Residual Risk After Applying the CVE-2026-98438 Linux MPTCP Fix

Applying the CVE-2026-98438 fix to Linux MPTCP leaves residual risk. The patch addresses an uninitialized checksum condition, but it does not resolve all operational concerns about MPTCP services. After installation, teams must verify the running kernel, the intended protocol path, and evidence confirming normal teardown remains intact.

The bug, introduced in Linux 7.1 and fixed in 7.2.7 and 7.3-rc3, results from an uninitialized resource, leading to potential packet corruption, dropped packets, and denial of service. The patch eliminates a deterministic initial state for the checksum algorithm, preventing malformed or rejected packets from being generated. Verification requires ensuring the kernel is patched, the correct kernel version is running, and MPTCP traffic is tested with specific conditions.

Residual risk persists due to the possibility of unrelated MPTCP bugs, network policy mistakes, or peers mishandling teardown. The patch does not constitute a comprehensive hardening review for the protocol stack. Organizations must confirm the operating state, test with MPTCP traffic, and monitor production for any increase in teardown errors.

External exposure should be prioritized based on factors like load balancer configuration, service requirements, and vendor fixes. The patch eliminates the known DATA_FIN checksum defect, but the final residual-risk assessment depends on specific operational configurations and peer implementations.