CVE-2026-64009

/// Technical 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

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References