CVE-2026-53240 Overview
CVE-2026-53240 is a use-after-free vulnerability in the Linux kernel's IP Traffic Flow Security (IPTFS) implementation within the xfrm subsystem. The flaw resides in the __input_process_payload() function, where a concurrent CPU or the drop_timer hrtimer can free the first_skb socket buffer while another code path still references it. Operations such as pskb_trim, ip_summed, and consume_skb then act on freed memory in the skbuff_head_cache slab. The issue has been resolved upstream via three stable-tree commits.
Critical Impact
Concurrent reassembly paths can free the first_skb between an unlocked pointer read and its subsequent use, producing a use-after-free condition in kernel slab memory.
Affected Products
- Linux kernel — xfrm IPTFS subsystem
- Distributions shipping affected stable kernel branches prior to the fixes referenced in commits 8d9a79f, eb48730b, and ff2ee35b
- Systems configured with XFRM IPTFS tunnels performing partial reassembly
Discovery Timeline
- 2026-06-25 - CVE-2026-53240 published to NVD
- 2026-06-25 - Last updated in NVD database
Technical Details for CVE-2026-53240
Vulnerability Analysis
The vulnerability exists in __input_process_payload() within the kernel's xfrm IPTFS code path. When the function begins a partial reassembly, it stores first_skb into xtfs->ra_newskb while holding drop_lock. It then releases the lock and exits the processing loop. After the loop, the function re-examines xtfs->ra_newskb without reacquiring the lock to determine whether first_skb is still owned by the caller.
The unlocked read introduces a race window. A concurrent CPU executing iptfs_reassem_cont(), or the drop_timer hrtimer firing, can finish the reassembly, set xtfs->ra_newskb to NULL, and free the underlying sk_buff. The post-loop check first_skb && first_iplen && !defer && first_skb != xtfs->ra_newskb then evaluates true because first_skb no longer matches the now-NULL ra_newskb. Subsequent calls to pskb_trim, the ip_summed assignment, and consume_skb operate on the freed skb.
Root Cause
The root cause is a missing synchronization invariant between the producer and consumer of xtfs->ra_newskb. The code relies on a pointer comparison performed outside the lock that previously protected the assignment. Once the lock is dropped, ownership of the skb can transfer to the reassembly state machine, which is permitted to free it. This is a classic Use-After-Free [CWE-416] driven by a Race Condition [CWE-362].
Attack Vector
Triggering the condition requires concurrent IPTFS reassembly activity on an active XFRM tunnel. An attacker capable of injecting or influencing IPTFS-encapsulated packets to a vulnerable endpoint could drive reassembly state transitions while the drop_timer is armed, increasing the probability of the race. Successful exploitation corrupts memory in the skbuff_head_cache slab, which can lead to kernel memory corruption, information disclosure, or denial of service depending on heap state.
The upstream fix replaces the unlocked read with a local boolean computed before releasing drop_lock. The flag records whether first_skb was handed to the reassembly state in the current call using the stable pointer equality first_skb == skb. Refer to the kernel commits 8d9a79f, eb48730b, and ff2ee35b for the full patch text.
Detection Methods for CVE-2026-53240
Indicators of Compromise
- Kernel oops or panic messages referencing __input_process_payload, iptfs_reassem_cont, pskb_trim, or consume_skb in stack traces.
- KASAN reports flagging use-after-free in skbuff_head_cache on hosts running XFRM IPTFS tunnels.
- Unexpected drops or corruption of IPTFS-encapsulated traffic correlated with hrtimer drop_timer activity.
Detection Strategies
- Enable KASAN on test or staging kernels to surface use-after-free conditions in the xfrm IPTFS path.
- Compare running kernel versions against the patched stable branches containing commits 8d9a79f, eb48730b, and ff2ee35b.
- Monitor kernel ring buffer (dmesg) for warnings emitted from net/xfrm/xfrm_iptfs.c during high reassembly load.
Monitoring Recommendations
- Centralize kernel logs and alert on BUG:, KASAN:, or general protection fault entries naming IPTFS or XFRM symbols.
- Track XFRM state changes and IPTFS tunnel counters via ip xfrm and nstat to detect abnormal reassembly activity.
- Inventory kernel build versions across the fleet and flag hosts running unpatched branches.
How to Mitigate CVE-2026-53240
Immediate Actions Required
- Apply the upstream kernel patches referenced in commits 8d9a79f, eb48730b, and ff2ee35b or upgrade to a distribution kernel that has incorporated them.
- Identify hosts using XFRM IPTFS tunnels and prioritize them for patching.
- Reboot patched hosts to ensure the corrected kernel is active, as xfrm code paths cannot be hot-patched in most deployments.
Patch Information
The fix is committed to the Linux stable tree across three references: Kernel Git Commit 8d9a79f, Kernel Git Commit eb48730b, and Kernel Git Commit ff2ee35b. The patch replaces the unlocked read of xtfs->ra_newskb with a local boolean set under drop_lock before the loop break, eliminating the race window.
Workarounds
- Disable XFRM IPTFS tunnels on affected hosts until the patched kernel is deployed.
- Restrict ingress of IPTFS-encapsulated traffic to trusted peers via firewall rules where IPTFS cannot be disabled.
- Limit exposure of XFRM endpoints to untrusted networks to reduce the probability of attacker-driven reassembly races.
# Verify whether the running kernel includes the fix commits
uname -r
zcat /proc/config.gz 2>/dev/null | grep -i XFRM_IPTFS
# Inspect active XFRM state and policies
ip -s xfrm state
ip xfrm policy
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
