CVE-2021-3493: 
Linux Debian vulnerability analysis and mitigation

CVE-2021-3493 is a privilege escalation vulnerability discovered in the overlayfs implementation of the Ubuntu Linux kernel. The vulnerability was disclosed on April 15, 2021, affecting Ubuntu versions from 14.04 to 20.10 LTS. The issue stems from improper validation of file capabilities with respect to user namespaces in the overlayfs implementation, combined with Ubuntu's specific patch that enables unprivileged overlay mounts (Ubuntu Security, OSS Security).

The vulnerability exists due to the overlayfs implementation not properly validating the setting of file capabilities on files in an underlying file system with respect to user namespaces. The issue arose because Ubuntu's kernel implementation did not make the necessary call to cap_convert_nscap for validation before sending requests to the file system. This was fixed by adding the verification call to cap_convert_nscap into the vfs_setxattr function code (Kernel Commit). The vulnerability has been assigned a CVSS v3.1 base score of 7.8 (High) (NVD).

The vulnerability allows a local attacker to gain elevated privileges on affected systems. Due to the combination of unprivileged user namespaces and Ubuntu's patch allowing unprivileged overlay mounts, an attacker could exploit this vulnerability to escalate their privileges to root level access (AttackerKB).

The vulnerability has been confirmed to be exploitable and has been actively exploited in the wild. It has been added to CISA's Known Exploited Vulnerabilities Catalog, indicating its significant threat level. The vulnerability requires local access and low privileges to exploit, making it particularly dangerous in multi-user environments (AttackerKB).

The vulnerability has been patched in multiple Ubuntu versions. Users should update their systems to the following versions: Ubuntu 20.10 (5.8.0-50.56), Ubuntu 20.04 LTS (5.4.0-72.80), Ubuntu 18.04 LTS (4.15.0-142.146), and Ubuntu 16.04 LTS (4.4.0-209.241). The fix involves updating the kernel packages through the standard system update process (Ubuntu Security Notice).