CVE-2020-36791 is a vulnerability identified in the Linux kernel's network scheduler (net_sched) subsystem, specifically related to the handling of hash allocations in the cls_tcindex classifier. The issue arises from improper updating of the cp->alloc_hash variable after a hash allocation operation (tcindex_alloc_perfect_hash()). In the affected code, the calculation of cp->hash was moved before the allocation function, but cp->alloc_hash was not updated accordingly. This discrepancy can lead to an out-of-bounds (OOB) access condition when the system references cp->alloc_hash, which no longer accurately reflects the allocated size. Out-of-bounds accesses in kernel code can cause memory corruption, leading to system instability, crashes (kernel panic), or potentially exploitable conditions for privilege escalation or denial of service. The vulnerability was addressed by ensuring cp->alloc_hash is updated immediately after the allocation function to maintain consistency and prevent OOB access. The affected versions correspond to specific Linux kernel commits prior to the fix. There are no known exploits in the wild as of the publication date, and no CVSS score has been assigned. The vulnerability is technical and low-level, impacting the kernel's network scheduling functionality, which is critical for managing network traffic and quality of service on Linux systems.

For European organizations, the impact of CVE-2020-36791 depends largely on their use of Linux-based systems, particularly those running custom or older kernel versions that include the vulnerable commits. The network scheduler is integral to managing network traffic, so exploitation could lead to kernel crashes or instability, potentially disrupting network services or critical infrastructure operations. In environments where Linux servers handle sensitive data or provide essential services (e.g., telecommunications, finance, government, cloud providers), an attacker exploiting this vulnerability could cause denial of service or attempt privilege escalation to gain unauthorized access. Although no public exploits are known, the presence of an out-of-bounds access in kernel code is a serious concern, as it could be leveraged in targeted attacks. European organizations with high reliance on Linux servers, embedded devices, or network appliances should consider this vulnerability significant, especially in sectors where uptime and security are paramount.

To mitigate CVE-2020-36791 effectively, European organizations should: 1) Apply the official Linux kernel patches that address this vulnerability as soon as they are available and tested in their environments. This involves updating to kernel versions that include the fix commit (post-599be01ee567). 2) For organizations using custom or long-term support (LTS) kernels, backport the patch to their maintained versions to ensure protection. 3) Conduct thorough testing of network scheduler functionality after patching to avoid regressions. 4) Implement kernel hardening measures such as Kernel Address Space Layout Randomization (KASLR), Kernel Page Table Isolation (KPTI), and use of security modules like SELinux or AppArmor to reduce the risk of exploitation. 5) Monitor system logs and kernel messages for unusual behavior or crashes related to net_sched components. 6) Limit access to systems running vulnerable kernels, especially restricting unprivileged user access, to reduce the attack surface. 7) Maintain an inventory of Linux kernel versions deployed across the organization to identify and prioritize vulnerable systems for patching.