CVE-2026-23111 netfilter: nf_tables: fix inverted genmask check in nft_map_catchall_activate()
Local attackers can exploit a use-after-free in Linux kernel nftables to achieve privilege escalation via user namespaces.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-23111?
CVE-2026-23111 is a high-severity use after free vulnerability in Linux kernel, published on 13 Feb 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
netfilter: nf_tables: fix inverted genmask check in nft_map_catchall_activate()
nft_map_catchall_activate() has an inverted element activity check compared to its non-catchall counterpart nft_mapelem_activate() and compared to what is logically required.
nft_map_catchall_activate() is called from the abort path to re-activate catchall map elements that were deactivated during a failed transaction. It should skip elements that are already active (they don't need re-activation) and process elements that are inactive (they need to be restored). Instead, the current code does the opposite: it skips inactive elements and processes active ones.
Compare the non-catchall activate callback, which is correct:
nft_mapelem_activate(): if (nft_set_elem_active(ext, iter->genmask)) return 0; /* skip active, process inactive */
With the buggy catchall version:
nft_map_catchall_activate(): if (!nft_set_elem_active(ext, genmask)) continue; /* skip inactive, process active */
The consequence is that when a DELSET operation is aborted, nft_setelem_data_activate() is never called for the catchall element. For NFT_GOTO verdict elements, this means nft_data_hold() is never called to restore the chain->use reference count. Each abort cycle permanently decrements chain->use. Once chain->use reaches zero, DELCHAIN succeeds and frees the chain while catchall verdict elements still reference it, resulting in a use-after-free.
This is exploitable for local privilege escalation from an unprivileged user via user namespaces + nftables on distributions that enable CONFIG_USER_NS and CONFIG_NF_TABLES.
Fix by removing the negation so the check matches nft_mapelem_activate(): skip active elements, process inactive ones.
Which versions are affected by CVE-2026-23111?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-23111?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-23111 being exploited?
There is no known exploitation of CVE-2026-23111 in the wild and no public exploit code.
Public exploit code: 10 proofs of concept on GitHub.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-77483, BELL-CVE-2026-23111, DEBIAN-CVE-2026-23111, GHSA-jh9m-9mr6-3ghc, ROOT-OS-DEBIAN-11-CVE-2026-23111, ROOT-OS-DEBIAN-12-CVE-2026-23111 and 9 more.