HIGH
netfilter Timeout Object UAF
CVE-2026-31665
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
KernelScan AI7.8HIGH
01Description
In the Linux kernel, the following vulnerability has been resolved: netfilter: nft_ct: fix use-after-free in timeout object destroy nft_ct_timeout_obj_destroy() frees the timeout object with kfree() immediately after nf_ct_untimeout(), without waiting for an RCU grace period. Concurrent packet processing on other CPUs may still hold RCU-protected references to the timeout object obtained via rcu_dereference() in nf_ct_timeout_data(). Add an rcu_head to struct nf_ct_timeout and use kfree_rcu() to defer freeing until after an RCU grace period, matching the approach already used in nfnetlink_cttimeout.c. KASAN report: BUG: KASAN: slab-use-after-free in nf_conntrack_tcp_packet+0x1381/0x29d0 Read of size 4 at addr ffff8881035fe19c by task exploit/80 Call Trace: nf_conntrack_tcp_packet+0x1381/0x29d0 nf_conntrack_in+0x612/0x8b0 nf_hook_slow+0x70/0x100 __ip_local_out+0x1b2/0x210 tcp_sendmsg_locked+0x722/0x1580 __sys_sendto+0x2d8/0x320 Allocated by task 75: nft_ct_timeout_obj_init+0xf6/0x290 nft_obj_init+0x107/0x1b0 nf_tables_newobj+0x680/0x9c0 nfnetlink_rcv_batch+0xc29/0xe00 Freed by task 26: nft_obj_destroy+0x3f/0xa0 nf_tables_trans_destroy_work+0x51c/0x5c0 process_one_work+0x2c4/0x5a0
02KernelScan AI Analysis
Risk summary
High severity vulnerability allowing potential memory corruption and system crashes through use-after-free conditions during network packet processing. Attackers with network access could potentially trigger this by manipulating netfilter timeout objects while network traffic is active.
Vulnerability analysis
Summary: Use-after-free vulnerability in netfilter nft_ct timeout object destruction due to improper RCU synchronization
Root Cause: Missing RCU synchronization - the timeout object is freed immediately without respecting RCU grace periods, while concurrent readers may still access it through RCU-protected references
Attack Surface: Network-accessible through TCP packet processing. Requires ability to trigger netfilter timeout object creation and destruction while concurrent network traffic is being processed
Fix Mechanism: Added rcu_head field to struct nf_ct_timeout and replaced kfree() with kfree_rcu() to defer freeing until after RCU grace period, ensuring all RCU readers complete before the object is freed
Details: The vulnerability occurs in nft_ct_timeout_obj_destroy() which immediately frees timeout objects with kfree() after calling nf_ct_untimeout(), without waiting for an RCU grace period. Concurrent packet processing on other CPUs may still hold RCU-protected references to the timeout object obtained via rcu_dereference() in nf_ct_timeout_data(). This creates a race condition where the timeout object can be accessed after being freed, leading to use-after-free conditions during TCP packet processing in nf_conntrack_tcp_packet().
03Fix Versions
| Branch | Fixed in | Patch commit |
|---|---|---|
| 5.10 | 5.10.253 | c458fc1c278a |
| 5.15 | 5.15.203 | c581e5c8f2b5 |
| 6.1 | 6.1.169 | f16fe84879a5 |
| 6.12 | 6.12.82 | aa7cfa16f98f |
| 6.18 | 6.18.23 | b42aca3660dc |
| 6.19 | 6.19.13 | d0983b48c10d |
| 6.6 | 6.6.135 | 070abdf1b043 |
| mainline | 7.0 | f8dca15a1b19 |