CVE-2021-46999 is a vulnerability in the Linux kernel's SCTP (Stream Control Transmission Protocol) implementation. The issue arises from a use-after-free bug related to the handling of duplicate COOKIE-ECHO chunks during SCTP association updates. Specifically, when processing a duplicate COOKIE-ECHO chunk in the function sctp_sf_do_dupcook_a(), both COOKIE-ACK and SHUTDOWN chunks are allocated using the transport from the new association (new_asoc). However, later in the state machine, the old association (old_asoc) is used to send these chunks, and the old_asoc's shutdown_last_sent_to pointer is set to the transport associated with the SHUTDOWN chunk, which actually belongs to the new_asoc. After the new_asoc is freed and the old_asoc's T2 timer expires, the old_asoc's shutdown_last_sent_to pointer, which now points to freed memory, is accessed in sctp_sf_t2_timer_expire(). This results in a use-after-free condition that can cause a kernel panic or general protection fault, leading to a denial of service (DoS). The fix involves updating the association earlier in the processing flow so that the COOKIE-ACK and SHUTDOWN chunks are allocated with the updated old_asoc, ensuring chunks are not sent from one association while being allocated from another. This correction prevents the use-after-free scenario and stabilizes SCTP association handling. The vulnerability affects multiple Linux kernel versions identified by specific commit hashes and was publicly disclosed in February 2024. There are no known exploits in the wild at this time, and no CVSS score has been assigned yet.

For European organizations, this vulnerability poses a risk primarily to systems running Linux kernels with SCTP enabled, which is commonly used in telecommunications, signaling, and certain enterprise network environments. Exploitation can lead to kernel panics and system crashes, resulting in denial of service conditions. This can disrupt critical infrastructure, especially in telecom operators, financial institutions, and data centers relying on Linux-based systems for high availability. The impact on confidentiality and integrity is limited since the vulnerability is a use-after-free leading to crashes rather than arbitrary code execution. However, availability impact can be significant if exploited on production servers or network devices. Given the widespread use of Linux in European IT infrastructure, especially in cloud services and telecom sectors, this vulnerability could cause operational disruptions if not patched promptly. The lack of known exploits suggests limited immediate threat, but the complexity of the bug and its kernel-level nature means that sophisticated attackers or accidental triggers could cause outages.

European organizations should prioritize updating their Linux kernels to versions that include the patch fixing CVE-2021-46999. Since the vulnerability involves SCTP, organizations should audit their environments to identify systems with SCTP enabled and assess their exposure. If SCTP is not required, disabling the SCTP kernel module can reduce the attack surface. For systems where SCTP is critical, ensure kernel updates are applied promptly. Additionally, monitoring kernel logs for signs of general protection faults or panics related to SCTP can help detect attempted exploitation or instability. Network segmentation and limiting exposure of SCTP ports to untrusted networks can further reduce risk. Organizations should also test patches in staging environments to avoid unexpected disruptions. Given the complexity of the issue, collaboration with Linux distribution vendors and telecom equipment providers is recommended to ensure comprehensive remediation.