CVE-2021-46912: 
Linux Kernel vulnerability analysis and mitigation

A vulnerability was discovered in the network sub-component of the Linux Kernel where the tcp_allowed_congestion_control is global and writable, allowing writes in any net namespace to leak into all other net namespaces (Red Hat CVE). The vulnerability has been assigned CVE-2021-46912 and has a CVSS v3 score of 5.5, indicating a moderate severity level.

The vulnerability exists because tcp_available_congestion_control and tcp_allowed_congestion_control are the only sysctls in ipv4_net_table (the per-netns sysctl table) with a NULL data pointer. Their handlers (proc_tcp_available_congestion_control and proc_allowed_congestion_control) have no other way of referencing a struct net, causing them to operate globally (Kernel Commit). The CVSS vector is CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N, indicating local access required with low attack complexity (Red Hat CVE).

The vulnerability allows information to leak between network namespaces, potentially compromising the isolation between different network environments. This could lead to unauthorized access to network configuration settings across namespace boundaries (Red Hat CVE).

The issue has been fixed by making tcp_allowed_congestion_control readonly in non-init network namespaces. The fix involves updating the data pointer logic to handle NULL pointers differently and forcing these entries to be read-only in non-init_net namespaces (Kernel Commit).