CVE-2026-80848 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
xfrm: espintcp: fix UAF during close
ZDI reported and analyzed a race condition during close for espintcp sockets:
espintcp_close() frees emsg->skb via kfree_skb() without holding any socket lock. Concurrently, the xfrm_trans_reinject work queue invokes esp_output_tcp_finish() -> espintcp_push_skb() -> espintcp_push_msgs() -> skb_send_sock_locked(), which reads the same skb as a data source.
Fix this by adding a synchronize_rcu() call after resetting sk_prot, since esp_output_tcp_finish() runs under RCU and won't use a socket with sk_prot == &tcp_prot. Simply taking the socket lock in espintcp_close() could lead to leaks, if esp_output_tcp_finish() re-adds an skb in the slot we just freed. After this, the existing barrier() is no longer needed.
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Analysis
by VulDB Data Team • 09/04/2026
The Linux kernel vulnerability identified as a use-after-free (UAF) condition within the ESP-in-TCP implementation of the XFRM subsystem represents a critical race condition during socket closure operations. This flaw was reported and analyzed by Zero Day Initiative, highlighting a concurrency issue where memory safety is compromised due to improper synchronization between resource deallocation and ongoing asynchronous processing tasks. The vulnerability specifically affects the espintcp protocol handler, which encapsulates IPsec ESP packets within TCP segments to facilitate tunneling over networks that may block UDP traffic or require specific transport-layer characteristics.
The technical root cause lies in the interaction between the socket close routine and a background work queue mechanism. When an espintcp socket is closed, the function espintcp_close() proceeds to free the associated sk_buff structure via kfree_skb(). This deallocation occurs without holding any form of socket lock or synchronization primitive that would prevent concurrent access to the same memory region. Simultaneously, the xfrm_trans_reinject work queue executes a chain of functions including esp_output_tcp_finish(), which calls espintcp_push_skb() and subsequently espintcp_push_msgs(). These functions eventually invoke skb_send_sock_locked(), attempting to read from the very sk_buff that has just been freed by the close routine. This creates a classic use-after-free scenario where kernel code accesses memory that is no longer valid, potentially leading to data corruption or arbitrary code execution if an attacker can control the contents of the reclaimed memory slab.
The operational impact of this vulnerability is severe due to its potential for privilege escalation and system instability. A local unprivileged user who can create and close espintcp sockets may trigger this race condition by carefully timing socket closure with network activity that activates the reinjection work queue. Successful exploitation could allow an attacker to read kernel memory, overwrite critical data structures, or crash the system through a denial of service. The complexity arises from the fact that simple locking strategies are insufficient; taking the standard socket lock in espintcp_close() is problematic because it may lead to resource leaks if esp_output_tcp_finish() re-adds an sk_buff into the slot immediately after it has been freed, creating another class of memory management errors.
To mitigate this vulnerability, the Linux kernel maintainers implemented a fix that leverages Read-Copy-Update (RCU) synchronization mechanisms rather than traditional locking. The solution involves adding a synchronize_rcu() call immediately after resetting the sk_prot field in the socket structure. This approach is effective because esp_output_tcp_finish() operates under RCU read-side critical sections, ensuring it will not access or use a socket once its protocol handler has been cleared to &tcp_prot. By waiting for all pre-existing RCU readers to complete before proceeding with cleanup, the kernel ensures that no concurrent thread can be reading from the freed memory. This method avoids the potential deadlocks and leak scenarios associated with traditional mutexes in this specific context. The existing barrier() instruction used previously is rendered unnecessary by this robust synchronization strategy.
From a classification perspective, this vulnerability aligns with CWE-416: Use After Free, as it involves accessing memory after it has been made available for reuse without proper safeguards against concurrent access. In the MITRE ATT&CK framework, this type of flaw facilitates privilege escalation techniques where an attacker leverages kernel vulnerabilities to gain higher-level system permissions. The fix demonstrates a sophisticated understanding of Linux kernel concurrency models, utilizing RCU grace periods to ensure memory safety in asynchronous network processing paths. System administrators should apply available kernel updates that include this patch to prevent exploitation on affected systems.