CVE-2026-98438 mptcp: options: fix uninit-value in mptcp_write_data_fin
An attacker can read uninitialized kernel memory via crafted MPTCP packets, potentially exposing sensitive data.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-98438?
CVE-2026-98438 is a vulnerability in Linux, published on 18 Oct 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:
mptcp: options: fix uninit-value in mptcp_write_data_fin
When sending a DATA_FIN without data, and because the DATA_FIN occupies 1 octet of the connection-level sequence space [1], it is then required to add a DSS mapping with specific values.
If the checksum has been negotiated, it also needs to be computed, and included in the outgoing packet, and thus the initial csum data needs to be reset to 0 as well. This is no longer the case since commit cfcceb7a39fc ("tcp: shrink per-packet memset in __tcp_transmit_skb()"), because the whole ext_copy structure is no longer zeroed by default.
This seems to be the only case where use_map is changed and set afterwards, so initialising the csum field only in this case, along with other fields for this specific case.
Which versions are affected by CVE-2026-98438?
How do I fix CVE-2026-98438?
Apply the vendor's patch.
Is CVE-2026-98438 being exploited?
There is no known exploitation of CVE-2026-98438 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-98438, GHSA-232g-622w-prrg, UBUNTU-CVE-2026-98438.