CVE-2024-26800 is a use-after-free vulnerability identified in the Linux kernel's TLS (Transport Layer Security) implementation, specifically related to the handling of asynchronous decryption operations. The flaw arises in the tls_do_decryption function when a decrypt request is sent to the backlog and the crypto_aead_decrypt function returns an -EBUSY error code, indicating the cryptographic operation is temporarily unavailable. In this scenario, tls_do_decryption waits for all asynchronous decryptions to complete. If one of these decryptions fails, tls_do_decryption returns -EBADMSG, causing tls_decrypt_sg to enter an error handling path where it releases all memory pages associated with the decryption. However, these pages have already been released by the asynchronous callback tls_decrypt_done, leading to a use-after-free condition. This memory mismanagement can potentially be exploited to cause kernel crashes or arbitrary code execution within the kernel context. The vulnerability is rooted in the incorrect handling of the -EBUSY return code, which differs from the true asynchronous case indicated by -EINPROGRESS. The fix involves introducing an async_done flag to notify tls_decrypt_sg that the memory has already been freed, preventing double-free and use-after-free scenarios. This vulnerability affects multiple Linux kernel versions as indicated by the affected commit hashes, and it was publicly disclosed on April 4, 2024. There are no known exploits in the wild at the time of publication, and no CVSS score has been assigned yet.

For European organizations, this vulnerability poses a significant risk due to the widespread use of Linux in servers, cloud infrastructure, and embedded systems across various industries including finance, telecommunications, healthcare, and government. Exploitation could lead to kernel crashes resulting in denial of service or, more critically, privilege escalation and arbitrary code execution at the kernel level, compromising system confidentiality, integrity, and availability. This is particularly concerning for organizations relying on TLS for secure communications, as the flaw resides in the TLS decryption path. Attackers with the ability to send crafted TLS traffic to vulnerable systems could potentially trigger the vulnerability. Given the kernel-level impact, successful exploitation could allow attackers to bypass security controls, access sensitive data, or disrupt critical services. The absence of known exploits currently provides a window for proactive patching, but the severity of the flaw demands urgent attention to prevent future exploitation attempts.

European organizations should prioritize updating their Linux kernel to versions that include the patch for CVE-2024-26800 as soon as vendor updates become available. Until patches are applied, organizations should: 1) Monitor network traffic for unusual TLS-related anomalies that could indicate exploitation attempts. 2) Restrict access to critical Linux systems to trusted networks and users to reduce exposure. 3) Employ kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR) and enable security modules like SELinux or AppArmor to limit potential damage from kernel exploits. 4) Conduct thorough testing of kernel updates in staging environments to ensure compatibility and stability before deployment. 5) Maintain up-to-date intrusion detection and prevention systems capable of recognizing exploit patterns targeting kernel vulnerabilities. 6) Review and tighten TLS configurations to minimize attack surface, including disabling deprecated or vulnerable cipher suites. These measures, combined with timely patching, will mitigate the risk posed by this vulnerability.