CVE-2025-0620 is a vulnerability identified in Samba version 4.21.0 affecting the smbd service daemon responsible for handling SMB protocol file sharing. The core issue is that when an SMB session expires and the client attempts to re-authenticate, the smbd daemon does not refresh the user's group membership information. Consequently, if a user's group membership has changed (e.g., removal from a group that grants access to certain file shares), the daemon continues to grant access based on outdated group data until the client fully disconnects and reconnects. This flaw can lead to unauthorized access to sensitive file shares, compromising confidentiality. The vulnerability has a CVSS 3.1 base score of 4.9 (medium severity), with an attack vector of network (AV:N), low attack complexity (AC:L), requiring high privileges (PR:H), no user interaction (UI:N), unchanged scope (S:U), high confidentiality impact (C:H), and no impact on integrity or availability (I:N/A:N). No known exploits have been reported in the wild, but the flaw presents a risk in environments where group memberships are frequently updated and SMB sessions are long-lived. The vulnerability highlights the importance of session management and dynamic permission evaluation in SMB services. Since Samba is widely used in enterprise and government networks for file sharing, this vulnerability could expose sensitive data if not addressed promptly.

For European organizations, the primary impact is unauthorized disclosure of sensitive files due to stale group membership information during SMB session re-authentication. This can lead to data breaches, especially in sectors handling confidential or regulated data such as finance, healthcare, and government. The flaw does not affect data integrity or availability, but the confidentiality breach could result in regulatory penalties under GDPR and damage to organizational reputation. Organizations with complex group-based access controls and long-lived SMB sessions are particularly vulnerable. The risk is elevated in environments where users are frequently added or removed from groups controlling access to sensitive shares. Since exploitation requires network access and high privileges, insider threats or compromised accounts pose a significant risk vector. The absence of known exploits provides a window for mitigation, but the medium severity score indicates that the vulnerability should not be ignored.

1. Apply official patches or updates from Samba maintainers as soon as they are released to ensure the smbd daemon correctly refreshes group memberships on session re-authentication. 2. Implement policies to force SMB clients to disconnect and reconnect periodically, reducing the window of exposure caused by stale sessions. 3. Monitor SMB session durations and re-authentication events to detect anomalous long-lived sessions that may exploit this flaw. 4. Enforce strict access controls and minimize the number of users with high privileges that can maintain SMB sessions. 5. Use network segmentation to limit SMB traffic to trusted segments and reduce exposure to unauthorized network actors. 6. Regularly audit group memberships and access permissions to ensure they reflect current organizational policies. 7. Consider deploying additional file access monitoring and alerting solutions to detect unauthorized access attempts. 8. Educate IT staff about the importance of session management and the risks posed by stale authentication data.