CVE-2026-80847

/// Technical description

In the Linux kernel, the following vulnerability has been resolved:

tcp: clamp route advmss to TCP_MIN_MSS

tcp_select_initial_window() assumes that callers never pass an MSS

smaller than 1, but route-derived advmss values can violate that

assumption.

A too-small explicit RTAX_ADVMSS is one way to get there, but it is not

the only one. The same divide-by-zero can also be reached through the

"default advmss" path when RTAX_ADVMSS is left at 0 and the effective

advmss is later driven down by route MTU and min_adv_mss.

Introduce a tcp_dst_advmss() helper that clamps route advmss to

TCP_MIN_MSS before TCP consumes it, and use it in the TCP paths that

derive advmss from dst metrics. This keeps the effective MSS from

dropping to zero before tcp_select_initial_window() rounds the receive

window.

/// 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