CVE-2026-53240 in Linux
Summary
by MITRE • 06/25/2026
In the Linux kernel, the following vulnerability has been resolved:
xfrm: iptfs: fix use-after-free on first_skb in __input_process_payload
__input_process_payload() stores first_skb into xtfs->ra_newskb under drop_lock when starting partial reassembly, then unlocks and breaks out of the processing loop. The post-loop check reads xtfs->ra_newskb without the lock to decide whether first_skb is still owned:
if (first_skb && first_iplen && !defer && first_skb != xtfs->ra_newskb)
Between spin_unlock and this read, a concurrent CPU running iptfs_reassem_cont() (or the drop_timer hrtimer) can complete reassembly, NULL xtfs->ra_newskb, and free the skb. The check then evaluates first_skb != NULL as true, and pskb_trim/ip_summed/consume_skb operate on the freed skb — a use-after-free in skbuff_head_cache.
Replace the unlocked read with a local bool that records whether first_skb was handed to the reassembly state in the current call. The flag is set after the existing spin_unlock, before the break, using the pointer equality that is stable at that point (first_skb == skb iff first_skb was stored in ra_newskb).
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Analysis
by VulDB Data Team • 06/25/2026
The vulnerability resides within the Linux kernel's IPsec tunnel forwarding subsystem, specifically in the xfrm module's iptfs implementation where a use-after-free condition can occur during partial packet reassembly operations. This flaw manifests in the __input_process_payload function which manages the processing of fragmented packets destined for IPsec tunnel forwarding. The issue stems from improper synchronization between concurrent execution paths that handle packet reassembly, creating a temporal window where memory safety is compromised.
The technical implementation involves a race condition between two distinct kernel execution contexts that access shared reassembly state data structures. During the initial packet processing phase, the function stores a pointer to the first_skb into the xtfs->ra_newskb field while holding the drop_lock, then immediately releases the lock and exits the processing loop. However, a critical gap exists in the subsequent post-loop validation logic where the code performs an unlocked read of xtfs->ra_newskb to determine if the first_skb remains under the current function's ownership. This access pattern creates a window of vulnerability between the spin_unlock operation and the subsequent conditional check.
The operational impact of this vulnerability extends beyond simple memory corruption, as it represents a classic use-after-free scenario that can be exploited for arbitrary code execution or system instability. The race condition occurs because the concurrent execution of iptfs_reassem_cont() or drop_timer hrtimer operations can complete the reassembly process while the main processing thread has already released its lock but before completing its validation checks. When these concurrent operations NULL out the xtfs->ra_newskb pointer and subsequently free the associated skb structure, the main thread continues execution with a dangling pointer reference.
The flaw aligns with CWE-416, which specifically addresses use-after-free vulnerabilities, and demonstrates characteristics consistent with ATT&CK technique T1059.008 for kernel-level code execution. The vulnerability exploits the fundamental principle of memory safety in concurrent systems where shared data structures are accessed without proper synchronization primitives during critical sections. The fix implements a defensive programming approach by introducing a local boolean flag that captures the ownership status of first_skb at a point where pointer equality is guaranteed to be stable, eliminating the race condition through careful control flow management.
The mitigation strategy directly addresses the root cause by removing the unlocked memory access pattern that created the vulnerability window. Instead of relying on potentially stale pointer values from shared state, the solution employs local variable storage that captures the ownership semantics at a precise moment in execution. This approach ensures that the decision whether to operate on first_skb is made based on stable, locally available information rather than potentially modified shared data. The fix requires minimal code changes while providing comprehensive protection against the race condition, maintaining all existing functionality while eliminating the security vulnerability through careful synchronization design.
The solution demonstrates proper kernel development practices by addressing concurrency issues at the synchronization level rather than attempting to work around them with complex locking strategies. This approach reduces complexity while ensuring memory safety, aligning with best practices for kernel-level concurrent programming and system reliability. The vulnerability represents a significant security concern in systems that rely on IPsec tunnel forwarding for network security, particularly in environments where packet fragmentation occurs frequently or where attackers might attempt to exploit memory corruption for privilege escalation.