CVE-2020-25718 Overview
A flaw was found in the way Samba, when operating as an Active Directory Domain Controller, handles Read-Only Domain Controller (RODC) authentication. This vulnerability allows an RODC to print administrator tickets, effectively enabling privilege escalation within the Active Directory environment. The issue stems from missing authorization checks (CWE-862) that fail to properly restrict RODC capabilities.
Critical Impact
An attacker with control over an RODC can generate Kerberos tickets for privileged accounts, including domain administrators, potentially leading to complete domain compromise.
Affected Products
- Samba (when configured as an Active Directory Domain Controller)
- Fedora 35
Discovery Timeline
- 2022-02-18 - CVE-2020-25718 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2020-25718
Vulnerability Analysis
This vulnerability affects Samba's implementation of Active Directory Domain Controller functionality, specifically in how it handles Read-Only Domain Controllers (RODCs). RODCs are designed to be deployed in branch offices or less secure locations where full domain controller functionality is not required or appropriate. By design, RODCs should have limited capabilities and should not be able to generate tickets for sensitive accounts.
The flaw allows an RODC to bypass these restrictions and print administrator tickets. In a properly configured Active Directory environment, RODCs maintain a filtered attribute set and can only cache credentials for explicitly allowed accounts. This vulnerability circumvents those security boundaries.
Root Cause
The root cause is a Missing Authorization vulnerability (CWE-862) in Samba's RODC handling code. The authorization checks that should prevent an RODC from generating tickets for privileged accounts are either missing or improperly implemented. This allows the RODC to exceed its intended security boundaries and perform actions reserved for full domain controllers.
Attack Vector
The attack requires network access and low-level privileges—specifically, an attacker must have control over or compromise an RODC within the domain. Once an attacker has compromised an RODC, they can exploit this vulnerability to generate Kerberos tickets for domain administrator accounts. The exploitation does not require user interaction and can be performed entirely over the network.
The attack flow typically involves:
- Compromising or gaining control of an RODC in the domain
- Exploiting the missing authorization checks to request administrator tickets
- Using the forged tickets to authenticate as a domain administrator
- Achieving full domain compromise
Detection Methods for CVE-2020-25718
Indicators of Compromise
- Unexpected Kerberos ticket requests originating from RODC servers for privileged accounts
- Authentication events showing administrator account access initiated from RODC systems
- Anomalous TGT (Ticket Granting Ticket) generation patterns from read-only domain controllers
- Audit logs showing privilege escalation activities correlated with RODC-sourced tickets
Detection Strategies
- Monitor Kerberos authentication logs for ticket requests from RODCs that reference accounts outside their password replication policy
- Implement alerting on any administrator account authentications that originate from or pass through RODC systems
- Review Active Directory audit logs for unexpected privilege delegation events
- Deploy network monitoring to detect anomalous Kerberos traffic patterns between RODCs and domain resources
Monitoring Recommendations
- Enable verbose Kerberos logging on all domain controllers including RODCs
- Configure SIEM rules to correlate RODC authentication events with privileged account access
- Regularly audit RODC password replication policies and compare against actual ticket generation activity
- Implement real-time alerting for any Domain Admin or Enterprise Admin ticket requests from RODC sources
How to Mitigate CVE-2020-25718
Immediate Actions Required
- Update Samba to the latest patched version immediately on all systems functioning as Active Directory Domain Controllers
- Audit all RODC deployments in your environment and verify their security posture
- Review RODC password replication policies to ensure they follow least-privilege principles
- Consider temporarily demoting RODCs in high-risk environments until patches can be applied
Patch Information
Samba has released security updates to address this vulnerability. Refer to the Samba Security Advisory CVE-2020-25718 for specific patch versions and upgrade instructions. Fedora users should apply updates through their standard package management system.
Additional resources:
Workarounds
- Restrict network access to RODC systems using firewall rules to limit potential attack surface
- Implement strict network segmentation between RODCs and other domain controllers
- Remove RODCs from service if they are not operationally critical until patching is complete
- Enable enhanced audit logging on all domain controllers to detect exploitation attempts
# Configuration example
# Verify Samba version for patch status
samba --version
# Check if system is configured as AD DC
testparm -s | grep "server role"
# Review RODC status in your domain
samba-tool rodc preload --help
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
