CVE-2026-80847 tcp: clamp route advmss to TCP_MIN_MSS
A remote attacker can cause a denial of service in the Linux kernel by triggering a divide-by-zero error via crafted TCP route metrics.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-80847?
CVE-2026-80847 is a denial of service vulnerability in Linux, published on 4 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-80847?
How do I fix CVE-2026-80847?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-80847 being exploited?
There is no known exploitation of CVE-2026-80847 in the wild and no public exploit code.
Sources
Also known as AZL-99588, BELL-CVE-2026-80847, DEBIAN-CVE-2026-80847, GHSA-m3f3-9j4q-c4f2, UBUNTU-CVE-2026-80847.