CVE-2021-47245 is a vulnerability identified in the Linux kernel's netfilter synproxy module, specifically in the TCP option parsing logic. The synproxy_parse_options function is responsible for parsing TCP options during SYN proxying, a technique used to mitigate SYN flood attacks by validating TCP connection requests before passing them to the target system. The vulnerability arises when the length of the TCP options is set to 1 byte. In this scenario, the parser enters a loop and reads one byte of the opcode. If the opcode is neither TCPOPT_EOL (end of options list) nor TCPOPT_NOP (no operation), the parser attempts to read an additional byte, which exceeds the declared length of 1, resulting in an out-of-bounds read. This out-of-bounds read can lead to undefined behavior, including potential memory corruption or information disclosure. The fix implemented includes an early return when the length is less than zero to prevent calls to skb_header_pointer with negative lengths, thereby preventing the out-of-bounds access. This vulnerability is similar in nature to a previous fix in the IPv4 TCP input code, indicating a pattern of careful bounds checking needed in TCP option parsing. The vulnerability affects Linux kernel versions containing the specified commit hashes prior to the patch and was publicly disclosed on May 21, 2024. No known exploits are currently reported in the wild. No CVSS score has been assigned yet.

For European organizations, the impact of CVE-2021-47245 can be significant depending on their reliance on Linux-based infrastructure, particularly those using netfilter's synproxy feature for network security. Exploitation could allow attackers to cause memory corruption or potentially execute arbitrary code within the kernel context, leading to system crashes (denial of service) or privilege escalation. This could disrupt critical services, especially in sectors such as finance, telecommunications, healthcare, and government, where Linux servers and network appliances are prevalent. Additionally, compromised systems could be leveraged as footholds for lateral movement within corporate networks. Although no active exploits are known, the vulnerability's presence in the kernel's networking stack makes it a potential target for attackers seeking to bypass SYN flood protections or destabilize network infrastructure. The impact on confidentiality, integrity, and availability is therefore medium to high, depending on the environment and exposure of vulnerable systems.

European organizations should prioritize applying the official Linux kernel patches that address CVE-2021-47245 as soon as they become available from their Linux distribution vendors. Since the vulnerability lies in the synproxy TCP option parser, organizations that do not use synproxy can consider disabling this feature as a temporary mitigation. Network administrators should audit their firewall and netfilter configurations to identify if synproxy is enabled and assess exposure. Additionally, monitoring network traffic for anomalous TCP option patterns that could trigger the vulnerability may help detect exploitation attempts. Employing kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR) and enabling security modules like SELinux or AppArmor can reduce exploitation risk. Regular vulnerability scanning and patch management processes should be enforced to ensure timely updates. Finally, organizations should maintain robust network segmentation and intrusion detection systems to limit the impact of any potential compromise.