CVE-2020-25717: Samba Privilege Escalation Vulnerability

CVE-2020-25717 Overview

A flaw was found in the way Samba maps domain users to local users. An authenticated attacker could use this flaw to cause possible privilege escalation. This vulnerability affects Samba's Active Directory Domain Controller functionality and its handling of user principal names, allowing authenticated users to potentially gain unauthorized access to resources.

Critical Impact

Authenticated attackers can exploit improper domain user mapping to escalate privileges and gain unauthorized access to sensitive resources and systems.

Affected Products

• Samba (multiple versions)
• Red Hat Enterprise Linux 7.0 and 8.0 (including EUS, AUS, TUS variants)
• Red Hat Gluster Storage 3.0 and 3.5
• Red Hat OpenStack 13, 16.1, and 16.2
• Red Hat Virtualization 4.0
• Debian Linux 9.0 and 10.0
• Fedora 33, 34, and 35
• Canonical Ubuntu Linux 18.04 LTS, 20.04 LTS, 21.04, and 21.10

Discovery Timeline

• 2022-02-18 - CVE CVE-2020-25717 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-25717

Vulnerability Analysis

This vulnerability stems from an improper input validation flaw (CWE-20) in Samba's domain user to local user mapping mechanism. When Samba is configured as a domain controller or domain member, it must map domain users to local system accounts for access control purposes. The vulnerability exists in how Samba handles this mapping process.

The core issue involves insufficient validation of user principal names (UPNs) during the mapping process. An authenticated attacker can manipulate this mapping to associate their domain account with a different local user, potentially gaining access to resources and privileges they should not have. This is particularly dangerous in Active Directory environments where Samba is used as a domain controller.

The vulnerability is network-exploitable and requires low complexity to attack, though it does require the attacker to have valid domain authentication credentials. Successful exploitation can result in unauthorized read and write access to sensitive data and system configurations.

Root Cause

The root cause is improper input validation in Samba's user mapping functionality. Specifically, Samba fails to properly validate and sanitize user principal names when mapping domain users to local Unix accounts. This allows authenticated attackers to craft malicious input that tricks Samba into mapping their domain account to a different local user with elevated privileges.

Attack Vector

The attack vector is network-based and requires an authenticated domain user. The attacker must have valid credentials to authenticate to the Samba domain, but once authenticated, they can exploit the improper user mapping to escalate their privileges.

The attack flow involves:

1. Authenticating to the Samba domain with valid but low-privileged credentials
2. Manipulating the user principal name or related attributes
3. Exploiting the improper mapping validation to associate their account with a higher-privileged local user
4. Gaining access to resources and privileges assigned to the mapped local user

For technical details on the exploitation mechanism, refer to the Samba Security Advisory.

Detection Methods for CVE-2020-25717

Indicators of Compromise

• Unusual user authentication events where domain users gain unexpected local user mappings
• Discrepancies between domain user permissions and accessed resources
• Log entries showing users accessing resources beyond their normal authorization scope
• Unexpected changes to user mapping configurations in smb.conf

Detection Strategies

• Monitor Samba authentication logs for anomalous user mapping behavior
• Implement audit rules to track changes to user principal names and mapping configurations
• Review log.smbd and log.winbindd for suspicious authentication patterns
• Deploy SentinelOne agents to detect privilege escalation attempts and anomalous process behavior

Monitoring Recommendations

• Enable verbose logging in Samba configuration with log level = 3 for authentication events
• Configure centralized log collection for all Samba servers
• Set up alerts for authentication events that result in unexpected privilege grants
• Regularly audit user mappings between domain accounts and local users

How to Mitigate CVE-2020-25717

Immediate Actions Required

• Update Samba to the latest patched version available for your distribution
• Review and restrict domain user mapping configurations
• Audit current user mappings for any suspicious or unexpected associations
• Consider limiting network access to Samba services to trusted networks only

Patch Information

Security patches are available from all major Linux distributions. Organizations should apply vendor-specific updates:

• Samba: Official patches available via the Samba Security Advisory
• Red Hat: Updates tracked in Red Hat Bugzilla #2019672
• Gentoo: Advisory GLSA-202309-06 provides patch information via the Gentoo Security Advisory

Workarounds

• Configure min domain uid to a high value to restrict which local users can be mapped
• Use explicit user mapping with username map to control domain-to-local user associations
• Implement idmap config with restrictive ranges to limit potential mapping targets
• Consider adding require membership of restrictions to limit authenticated user access

# Example Samba configuration hardening
# Add to smb.conf [global] section

# Set minimum UID for domain user mapping
min domain uid = 1000

# Restrict idmap range to limit mapping targets
idmap config * : range = 10000-999999

# Enable logging for authentication events
log level = 3 auth:5

# Require specific group membership for access
# require membership of = DOMAIN\AllowedGroup