CVE-2025-40173 in Linux
Summary
by MITRE • 11/12/2025
In the Linux kernel, the following vulnerability has been resolved:
net/ip6_tunnel: Prevent perpetual tunnel growth
Similarly to ipv4 tunnel, ipv6 version updates dev->needed_headroom, too. While ipv4 tunnel headroom adjustment growth was limited in commit 5ae1e9922bbd ("net: ip_tunnel: prevent perpetual headroom growth"), ipv6 tunnel yet increases the headroom without any ceiling.
Reflect ipv4 tunnel headroom adjustment limit on ipv6 version.
Credits to Francesco Ruggeri, who was originally debugging this issue and wrote local Arista-specific patch and a reproducer.
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Analysis
by VulDB Data Team • 02/08/2026
The vulnerability CVE-2025-40173 addresses a critical issue in the Linux kernel's IPv6 tunneling implementation that could lead to resource exhaustion and system instability. This flaw specifically affects the ip6_tunnel subsystem where the kernel fails to properly limit the growth of network device headroom parameters, creating a potential denial of service vector through perpetual tunnel expansion. The vulnerability stems from an inconsistency between IPv4 and IPv6 tunneling mechanisms, where the IPv4 implementation already received a fix to prevent unbounded headroom growth while the IPv6 counterpart remained vulnerable to unlimited expansion. The issue was identified and initially debugged by Francesco Ruggeri who developed both a local Arista-specific patch and a reproducer demonstrating the problematic behavior.
The technical flaw manifests in the net/ip6_tunnel subsystem where IPv6 tunnels continuously increase the dev->needed_headroom parameter without imposing any upper bounds on this growth. This parameter represents the amount of headroom that network devices need to accommodate additional headers when packets traverse tunnel boundaries. Unlike the IPv4 tunnel implementation which was patched in commit 5ae1e9922bbd to prevent perpetual headroom growth, the IPv6 version lacks this crucial limitation, allowing the headroom value to increase indefinitely with each tunnel operation. The root cause lies in the missing ceiling mechanism that would cap the headroom growth, enabling malicious actors to exploit this behavior through repeated tunnel operations.
From an operational impact perspective, this vulnerability creates a significant risk of system resource exhaustion and potential denial of service conditions. As the headroom parameter grows without bounds, it consumes increasing amounts of memory and system resources, eventually leading to performance degradation or complete system instability. Network administrators may observe gradual system slowdowns, memory allocation failures, or even kernel panics as the tunneling subsystem consumes excessive resources. The vulnerability is particularly concerning in environments with high tunneling activity or where automated tunnel creation mechanisms exist, as the headroom growth can occur rapidly and silently. Attackers could potentially exploit this by creating multiple tunnels in succession, causing progressive resource consumption that eventually renders the system unusable.
The mitigation strategy involves implementing the same headroom growth limitation that was applied to the IPv4 tunnel implementation, ensuring consistency between both tunneling protocols. This requires modifying the IPv6 tunnel code to introduce a ceiling on dev->needed_headroom growth, similar to the approach taken in commit 5ae1e9922bbd. The fix should establish reasonable upper bounds for headroom expansion while maintaining the functionality of legitimate tunnel operations. Organizations should prioritize applying the kernel patches that implement this fix, particularly in environments where IPv6 tunneling is actively used. Additionally, monitoring systems should be configured to detect unusual headroom growth patterns that might indicate exploitation attempts, and network configurations should be reviewed to minimize unnecessary tunnel creation operations.
This vulnerability aligns with CWE-770, which describes allocation of resources without limits or appropriate checking, and represents a classic case of unbounded resource growth leading to denial of service conditions. The issue also maps to ATT&CK technique T1499.004, which covers network disruption through resource exhaustion attacks, and T1070.006, involving the manipulation of system resources to achieve malicious objectives. The security implications extend beyond simple resource consumption, as this vulnerability could enable attackers to systematically degrade network infrastructure performance or cause cascading failures in complex network environments where tunneling is extensively utilized for traffic management and routing.