local address. This fallback logic is designed to provide flexible routing options but contains a critical oversight in its validation process.
The fundamental flaw lies in the hash table management approach where all tunnel types including exact matches, local wildcard matches, and remote wildcard matches are stored within the same hash table structure called ip6n->tnls_r_l. This shared storage creates conditions where different tunnel configurations can collide in the hash table, making proper matching more complex. The vulnerability specifically affects the fallback search algorithm which fails to properly validate whether candidate tunnels actually possess the required wildcard characteristics before considering them as valid matches.
This security issue has significant operational implications for systems running Linux kernels with IPv6 virtual tunnel interfaces. Attackers could potentially exploit this flaw to manipulate packet routing through incorrect tunnel selections, leading to man-in-the-middle attacks, traffic redirection, or denial-of-service conditions. The vulnerability impacts network security policies that rely on precise tunnel configuration matching and can compromise the integrity of encrypted or isolated network segments.
The root cause maps directly to CWE-284 Access Control Bypass and CWE-129 Improper Validation of Array Indexes, as the system fails to properly validate tunnel candidate selection during the fallback mechanism. From an ATT&CK perspective, this vulnerability relates to T1071.004 Application Layer Protocol: DNS and T1566 Phishing, as it could enable attackers to redirect network traffic through unauthorized tunnel configurations. The fix requires implementing proper validation checks in the fallback search loop to ensure that candidate tunnels actually possess the wildcard address characteristics they claim to have before being considered valid matches.
Mitigation strategies should include immediate kernel updates from trusted sources to apply the patched version of vti6_tnl_lookup() function, proper network segmentation to limit exposure, and monitoring for anomalous routing patterns that might indicate exploitation attempts. System administrators should also consider implementing additional network access controls and packet filtering rules to detect and prevent unauthorized tunnel configuration modifications. The vulnerability demonstrates the importance of proper input validation in kernel space code and highlights the need for comprehensive testing of fallback mechanisms in security-critical systems.