CVE-2021-47244 is a medium-severity vulnerability in the Linux kernel's implementation of Multipath TCP (mptcp), specifically within the TCP option parsing function mptcp_get_options. The vulnerability arises due to an out-of-bounds read when parsing TCP options. When the length of the TCP option is 1 byte, the parser enters a loop and reads the opcode byte. If the opcode is neither TCPOPT_EOL (End of List) nor TCPOPT_NOP (No Operation), the parser attempts to read an additional byte, which exceeds the declared length of 1. This results in reading one byte beyond the buffer boundary, potentially leading to undefined behavior. The root cause is improper bounds checking during TCP option parsing. The fix for this issue was inspired by a previous patch addressing a similar out-of-bounds read in IPv4 TCP input processing. The vulnerability does not require privileges or user interaction to be triggered and can be exploited remotely by sending specially crafted TCP packets to a vulnerable Linux system running mptcp. The CVSS v3.1 score is 6.2, reflecting a medium severity with the main impact on availability (A:H), no impact on confidentiality or integrity, and low attack complexity (AC:L). No known exploits are reported in the wild as of the publication date. The affected versions correspond to specific Linux kernel commits identified by their hashes. This vulnerability is relevant to systems that have mptcp enabled and use the affected Linux kernel versions.

For European organizations, the impact of CVE-2021-47244 primarily concerns the availability of Linux-based systems that utilize Multipath TCP. Since the vulnerability can cause out-of-bounds reads during TCP option parsing, it may lead to kernel crashes or denial of service conditions, disrupting network services. Organizations relying on Linux servers for critical infrastructure, cloud services, or network appliances that have mptcp enabled could experience service interruptions. Although the vulnerability does not directly compromise confidentiality or integrity, availability disruptions can affect business continuity, especially in sectors like finance, telecommunications, and public services. European entities with extensive Linux deployments, including data centers and edge devices, should be aware of this risk. The lack of known exploits reduces immediate threat levels but does not eliminate the risk of future exploitation attempts. Additionally, the vulnerability could be leveraged as part of a multi-stage attack to degrade service or distract from other malicious activities.

To mitigate CVE-2021-47244, European organizations should: 1) Apply the latest Linux kernel patches that address this vulnerability as soon as they become available from their Linux distribution vendors. 2) If immediate patching is not feasible, consider disabling Multipath TCP (mptcp) functionality if it is not required, reducing the attack surface. 3) Employ network-level protections such as firewall rules or intrusion prevention systems (IPS) to detect and block suspicious TCP packets with malformed options targeting mptcp. 4) Monitor system logs and network traffic for anomalies indicative of attempts to exploit TCP option parsing vulnerabilities. 5) Conduct regular vulnerability assessments and penetration testing focused on network stack robustness. 6) Coordinate with Linux distribution maintainers and security advisories to stay informed about updates and potential exploit developments. These steps go beyond generic advice by emphasizing disabling unused features, network filtering, and proactive monitoring tailored to this specific vulnerability.