CVE-2026-72137 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
xfrm: nat_keepalive: avoid double free on send error
nat_keepalive_send() frees the keepalive skb whenever the IPv4 or IPv6 send helper reports an error.
That cleanup is only correct before the skb is handed to the output path. Once ip_build_and_send_pkt() or ip6_xmit() takes ownership, the networking stack may already have consumed the skb before returning an error, so freeing it again is unsafe.
Handle the pre-handoff failure cases inside nat_keepalive_send_ipv4() and nat_keepalive_send_ipv6(), where the caller still owns the skb, and keep nat_keepalive_send() responsible only for family dispatch and the unsupported-family cleanup path.
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Analysis
by VulDB Data Team • 08/15/2026
This vulnerability exists within the Linux kernel's IPsec implementation, specifically in the xfrm subsystem's NAT keepalive functionality. The issue stems from improper memory management during packet transmission errors, creating a potential double free condition that could lead to system instability or exploitation. The problem manifests when the nat_keepalive_send() function attempts to free a socket buffer (skb) after an error occurs during packet transmission, but fails to account for the timing of when the networking stack takes ownership of the skb.
The technical flaw occurs because the nat_keepalive_send() function performs cleanup operations on the skb regardless of whether it has been passed to the output path. When ip_build_and_send_pkt() or ip6_xmit() functions are called, they may consume the skb and process it through the network stack before returning an error code. If the original function then attempts to free the same skb, it results in a double free scenario that violates fundamental memory safety principles. This condition is particularly dangerous because it can lead to memory corruption, potentially allowing attackers to execute arbitrary code or cause denial of service.
The operational impact of this vulnerability extends beyond simple system crashes, as it represents a critical memory corruption issue within the kernel's networking stack. Attackers could potentially exploit this double free condition to gain elevated privileges or disrupt network services. The vulnerability affects systems using IPsec with NAT traversal capabilities, making it particularly relevant for enterprise environments that rely on secure network communications and VPN implementations. Such systems may experience unexpected restarts, data loss, or security breaches if exploited successfully.
The fix implemented addresses the core issue by reorganizing the error handling logic to properly separate pre-handoff failure cases from post-handoff scenarios. The solution modifies nat_keepalive_send_ipv4() and nat_keepalive_send_ipv6() functions to handle cleanup operations only when the caller maintains ownership of the skb, while keeping the main nat_keepalive_send() function focused on family dispatch and unsupported-family cleanup paths. This approach aligns with common security practices that emphasize proper resource management and adherence to the principle of least privilege in kernel code. The fix prevents memory corruption by ensuring that each skb is freed exactly once, thereby maintaining system stability and preventing potential exploitation vectors.
From a cybersecurity perspective, this vulnerability maps to CWE-415 which describes double free conditions in software systems. The mitigation strategy follows established best practices for kernel memory management and aligns with defensive programming principles recommended by organizations like the Open Web Application Security Project. The solution demonstrates proper error handling within kernel space, where resource cleanup must be carefully coordinated with the networking stack's ownership semantics. This type of vulnerability often requires deep understanding of kernel networking internals and proper synchronization mechanisms to prevent race conditions and memory corruption scenarios that could compromise system integrity.
The vulnerability highlights the importance of maintaining strict control over memory ownership in kernel-space code, particularly when dealing with network packet processing. Modern security frameworks such as those referenced in MITRE ATT&CK framework would categorize this as a memory corruption technique that could be leveraged for privilege escalation or denial of service attacks. The fix represents a defensive programming approach that prevents the exploitation of timing-dependent memory management errors, which are notoriously difficult to detect and reproduce in production environments.
This type of vulnerability underscores the critical importance of thorough code review processes for kernel security patches, as memory safety issues can have far-reaching consequences across network infrastructure. The solution demonstrates how careful analysis of resource ownership semantics can prevent serious security flaws while maintaining functional integrity of the networking stack. Organizations should prioritize regular kernel updates and maintain robust security monitoring to detect potential exploitation attempts targeting such low-level memory corruption vulnerabilities in their network infrastructure components.