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CVE-2026-64009
xfrm: Check for underflow in xfrm_state_mtu
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem net/xfrm. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-64009 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 | 6.1.176-1 | Update the kernel and reboot. Fixed in 6.1.176-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.94-1 | Update the kernel and reboot. Fixed in 6.12.94-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
Leo Lin reported OOB write issue in esp component: xfrm_state_mtu() returns u32 but performs its arithmetic in unsigned modulo-2^32 space using an attacker-influenced "header_len + authsize + net_adj" subtracted from a small "mtu" argument. A nobody user can install an IPv4 ESP tunnel SA with a large authentication key (XFRMA_ALG_AUTH_TRUNC, e.g. hmac(sha512), 64-byte key, 64-byte trunc), configure a small interface MTU (68 bytes), and set XFRMA_TFCPAD to a large value. When a single UDP datagram is then sent through the tunnel, xfrm_state_mtu() underflows to a near-2^32 value, and esp_output() consumes it as a signed int via: padto = min(x->tfcpad, xfrm_state_mtu(x, mtu_cached)) esp.tfclen = padto - skb->len (assigned to int) esp.tfclen ends up negative (e.g. -207). It is sign-extended to size_t when passed to memset() inside esp_output_fill_trailer(), producing a ~16 EB write of zeroes at skb_tail_pointer(skb). KASAN logs it as "Write of size 18446744073709551537 at addr ffff888...". Check for underflow and return 1. This causes the sendmsg attempt to fail with ENETUNREACH.
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/xfrm/xfrm_state.c
- What this is
- NETWORKING [IPSEC]MAINTAINERS section matched on net/xfrm/ · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_XFRM /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.
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sidy
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switch
- CONFIG_XFRM
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
- 2026-07-19 (3 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
- 0.14%, higher than 3.3% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/xfrm
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 2.6.22 | 5.10.259 | 8014f70c4e6e |
| 2.6.22 | 5.15.210 | 1021d2877b68 |
| 2.6.22 | 6.1.176 | 2a41b1b31c61 |
| 2.6.22 | 6.6.143 | fccd685b32df |
| 2.6.22 | 6.12.93 | 820e501be8ae |
| 2.6.22 | 6.18.35 | 82ac903e0b51 |
| 2.6.22 | 7.0.12 | 3db50ceeacb5 |
| 2.6.22 | 7.1 | 742b04d0550b |
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 vulnerability is triggered on the local ESP output path when a process sends a UDP datagram through an attacker-configured XFRM tunnel via sendmsg(); remote peers cannot install XFRM SAs, set interface MTU, or control XFRMA_TFCPAD on the victim system.
- AC:LAttack complexity, low
- The attacker fully controls all preconditions (ESP tunnel SA with large HMAC-SHA512 auth, MTU 68, XFRMA_TFCPAD, and UDP payload size) and can trigger the underflow deterministically with a single datagram as demonstrated in the fix commit.
- PR:LPrivileges required, low
- XFRM state/policy installation and interface MTU configuration require CAP_NET_ADMIN, which an unprivileged local user can obtain in a user namespace via unshare -Urn; this is the most severe reasonable deployment scenario on multi-tenant hosts.
- UI:NUser interaction, none
- Exploitation requires only the attacker's own netlink configuration and socket send operations; no action from another user or administrator is needed.
- S:UScope, unchanged
- The vulnerability corrupts kernel heap memory from a local attacker's security context, constituting standard kernel privilege escalation rather than crossing a VM or hypervisor boundary.
- C:HConfidentiality impact, high
- The bug produces a massive out-of-bounds write via memset() at the skb tail pointer; such heap corruption is exploitable for arbitrary kernel memory disclosure in addition to the demonstrated KASAN-detected write primitive.
- I:HIntegrity impact, high
- A negative tfclen sign-extended to size_t causes an enormous memset() OOB write at skb_tail_pointer(skb), providing a memory corruption primitive that can be leveraged for arbitrary kernel writes and code execution.
- A:HAvailability impact, high
- The OOB write reliably triggers kernel oops/panic (KASAN reports a write of size 18446744073709551537 bytes), causing immediate system crash and denial of service.