CVE-2026-74705 in Linux
Summary
by MITRE • 08/22/2026
In the Linux kernel, the following vulnerability has been resolved:
udp: fix potential use-after-free in tunnel segmentation
__skb_udp_tunnel_segment() gets the UDP header before ensuring the tunnel header is in the skb head. If the pull reallocates skb->head, the saved UDP header pointer is no longer valid.
Get the UDP header after the pull to avoid a potential use-after-free.
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Analysis
by VulDB Data Team • 08/22/2026
The Linux kernel vulnerability identified as a potential use-after-free within the udp tunnel segmentation logic stems from an incorrect ordering of memory access operations during packet processing. Specifically, the function __skb_udp_tunnel_segment() is responsible for handling UDP encapsulated packets by performing segmenting and checksum offloading tasks. The core technical flaw lies in how this routine retrieves pointers to protocol headers relative to buffer reallocation events. In network stack implementations, sk_buff structures often undergo memory management operations such as headroom adjustment or data pulling via functions like skb_pull(). These operations can trigger a reallocation of the underlying memory buffer if there is insufficient contiguous space at the beginning of the packet structure. When __skb_udp_tunnel_segment() retrieves a pointer to the UDP header before verifying that the tunnel headers are fully present and accessible within the sk_buff head, it assumes static memory addresses for these structures. If the subsequent pull operation reallocates the skb->head buffer due to insufficient room or alignment requirements, the previously saved pointer to the UDP header becomes stale. This results in a dangling pointer scenario where any further dereference of this address accesses invalid or freed memory regions, leading to undefined behavior that can manifest as kernel panics, data corruption, or potential privilege escalation if an attacker can control packet contents to trigger specific allocation paths.
From a security architecture perspective, this flaw represents a classic race condition between logical state verification and physical memory access patterns within the network driver layer. The vulnerability is categorized under CWE-416 which denotes use after free errors, specifically arising from improper handling of pointer validity following dynamic memory reallocation events inherent to packet buffer management in high-throughput networking scenarios. In terms of adversarial tactics, this type of kernel-level memory corruption aligns with MITRE ATT&CK technique T1055 related to process injection or system exploitation via local privilege escalation vectors where attackers might leverage such instability to crash the host for denial of service or potentially exploit subsequent state inconsistencies to gain unauthorized access. The impact extends beyond simple stability issues; in production environments handling high volumes of encapsulated traffic, particularly with VXLAN or GRE tunnels common in cloud and container networking stacks, this bug can be triggered remotely by sending malformed or specifically crafted UDP tunnel packets that force the kernel into reallocation paths while maintaining references to outdated memory locations.
Mitigation strategies primarily involve applying vendor-provided patches that reorder the header retrieval logic within __skb_udp_tunnel_segment(). The fix ensures that the UDP header pointer is obtained only after confirming that all necessary tunnel headers are present and that any required buffer adjustments have been safely completed without invalidating existing pointers. Administrators should prioritize updating their kernel versions to include this correction, particularly for systems actively participating in overlay networks or virtualized environments relying on UDP-based encapsulation protocols. Additionally, enabling runtime memory safety features such as KASAN can help detect similar issues during development and testing phases before they reach production deployments. Regular auditing of network stack configurations and monitoring for unexpected kernel oops messages related to udp tunneling modules are recommended operational practices to maintain resilience against these types of low-level memory management vulnerabilities in Linux-based infrastructure.