Potentially Dangerous ID Mapping Between Local And Domain Users In SSSD A Security Analysis

July 7, 2025 by StackCamp Team 92 views

Introduction

This article delves into a potentially critical security issue encountered when using SSSD (System Security Services Daemon) version 2.6.3 to facilitate SSH authentication via GSSAPI. Specifically, we will explore the dangerous implications of ID mapping between local and domain users, focusing on how a machine account's Kerberos ticket can be incorrectly mapped to the local root user, and how this behavior extends to other domain accounts, potentially leading to privilege escalation. The core issue arises when using SSSD to integrate with Active Directory, where misconfigurations or default behaviors can lead to domain users being mapped to local user accounts, thereby creating significant security vulnerabilities. This article provides a comprehensive analysis of the problem, its potential impact, and recommendations for mitigating these risks.

Problem Description

Using SSSD version 2.6.3, the latest available on Ubuntu packages, for SSH authentication via GSSAPI, a concerning behavior has been observed. When authenticating with the Kerberos ticket of a machine account (e.g., root$) from the asia.earth.local domain, the system incorrectly maps the authentication to the local root user. This means that an attacker who can compromise a machine account in the domain could potentially gain root access on the local system. To demonstrate the issue, consider the following SSSD configuration:

# /etc/sssd/sssd.conf
[sssd]
domains = asia.earth.local
config_file_version = 2
services = nss, pam

[domain/asia.earth.local]
default_shell = /bin/bash
krb5_store_password_if_offline = True
cache_credentials = False
krb5_realm = ASIA.EARTH.LOCAL
realmd_tags = manages-system joined-with-adcli 
id_provider = ad
fallback_homedir = /home/%u
ad_domain = asia.earth.local
use_fully_qualified_names = True
ldap_id_mapping = True
access_provider = simple
simple_allow_groups = Everyone@asia.earth.local, Domain Admins@asia.earth.local, Domain Users@asia.earth.local

This configuration file sets up SSSD to authenticate against the asia.earth.local Active Directory domain. The critical setting here is ldap_id_mapping = True, which instructs SSSD to attempt to map domain user IDs to local user IDs. While this setting can be useful in some contexts, it can also lead to the vulnerabilities described below. To further illustrate the problem, consider the following steps and their outcomes:

Obtain a Kerberos ticket for the root$ machine account:

Even without explicit configuration, the root$ machine account in the Active Directory domain can acquire a Kerberos ticket.
Attempt to SSH to the target system using the root user:
```
$ ssh 10.5.20.74 -l root id
uid=0(root) gid=0(root) groups=0(root)
```
The output clearly shows that the authentication is mapped to the local root user, indicating a severe security breach. The implications of this are significant. An attacker who manages to compromise the machine account’s credentials can effectively gain root access on the target system, potentially leading to data breaches, system compromise, and other malicious activities. This vulnerability is further exacerbated by the default Active Directory configuration, which allows domain users to create up to ten machine accounts.
Demonstrating the issue with a standard domain user:

The same behavior is observed even with standard domain accounts. For instance, if a domain user testuser1234 exists both in the asia.earth.local domain and as a local user on the system, the Kerberos-based authentication maps to the local user account. This can lead to confusion and potential privilege escalation if the local user has different permissions than the domain user.
```
$ kinit testuser1234@ASIA.EARTH.LOCAL
Password for testuser1234@ASIA.EARTH.LOCAL: 

$ ssh 10.5.20.74 -l testuser1234 id
uid=1003(testuser1234) gid=1003(testuser1234) groups=1003(testuser1234)
```
This result indicates that the domain user testuser1234 has been mapped to the local user testuser1234, which may not always be the desired behavior. In environments where local users and domain users share the same usernames, this can lead to significant authentication and authorization issues.

However, when authenticating using the fully qualified domain username, the correct domain user context is maintained:
```
$ ssh 'testuser1234@asia.earth.local'@10.5.20.74 id
```

testuser1234@asia.earth.local@10.5.20.74's password: uid=1753602609(testuser1234@asia.earth.local) gid=1753600513(domain users@asia.earth.local) groups=1753600513(domain users@asia.earth.local) ```

This outcome shows that specifying the full domain username prevents the incorrect mapping, highlighting the importance of using fully qualified names in certain contexts. However, the initial issue of mapping machine accounts to the local root user remains a critical concern.

Root Cause Analysis

The root cause of this issue lies in the ldap_id_mapping setting within the SSSD configuration. When set to True, SSSD attempts to map domain user IDs to local user IDs based on the username. In the case of the root$ machine account, SSSD strips the $ and attempts to map to a local user named root. Since a local root user almost always exists, the authentication incorrectly maps, granting domain-authenticated machine accounts root access on the local system. This behavior represents a significant security risk because it bypasses the intended security boundaries between the domain and the local system.

The default Active Directory configuration, allowing domain users to create up to 10 machine accounts, amplifies this risk. An attacker can create multiple machine accounts, potentially increasing their chances of successfully compromising one and gaining unauthorized access. The incorrect ID mapping coupled with the ability to create numerous machine accounts makes the system highly vulnerable.

Furthermore, the issue extends beyond machine accounts. Any domain user with a username that matches a local user can potentially be mapped incorrectly, leading to privilege escalation or unauthorized access to local resources. This is particularly problematic in environments where local user accounts and domain accounts are not strictly segregated, which is a common scenario in many organizations.

