CVE-2026-98438 Linux CVE debrief

Who should care

Defenders responsible for Linux kernel deployments using MPTCP should assess exposure and apply patches to mitigate potential risks. This includes verifying MPTCP usage, reviewing compensating controls, and monitoring for potential issues with DATA_FIN handling. Security teams and operators should prioritize patching and verifying affected systems to prevent exploitation of this vulnerability.

Why it matters

Defenders should prioritize verifying MPTCP usage and applying patches to mitigate potential risks associated with the Linux kernel vulnerability.

• Verify MPTCP DATA_FIN handling in Linux kernel deployments
• Assess exposure to potential issues with uninitialized values

Technical summary

The Linux kernel's MPTCP implementation had an uninitialized value issue when sending a DATA_FIN without data. The fix involves initializing specific fields for this case, including setting the csum field to 0 when the checksum has been negotiated. This change ensures proper handling of DATA_FIN without data in MPTCP, preventing potential issues with uninitialized values. The fix was applied to the Linux kernel to address this vulnerability. Defenders should prioritize verifying MPTCP usage and applying patches to mitigate potential risks.

Defensive priority

Defenders should prioritize verifying MPTCP usage and applying patches to mitigate potential risks.

Recommended defensive actions

• Verify MPTCP usage in Linux kernel deployments
• Apply patches to address the vulnerability
• Monitor for potential issues with MPTCP DATA_FIN handling
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The issue was resolved by initializing specific fields for the DATA_FIN case. The fix involves setting the csum field to 0 when the checksum has been negotiated. This change addresses the uninitialized value issue when sending a DATA_FIN without data in the Linux kernel's MPTCP implementation. The fix was applied to ensure proper initialization of fields in this specific case.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.