CVE-2021-47243 is a vulnerability identified in the Linux kernel's CAKE (Common Applications Kept Enhanced) queuing discipline (qdisc) implementation, specifically in the TCP option parsing logic. The flaw arises in the functions cake_get_tcpopt and cake_tcph_may_drop, where the TCP option parser can read one byte out of bounds when processing TCP options with a length of 1. The parser enters a loop and reads the opcode byte; if the opcode is neither TCPOPT_EOL (end of options list) nor TCPOPT_NOP (no operation), it attempts to read an additional byte, which exceeds the allocated length, leading to an out-of-bounds read. This can result in reading unintended memory contents, potentially leaking sensitive information or causing undefined behavior. The vulnerability was addressed by incorporating validation inspired by a prior fix (commit 9609dad263f8) that corrected similar out-of-bounds reads in IPv4 TCP input processing. Additionally, a second version of the fix added validation of the TCP header data offset (doff) in cake_get_tcphdr to prevent parsing of garbage data as a TCP header when doff is less than the minimum valid value of 5. Although this second issue did not cause a strict out-of-bounds memory access, it could lead to incorrect processing of TCP headers, potentially destabilizing the network stack. The vulnerability affects Linux kernel versions containing the specified commit hashes and is relevant to systems utilizing the CAKE qdisc for traffic shaping and network packet scheduling. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, this vulnerability poses a moderate risk primarily to systems running Linux kernels with the affected CAKE qdisc implementation enabled, especially those relying on advanced traffic shaping or quality of service controls. Potential impacts include information disclosure through out-of-bounds reads, which could leak kernel memory contents, and possible kernel instability or crashes if malformed TCP packets trigger the parsing errors. This could disrupt network services, degrade performance, or in rare cases, lead to denial of service conditions. Organizations with critical infrastructure, data centers, or cloud services running Linux-based routers, firewalls, or network appliances that employ CAKE qdisc are particularly at risk. While exploitation requires crafting specific TCP packets with malformed options, the lack of authentication or user interaction requirements means that attackers on the network path could attempt exploitation. The absence of known exploits suggests limited immediate threat, but the vulnerability's presence in widely used Linux kernels means it could be targeted in the future. European entities with stringent network performance and security requirements, such as telecom providers, financial institutions, and government agencies, should prioritize addressing this issue to maintain network integrity and confidentiality.

To mitigate CVE-2021-47243, European organizations should: 1) Apply the latest Linux kernel updates and patches that include the fix for this vulnerability, ensuring that the CAKE qdisc TCP option parsing logic is corrected. 2) Audit network devices and servers to identify those utilizing CAKE qdisc and verify kernel versions to confirm patch status. 3) Where immediate patching is not feasible, consider disabling CAKE qdisc or replacing it with alternative queuing disciplines that do not exhibit this vulnerability, to reduce attack surface. 4) Implement network-level protections such as ingress filtering and deep packet inspection to detect and block malformed TCP packets that could exploit this flaw. 5) Monitor network traffic for unusual TCP option patterns and anomalous behavior indicative of exploitation attempts. 6) Incorporate this vulnerability into vulnerability management and incident response plans to ensure timely detection and remediation. 7) Engage with Linux distribution vendors and security communities for updates and best practices related to this vulnerability. These steps go beyond generic advice by focusing on the specific affected component (CAKE qdisc) and emphasizing network-level controls and operational practices tailored to European organizational environments.