External risk intelligence

Linux Kernel MPTCP Timer Race Condition

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2025-40257

This vulnerability exists within the Linux kernel's MPTCP implementation, specifically affecting internal timer management logic. It is a low-level kernel code defect related to memory safety and concurrency, not a network-exposed service, application, or interface. It does not represent an externally reachable surface.

Use After Free

Halo Surface Signal: 1 out of 5 — much less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A recently resolved issue in the Linux kernel's MultiPath TCP (MPTCP) implementation could lead to a crash due to a race condition in timer management. This problem, identified by syzbot, has been addressed with RCU protection and improved variable naming, and is considered a low-level kernel code defect. The primary concern for leadership is to confirm if MPTCP is in use and exposed within the organization.

Fixes a kernel memory issue in networking code.
Concerns internal timer handling, not direct external access.
Confirm MPTCP usage and exposure in your environment.

Attack Path

How an attacker could exploit the issue

An attacker could trigger this vulnerability by sending specific network traffic that manipulates internal kernel timers within the MPTCP subsystem. This race condition, occurring during timer management, could lead to a use-after-free scenario.

Requires network access.
Triggers a race condition in timer handling.
Can lead to kernel memory corruption.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability in the Linux kernel's MPTCP component involves a race condition that could lead to a use-after-free error. This occurs when managing timers related to MPTCP's path management, potentially causing system instability or crashes under specific network conditions when MPTCP is active.

Kernel memory corruption
Race condition in timer management
System instability or crashes

Operational Fix

Recommended remediation, mitigation, and detection steps

This Linux kernel vulnerability requires careful triage by the team responsible for the kernel and its MPTCP implementation. The first step is to identify all systems running the affected kernel version, determine their exposure and criticality, and locate the accountable owner before planning remediation.

Kernel and MPTCP owners should manage this.
Verify MPTCP reachability and system criticality.
Plan remediation based on confirmed risk.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is the Linux MPTCP component?

MultiPath TCP (MPTCP) is a Linux kernel extension that allows a single data stream to be split across multiple network paths simultaneously, such as using both Wi-Fi and cellular connections at once. It improves network resilience and performance by enabling connections to stay active even if one path fails, by allowing data to flow over alternative routes seamlessly.

What does the race condition in CVE-2025-40257 mean?

This is a memory safety issue classified as a use-after-free weakness. In plain terms, the kernel mistakenly tries to access or stop a timer that has already been deleted or freed from memory. Because two different processes attempt to handle the same timer simultaneously without proper coordination, it creates a race condition that can lead to kernel instability or system crashes.

How is this timer issue triggered?

The flaw is triggered when the kernel's MPTCP subsystem processes specific network traffic that forces an interaction with its internal timer management. It requires MPTCP to be active and experiencing specific network timing conditions to manifest. Simply sending arbitrary network packets does not guarantee the race condition will occur; it depends on the precise, concurrent execution of internal kernel worker tasks.

Is this vulnerability reachable from the internet?

According to Halo Surface Signal, this vulnerability is very unlikely to be reachable from the internet. While the CVSS score is high, the bug exists deep within internal kernel memory management logic for MPTCP timers, not in an externally exposed application or service. It is a low-level software defect rather than an easily accessible network-facing interface.

What is the first step for my team to take?

Begin by auditing your environment to identify which systems have MPTCP enabled. Once you have an inventory, coordinate with the infrastructure or kernel engineering teams to assess the criticality of these specific machines. Prioritize systems where MPTCP is actively utilized and plan for updates as part of your standard kernel maintenance lifecycle.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.