CVE-2026-64009 xfrm: Check for underflow in xfrm_state_mtu
A local attacker can trigger an integer underflow in the Linux kernel's xfrm subsystem, leading to an out-of-bounds write and potential code execution or system crash.
- 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-2026-64009?
CVE-2026-64009 is a high-severity vulnerability in Linux kernel, published on 19 Jul 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:
xfrm: Check for underflow in xfrm_state_mtu
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.
Which versions are affected by CVE-2026-64009?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-64009?
Apply the vendor's patch.
Is CVE-2026-64009 being exploited?
There is no known exploitation of CVE-2026-64009 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2026-64009, DEBIAN-CVE-2026-64009, GHSA-666q-fgc6-3x4w, ROOT-OS-UBUNTU-2204-CVE-2026-64009, UBUNTU-CVE-2026-64009.