CVE-2025-21707 is a vulnerability identified in the Linux kernel's implementation of Multipath TCP (MPTCP), a protocol extension that allows a single TCP connection to use multiple paths to maximize resource usage and increase redundancy. The vulnerability arises from improper initialization and clearing of bitfields that track the status of received MPTCP suboptions during packet parsing. Specifically, while the bitmask representing received suboptions is zeroed before parsing, certain additional bitfields that carry per-suboption information are not consistently cleared depending on the suboption parsed. This fragile state management leads to uninitialized memory usage, as detected by the Kernel Memory Sanitizer (KMSAN), which reports uninitialized value usage in functions such as __mptcp_expand_seq, mptcp_expand_seq, and ack_update_msk within the MPTCP options processing code. The uninitialized values can propagate through the TCP input processing stack, potentially causing undefined behavior, kernel crashes, or memory corruption. Although the detailed exploitability is not explicitly stated and no known exploits are reported in the wild, the vulnerability could be triggered by crafted network packets that exploit the inconsistent bitfield clearing during MPTCP option parsing. This flaw is rooted in the complex handling of MPTCP suboptions and the fragile state management of their status bits, which could be leveraged to destabilize the kernel networking stack or cause denial of service. The affected versions correspond to specific Linux kernel commits prior to the fix, and the issue was publicly disclosed on February 27, 2025. No CVSS score has been assigned yet, and no patches or exploits are currently linked.

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions with MPTCP enabled or in use. MPTCP is increasingly used in environments requiring high availability and bandwidth aggregation, such as data centers, cloud infrastructure, telecom networks, and enterprise servers. Exploitation could lead to kernel crashes or denial of service, impacting the availability of critical services. In worst cases, memory corruption might be leveraged for privilege escalation or arbitrary code execution, though such exploitation would require further research. Given the widespread use of Linux in European government, financial, industrial, and telecommunications sectors, the vulnerability could disrupt essential services or infrastructure if exploited. Additionally, organizations relying on multi-path networking for redundancy or performance might face increased risk. The lack of known exploits reduces immediate threat but does not eliminate the risk, especially as attackers may develop exploits once patches are available. The vulnerability's impact on confidentiality and integrity is less direct but cannot be ruled out if memory corruption leads to information disclosure or kernel compromise.

European organizations should prioritize updating their Linux kernels to versions where this vulnerability is patched. Since no patch links are provided, monitoring official Linux kernel repositories and security advisories for updates addressing CVE-2025-21707 is critical. In the interim, organizations can mitigate risk by disabling MPTCP if it is not required, thereby eliminating the attack surface related to this vulnerability. Network-level mitigations include filtering or monitoring suspicious TCP options that could carry malformed MPTCP suboptions, using intrusion detection systems tuned to detect anomalous MPTCP traffic patterns. For environments where MPTCP is essential, applying kernel hardening techniques such as enabling Kernel Address Sanitizer (KASAN) or Kernel Memory Sanitizer (KMSAN) during testing can help detect similar issues proactively. Additionally, rigorous network segmentation and limiting exposure of vulnerable systems to untrusted networks reduce exploitation likelihood. Organizations should also prepare incident response plans to handle potential denial of service or kernel compromise scenarios stemming from this vulnerability.