CVE-2026-53000 Overview
CVE-2026-53000 affects the Linux kernel's netfilter Network Address Translation (NAT) subsystem. The vulnerability stems from improper release of nf_hook_ops structures used by the NAT dispatcher. The fix updates nf_nat_register_fn() to use kfree_rcu when releasing ops, ensuring deferred freeing consistent with how other netfilter hooks are handled.
The issue surfaces because the netfilter hook dump interface added in v5.14 peeks into nf_hook_ops structures stored at the tail of the datapath pointer-array blob. NAT hooks are invoked indirectly through the central NAT dispatcher, and their ops structures must be deferred to avoid use-after-free conditions when partial registration fails or when nfnetlink_hook exposes them.
Critical Impact
A race between NAT hook teardown and concurrent readers of the netfilter hook dump interface can lead to use-after-free access of nf_hook_ops memory in kernel space.
Affected Products
- Linux kernel versions containing the netfilter NAT hook dump exposition introduced in v5.14
- Linux kernel stable trees referenced in the upstream commits
- Distributions shipping the affected mainline kernel before the fix was backported
Discovery Timeline
- 2026-06-24 - CVE-2026-53000 published to NVD
- 2026-06-24 - Last updated in NVD database
Technical Details for CVE-2026-53000
Vulnerability Analysis
The Linux netfilter subsystem registers packet processing callbacks through nf_hook_ops structures. For performance, the datapath does not directly reference the original ops; instead it uses a compact pointer-array blob with the ops appended at the tail. Historically this design tolerated immediate freeing of the original ops because the datapath did not consult them after registration.
Linux kernel 5.14 introduced a userspace facility that dumps active netfilter hooks via nfnetlink_hook. This dumper reads from the trailing nf_hook_ops data in the live blob. NAT hooks are special: they are invoked indirectly through a central NAT dispatcher and remain invisible to the current dumper. Once the dumper begins enumerating NAT ops, immediate freeing of those structures becomes unsafe.
Root Cause
The root cause is missing read-copy-update (RCU) deferral when releasing nf_hook_ops allocated by nf_nat_register_fn(). Concurrent readers in the netfilter hook dump path can observe these ops while the registration error path or teardown logic frees them synchronously. The fix replaces synchronous kfree with kfree_rcu, aligning NAT hook lifetime with the RCU grace period observed by readers.
Attack Vector
Triggering the condition requires the ability to cause partial registration failures of NAT hooks while another thread reads the netfilter hook table through nfnetlink_hook. Both operations typically require privileges associated with network configuration (CAP_NET_ADMIN) in the relevant network namespace. The vulnerability is therefore a local kernel memory-safety issue rather than a remote attack surface.
For exploitation mechanics, see the upstream commits: kernel commit 32fdd2e3, kernel commit 3c7511f3, and kernel commit 6eda0d77.
Detection Methods for CVE-2026-53000
Indicators of Compromise
- Kernel oops or KASAN: use-after-free reports referencing nf_nat_register_fn, nf_hook_run, or nfnetlink_hook_dump
- Unexpected kernel panics on hosts performing concurrent iptables, nftables, or conntrack reconfiguration
- Audit log entries showing nfnetlink_hook dump requests coinciding with NAT rule reloads
Detection Strategies
- Monitor kernel ring buffer (dmesg) and journalctl -k output for netfilter-related slab corruption or RCU stall warnings
- Compare running kernel version (uname -r) against vendor advisories listing the three stable commits referenced above
- Audit kernel build configuration to confirm CONFIG_NF_NAT and CONFIG_NETFILTER_NETLINK_HOOK are both enabled, which is the prerequisite combination for the race
Monitoring Recommendations
- Track processes invoking NFNL_SUBSYS_HOOK netlink operations alongside NAT configuration changes
- Alert on repeated CAP_NET_ADMIN usage in non-administrative containers that load netfilter rules
- Enable kernel hardening telemetry (KASAN or KFENCE) on canary nodes to surface memory-safety regressions early
How to Mitigate CVE-2026-53000
Immediate Actions Required
- Apply the upstream stable kernel updates that include commits 32fdd2e3, 3c7511f3, and 6eda0d77
- Subscribe to your Linux distribution's security tracker for backported fixes and reboot patched hosts
- Restrict CAP_NET_ADMIN in untrusted containers and user namespaces to reduce the local attack surface
Patch Information
The fix updates nf_nat_register_fn() to release nf_hook_ops via kfree_rcu, deferring memory reclamation until all RCU readers, including the nfnetlink_hook dump path, have completed. The patch also corrects the error path so that partial hook exposition does not free ops that may still be observed. Refer to the upstream commits 32fdd2e3, 3c7511f3, and 6eda0d77 for the exact source changes.
Workarounds
- Disable the netfilter hook dump interface by ensuring nfnetlink_hook is not loaded if it is not required: rmmod nfnetlink_hook and blacklist via /etc/modprobe.d/
- Limit which user namespaces can configure netfilter by setting kernel.unprivileged_userns_clone=0 on distributions that expose it
- Avoid concurrent NAT rule reloads and hook enumeration on production hosts until the patched kernel is deployed
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
