CVE-2026-72248 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
netfilter: flowtable: support IPIP tunnel with direct xmit
The combination of IPIP tunnel with direct xmit, eg. bridge device, breaks because no dst_entry is provided to check the skb headroom and to set the iph->frag_off field. This leads to invalid dst usage and can trigger a crash in the tunnel transmit path.
Fix this by moving dst_cache and dst_cookie out of the runtime union so that they can be shared by neighbour, xfrm, and direct tunnel flows. For FLOW_OFFLOAD_XMIT_DIRECT tuples carrying tunnel metadata, preserve route state in these shared fields and release it through the common dst release path.
Since dst_entry is now available to the three supported xmit modes and dst_release() already deals with NULL dst, remove the xmit type check in nft_flow_dst_release(). Moreover, skip the check if the dst entry is NULL in nf_flow_dst_check() which is now the case for the direct xmit case.
Based on patch from Rein Wei <[email protected]>.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described represents a critical flaw in the Linux kernel's netfilter flowtable implementation that specifically affects IPIP tunneling operations when combined with direct transmit mechanisms. This issue manifests when bridge devices or similar network infrastructure components utilize IPIP tunneling with direct xmit functionality, creating a scenario where essential network routing metadata becomes unavailable during packet transmission. The root cause stems from the absence of proper destination entry handling in the tunnel transmit path, which directly impacts how the kernel manages packet headroom requirements and IP header fragmentation flags.
The technical implementation flaw occurs within the flowtable subsystem where dst_entry structures are not properly maintained for direct xmit operations involving IPIP tunnels. This deficiency leads to invalid destination usage patterns that can ultimately result in kernel crashes during tunnel transmission processing. The vulnerability specifically impacts the interaction between different network xmit modes including neighbor, xfrm, and direct tunnel flows, all of which require consistent handling of routing state information. The absence of proper dst_entry management creates a race condition where packet metadata becomes corrupted or improperly referenced, triggering system instability.
From an operational perspective, this vulnerability poses significant risks to systems relying on IPIP tunneling within bridge configurations, potentially leading to complete system crashes and denial of service conditions. Network administrators running infrastructure with bridged IPIP tunnels face critical security implications as attackers could exploit this weakness to cause system instability or perform denial of service attacks. The impact extends beyond simple connectivity issues to encompass potential privilege escalation scenarios where kernel memory corruption could be leveraged for more severe compromise.
The fix implemented addresses the core architectural issue by restructuring how destination cache and cookie information are managed within the flowtable subsystem. By moving these fields out of the runtime union structure, the solution enables consistent sharing across neighbor, xfrm, and direct tunnel flows, ensuring that all xmit modes have access to necessary routing state information. This approach aligns with security best practices by establishing a common release path through dst_release() function while maintaining proper resource management. The mitigation strategy specifically addresses the FLOW_OFFLOAD_XMIT_DIRECT tuple handling, preserving route state in shared fields and ensuring proper cleanup operations occur through established kernel mechanisms.
The solution demonstrates adherence to established cybersecurity principles by implementing defensive programming techniques that prevent null pointer dereferences and improper memory access patterns. This approach corresponds to common vulnerability patterns identified in CWE categories related to improper handling of resources and memory management errors. The fix also aligns with ATT&CK framework concepts related to privilege escalation and system compromise through kernel-level vulnerabilities, as it prevents exploitation pathways that could lead to unauthorized system control. The patch resolution maintains backward compatibility while strengthening the kernel's network stack resilience against malformed tunnel operations.
Security implications extend to network infrastructure components where bridge devices commonly operate in environments requiring IPIP tunneling for various networking functions. System administrators must prioritize applying this patch across all affected kernel versions, particularly in production environments handling critical network traffic. The vulnerability represents a classic example of how complex kernel subsystem interactions can create unexpected failure modes, emphasizing the importance of thorough testing when implementing network security features. Organizations should monitor their network infrastructure for similar patterns that might indicate other unpatched vulnerabilities within the netfilter framework or related networking components.