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CVE-2026-72250
netfilter: nf_conntrack_reasm: guard mac_header adjustment after IPv6 defrag
Published 2026-08-15 (2 months ago) by the Linux kernel CNA. Subsystem net/ipv6. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-72250 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 | fixed | 6.1.180-1 | Update the kernel and reboot. Fixed in 6.1.180-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.100-1 | Update the kernel and reboot. Fixed in 6.12.100-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
nf_ct_frag6_reasm() slides the packet head forward to drop the IPv6 fragment header and then unconditionally advances skb->mac_header: skb->mac_header += sizeof(struct frag_hdr); On the NF_INET_LOCAL_OUT defrag path the skb has no link-layer header yet, so skb->mac_header is still the "not set" sentinel (u16)~0U. Adding sizeof(struct frag_hdr) wraps it to a small value (0xffff + 8 == 7), after which skb_mac_header_was_set() wrongly reports a MAC header is present and skb_mac_header() points into the headroom. The reassembler has done this unconditional add since it was introduced; it was harmless while mac_header was a bare pointer, but wrong once mac_header became a u16 offset whose unset state is the ~0U sentinel tested by skb_mac_header_was_set(). The sibling net/ipv6/reassembly.c does the same relocation and does guard the adjustment; mirror the guard here.
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/ipv6/netfilter/nf_conntrack_reasm.c
- What this is
- NETFILTERMAINTAINERS section matched on net/*/netfilter/ · netfilter-devel@vger.kernel.org · Maintained
- Kernel module
- nf_defrag_ipv6
Check it on the machine
grep -wE 'CONFIG_NETFILTER|CONFIG_NF_DEFRAG_IPV6' /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_defrag_ipv6Prints 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, sidm
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_NETFILTER, CONFIG_NF_DEFRAG_IPV6
- 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
- 2026-08-15 (2 months ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.17%, higher than 5.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/ipv6
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 2.6.15 | 5.10.261 | 6e8cd710ca35 |
| 2.6.15 | 5.15.212 | 2a95ec21824a |
| 2.6.15 | 6.1.178 | bbcdef2061b1 |
| 2.6.15 | 6.6.145 | cd0d7bbc027b |
| 2.6.15 | 6.12.97 | 53ef70a31542 |
| 2.6.15 | 6.18.40 | a58230f3a7c4 |
| 2.6.15 | 7.1.5 | 00bdce2fda7e |
| 2.6.15 | 7.2 | 3b08fed5b7e0 |
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 bug triggers only when nf_ct_frag6_reasm() runs on NF_INET_LOCAL_OUT with skb->mac_header still unset (~0U). That requires injecting pre-fragmented IPv6 into the local output path (e.g., AF_INET6 SOCK_RAW), not standard remotely received PRE_ROUTING traffic where mac_header is already valid and the adjustment is correct.
- AC:LAttack complexity, low
- An attacker who can send raw IPv6 fragments can reliably complete conntrack reassembly by crafting matching fragment ID/source/destination tuples. nf_defrag_ipv6 is enabled automatically whenever IPv6 connection tracking is in use (typical on nftables/iptables hosts), so no rare race or admin-only timing is needed.
- PR:LPrivileges required, low
- Triggering handcrafted IPv6 fragments requires CAP_NET_RAW. On systems with unprivileged user namespaces, a local user can obtain CAP_NET_RAW (and CAP_NET_ADMIN for conntrack) via unshare -Urn, so required privileges are Low rather than High root-only access.
- UI:NUser interaction, none
- Exploitation requires only attacker-controlled fragmented IPv6 traffic and an active conntrack defrag path; it does not depend on any victim user action such as opening files, mounting filesystems, or clicking links.
- S:UScope, unchanged
- Impact is skb metadata corruption within the kernel network stack in the same security domain. It does not inherently cross VM, container runtime, or IOMMU boundaries without separate privilege-escalation exploitation.
- C:HConfidentiality impact, high
- Adding sizeof(frag_hdr) to the unset mac_header sentinel wraps to a small offset, so skb_mac_header_was_set() becomes true and skb_mac_header() points into skb headroom. Downstream netfilter/nft L2 helpers can then perform out-of-bounds reads, leaking stale kernel data from headroom.
- I:HIntegrity impact, high
- The bogus mac_header and inflated skb_mac_header_len() misdirect later L2 header access and nft payload/meta write paths to incorrect offsets in skb headroom or packet data, enabling memory corruption that can be leveraged for arbitrary kernel writes or control-flow hijacking.
- A:HAvailability impact, high
- Corrupted skb header state can cause kernel oops, panic, or fatal faults when subsequent LOCAL_OUT, netfilter, or output routines use the invalid MAC header pointer, and the associated memory corruption can crash the system even without full exploit development.