CVE-2024-40931 in Linux
Summary
by MITRE • 07/12/2024
In the Linux kernel, the following vulnerability has been resolved:
mptcp: ensure snd_una is properly initialized on connect
This is strictly related to commit fb7a0d334894 ("mptcp: ensure snd_nxt is properly initialized on connect"). It turns out that syzkaller can trigger the retransmit after fallback and before processing any other incoming packet - so that snd_una is still left uninitialized.
Address the issue explicitly initializing snd_una together with snd_nxt and write_seq.
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Analysis
by VulDB Data Team • 03/06/2025
The vulnerability CVE-2024-40931 affects the Linux kernel's Multipath TCP implementation and represents a critical initialization flaw that can lead to unpredictable behavior during connection establishment. This issue specifically targets the MPTCP (Multipath TCP) subsystem which enables TCP connections to utilize multiple paths simultaneously for improved performance and reliability. The vulnerability stems from improper initialization of the send unacknowledged sequence number (snd_una) variable during the connection setup process, creating a potential attack vector that could be exploited by malicious actors.
The technical flaw manifests when the MPTCP implementation fails to properly initialize the snd_una variable in conjunction with other critical sequence number variables during connection establishment. This occurs in scenarios where the system transitions from a multipath connection to a fallback to a single path, creating a window where snd_una remains uninitialized while other sequence numbers are properly set. The vulnerability is particularly concerning because it can be triggered by specific timing conditions that allow for retransmission events to occur before any incoming packets are processed, leaving the send unacknowledged sequence number in an undefined state.
The operational impact of this vulnerability extends beyond simple connection establishment failures and can potentially enable denial of service attacks or data corruption within MPTCP connections. When snd_una remains uninitialized, the TCP stack may make incorrect assumptions about the state of transmitted data, potentially causing the system to retransmit packets unnecessarily or fail to properly acknowledge received data. This behavior can disrupt network communications and create opportunities for attackers to exploit the inconsistent state of the TCP connection management. The vulnerability specifically relates to commit fb7a0d334894 which addressed similar initialization issues with snd_nxt but failed to account for the interdependencies between sequence number variables.
Security implications of this vulnerability align with CWE-665 and CWE-704 categories, representing improper initialization of critical variables and inadequate error handling within network protocol implementations. The issue demonstrates how seemingly minor initialization gaps in protocol stacks can have significant security consequences, particularly in complex multipath networking scenarios. From an ATT&CK framework perspective, this vulnerability could be leveraged for initial access or privilege escalation through network-based attacks, potentially enabling adversaries to disrupt services or manipulate network traffic flow. The vulnerability's exploitation potential is heightened in environments where MPTCP is actively used for high-performance networking and where systems are exposed to adversarial network conditions.
Mitigation strategies should focus on applying the kernel patch that explicitly initializes snd_una together with snd_nxt and write_seq variables during connection establishment. System administrators should prioritize updating their kernel versions to include the fix, particularly in environments where MPTCP is actively deployed. Network monitoring should be enhanced to detect anomalous sequence number behavior in TCP connections, as this could serve as an indicator of exploitation attempts. Additionally, organizations should consider implementing network segmentation and access controls to limit the potential impact of any successful exploitation attempts, while maintaining regular kernel updates to address similar initialization vulnerabilities that may exist in other protocol implementations.