CVE-2021-20254: 
Samba vulnerability analysis and mitigation

A vulnerability (CVE-2021-20254) was discovered in the Samba smbd file server affecting all versions since Samba 3.6.0. The vulnerability involves the mapping of Windows group identities (SIDs) into Unix group ids (gids), where a flaw in the code could allow reading data beyond the end of the array when a negative cache entry was added to the mapping cache (Samba Security). The issue was discovered when an unprivileged user at Linköping University was able to delete a file within a network share that they should not have had access to (NVD).

The vulnerability is classified with a CVSS v3.1 Base Score of 6.8 (MEDIUM) with the vector string CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N. The flaw specifically occurs in the code that maps Windows SIDs to Unix gids, where improper handling of negative cache entries could lead to out-of-bounds array reads. This could result in those invalid values being returned into the process token that stores group membership for a user (NVD, Samba Security).

The vulnerability primarily affects data confidentiality and integrity. When successfully exploited, it could allow unauthorized access to files and potentially lead to the disclosure of sensitive information or modification of data. In most cases, the flaw caused the calling code to crash, but in some instances, it allowed unprivileged users to access files they should not have permission to access (NetApp Security, Samba Security).

The vulnerability has been patched in Samba versions 4.14.4, 4.13.8, and 4.12.15. System administrators are advised to upgrade to these versions or apply the available security patches. No workarounds were available for this vulnerability, making upgrading the only solution (Samba Security, Debian LTS).