CVE-2026-64009

A flaw was found in the Linux kernel's xfrm (IPSec framework) component. A local, unprivileged user can trigger an integer underflow in the Encapsulating Security Payload (ESP) module's Maximum Transmission Unit (MTU) calculation. This underflow leads to a severe out-of-bounds write, which can cause memory corruption. The primary impact of this vulnerability is a denial of service, with potential for further system compromise.

An IMPORTANT severity flaw was found in the Linux kernel's xfrm (IPsec) framework as shipped in Red Hat Enterprise Linux and Red Hat In-Vehicle OS. A local, unprivileged attacker can trigger an integer underflow in the xfrm_state_mtu() function of the ESP module by installing an IPv4 ESP tunnel security association with a large authentication key, configuring a small interface MTU, and setting a large TFC padding value. The underflowed MTU result produces a negative padding length that is passed to memset() in esp_output_fill_trailer(), causing a severe out-of-bounds write and kernel memory corruption. The primary impact is denial of service, with potential for further system compromise. The kernel and kernel-rt packages in the affected Red Hat Enterprise Linux streams and the kernel packages in Red Hat In-Vehicle OS are affected, and a fix is being pursued for these products. Red Hat assigns this flaw a CVSS 3.1 base score of 7.0 (AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H) because the specific combination of security association, MTU, and padding configuration required makes the attack complex, whereas the upstream CVE record assigns 7.8 (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H).

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.