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CVE-2026-74579
netfilter: nft_payload: fix mask build for partial field offload
Published 2026-08-17 (2 months ago) by the Linux kernel CNA. Subsystem net/netfilter. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-74579 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixedDLA-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6466-1 · 2026-08-25 · text | 6.12.105-1 | Update the kernel and reboot. DSA-6466-1 shipped the fix on 2026-08-25, in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.8-1 | Update the kernel and reboot. Fixed in 7.1.8-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.8-1 | Update the kernel and reboot. Fixed in 7.1.8-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4788-1 · 2026-09-19 · text | 6.12.107-1~deb12u1 | Update the kernel and reboot. DLA-4788-1 shipped the fix on 2026-09-19, in 6.12.107-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
nft_payload_offload_mask() builds the offload match mask for a payload expression that covers only part of a header field. For a partial IPv6 address match (field_len = 16, priv_len = 1) that shift is 1 << 120, which is undefined on the 32-bit int operand. It also trims only one word, so the remaining words stay 0xffffffff (and when priv_len is a multiple of 4 the trim is skipped entirely), leaving the mask covering more bytes than the rule matches. UBSAN: shift-out-of-bounds in net/netfilter/nft_payload.c:278:20 shift exponent 120 is too large for 32-bit type 'int' ... The match is byte-granular and struct nft_data is zero-initialised, so the correct mask is simply the first priv_len bytes set to 0xff. Set those bytes directly and drop the word/shift trimming; this removes the undefined shift and no longer over-masks the trailing bytes.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
net/netfilter/nft_payload.c
- What this is
- NETFILTERMAINTAINERS section matched on net/netfilter/ · netfilter-devel@vger.kernel.org · Maintained
- Kernel module
- nf_tables
Check it on the machine
grep -wE 'CONFIG_NETFILTER|CONFIG_NF_TABLES' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
lsmod | grep -w nf_tablesPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sid, bookworm · linux-6.12m
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_NETFILTER, CONFIG_NF_TABLES
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-17 (2 months ago)source
- CVSS
- 7.1 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.12%, higher than 1.7% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/netfilter
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.10 | 5.10.265 | 3ee7b3f813b1 |
| 5.10 | 5.15.216 | 8720df4504e0 |
| 5.10 | 6.1.183 | 363c3a84a946 |
| 5.10 | 6.6.151 | b19b5d2e042c |
| 5.10 | 6.12.103 | a375d8ace807 |
| 5.10 | 6.18.44 | 16b553c46e34 |
| 5.10 | 7.1.8 | 630295d5bba1 |
| 5.10 | 7.2 | 39e88f28fb32 |
| 5.9.14 | 0 | 0 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4788-1 | 2026-09-19 | bookworm 6.12.107-1~deb12u1 | tracker |
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | tracker |
| DSA-6466-1 | 2026-08-25 | trixie 6.12.105-1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- The flaw is reached only when installing nftables rules via NETLINK_NETFILTER (NFT_MSG_NEWRULE) on NFT_CHAIN_HW_OFFLOAD netdev ingress chains; nft_flow_rule_create() calls nft_payload_offload_mask() during expr offload setup, which is a local netlink syscall path per kernel CNA guidance for nftables/netfilter.
- AC:LAttack complexity, low
- An attacker with CAP_NET_ADMIN can deterministically craft a partial-field payload match (e.g., priv_len=1 on IPv6 saddr with field_len=16) on an HW-offload chain; no race, memory layout, or other conditions outside attacker control are required to hit the undefined shift and incorrect mask build.
- PR:LPrivileges required, low
- All nf_tables netlink operations are gated by netlink_net_capable(skb, CAP_NET_ADMIN) in nfnetlink_rcv(), checked against the socket network namespace user_ns; unprivileged local users routinely obtain CAP_NET_ADMIN via user+network namespaces (unshare -Urn) on cloud hosts and containers.
- UI:NUser interaction, none
- No victim interaction is required; the attacker installs the malformed HW-offload nftables rule through their own netlink socket and triggers the bug during rule commit without needing another user to mount filesystems, open files, or take any action.
- S:UScope, unchanged
- Impact is confined to the host kernel and its hardware-offloaded netdev ingress filtering policy (incorrect TC flower masks, potential kernel crash); this is standard kernel/network-authority impact, not a VM escape, IOMMU bypass, or cross-security-boundary scope change.
- C:NConfidentiality impact, none
- The bug corrupts on-stack bitmask construction during rule installation and does not perform out-of-bounds reads, use-after-free, or other memory-disclosure primitives; no attacker-influenced kernel memory read path exists beyond incorrect mask values.
- I:HIntegrity impact, high
- Incorrect or UB-corrupted HW flower match masks misprogram NIC ingress ACLs on hardware-offloaded edge firewalls, causing accept/drop/redirect actions to apply to traffic contrary to administrator intent; this is exploitable integrity compromise of enforced network policy on internet-facing offload deployments.
- A:HAvailability impact, high
- The undefined shift (1<<120) triggers documented UBSAN shift-out-of-bounds at rule install and can panic/oops on UBSAN/trap kernels; misprogrammed HW offload masks can also blackhole or mis-drop production ingress traffic on netdev-offloaded chains, satisfying CNA high availability guidance.