Security Implications

The security implications of this incorrect ID mapping are profound. The ability for a domain-authenticated machine account to gain root access on a local system opens the door to a wide range of malicious activities. An attacker who successfully exploits this vulnerability can:

Gain Full System Control: Root access allows the attacker to perform any action on the system, including installing malware, modifying system configurations, and accessing sensitive data.
Bypass Security Controls: With root privileges, the attacker can disable security measures, such as firewalls, intrusion detection systems, and antivirus software.
Access Sensitive Data: The attacker can access any file on the system, including configuration files, databases, and user data.
Pivot to Other Systems: If the compromised system is part of a network, the attacker can use it as a launching point to attack other systems on the network.
Maintain Persistence: The attacker can create backdoors or other mechanisms to maintain access to the system even after the initial vulnerability is patched.
Data Breaches and Exfiltration: The attacker can exfiltrate sensitive data from the system, leading to data breaches and compliance violations.
Denial of Service (DoS): The attacker can render the system unusable by deleting critical files, crashing services, or consuming system resources.

The risk is not limited to machine accounts. The possibility of mapping domain users to local users with matching usernames can create confusion and security vulnerabilities. In scenarios where local users have more privileges than their domain counterparts, this mapping can lead to unauthorized access to resources. Conversely, if domain users are inadvertently mapped to local accounts with fewer privileges, it can disrupt legitimate workflows.

Mitigation Strategies

To mitigate the risks associated with this ID mapping issue, several strategies can be employed. The most effective approach is to disable ldap_id_mapping and instead rely on explicit ID mapping or other mechanisms to ensure proper authentication and authorization. Here are detailed steps and recommendations to address this vulnerability:

Disable ldap_id_mapping:

The most direct way to prevent the incorrect ID mapping is to set ldap_id_mapping = False in the SSSD configuration file (/etc/sssd/sssd.conf). This will prevent SSSD from attempting to map domain user IDs to local user IDs based on username matching.
```
[domain/asia.earth.local]
ldap_id_mapping = False
```
After making this change, restart the SSSD service to apply the new configuration:
```
sudo systemctl restart sssd
```
This ensures that domain users and machine accounts are not inadvertently mapped to local user accounts, thereby mitigating the primary security risk.
Use Explicit ID Mapping:

If specific users or groups require mapping between domain and local accounts, use explicit ID mapping. This can be achieved by defining specific mappings in the SSSD configuration file or using other mechanisms provided by SSSD. This approach provides more control and reduces the risk of unintended mappings.

For example, to map a specific domain user to a local user, you can use the ldap_user_map and ldap_group_map options in the SSSD configuration. However, this method requires careful planning and maintenance to ensure that mappings are accurate and up-to-date.
Implement Fully Qualified Names:

Encourage the use of fully qualified usernames (e.g., user@domain.com) when authenticating to systems. This ensures that the domain context is always preserved, reducing the risk of incorrect mappings. When users specify the full domain username, SSSD can accurately identify the domain account, even if a local user with the same username exists.

This can be enforced through policies and user training, ensuring that users are aware of the importance of using fully qualified names, especially in environments where local and domain users share usernames.
Regularly Audit User and Group Mappings:

Establish a process for regularly auditing user and group mappings in SSSD to identify and correct any misconfigurations. This helps prevent unauthorized access and ensures that user privileges are correctly assigned. Regular audits should include reviewing the SSSD configuration, user authentication logs, and any manual mappings that have been set up.

Automated tools and scripts can be used to assist in these audits, making the process more efficient and less prone to human error.
Limit Machine Account Creation:

Review and adjust the Active Directory policy that allows domain users to create machine accounts. Limiting the number of machine accounts that can be created reduces the attack surface and the potential for abuse. By default, Active Directory allows domain users to create up to ten machine accounts, which can be excessive in many environments.

Restrictive policies should be implemented to ensure that only authorized personnel can create machine accounts and that the number of accounts created is kept to a minimum.
Monitor Authentication Logs:

Implement robust monitoring of authentication logs to detect any suspicious activity, such as failed login attempts or unexpected user mappings. This can help identify potential security breaches early and allow for a timely response. Monitoring should include both local system logs and domain controller logs to provide a comprehensive view of authentication activity.

Security Information and Event Management (SIEM) systems can be used to centralize log collection and analysis, making it easier to detect and respond to security incidents.
Apply the Principle of Least Privilege:

Ensure that users and machine accounts are granted only the privileges necessary to perform their tasks. This reduces the potential damage that can be caused by a compromised account. The principle of least privilege should be applied consistently across all systems and applications, ensuring that users and accounts have only the minimum level of access required.

Regular reviews of user and account privileges should be conducted to ensure that they remain appropriate and that no unnecessary access has been granted.
Keep SSSD and Related Packages Updated:

Ensure that SSSD and all related packages are kept up-to-date with the latest security patches. Software updates often include fixes for known vulnerabilities, so keeping systems up-to-date is essential for maintaining security. Patch management processes should be in place to ensure that updates are applied promptly and consistently across the environment.

Automated update mechanisms can be used to streamline the patching process and reduce the risk of missed updates.

Conclusion

The potentially dangerous ID mapping issue between local and domain users in SSSD poses a significant security risk. By incorrectly mapping domain-authenticated machine accounts to the local root user, attackers can gain unauthorized access and compromise systems. This vulnerability, exacerbated by the default Active Directory configuration allowing users to create machine accounts, necessitates immediate attention and mitigation. Disabling ldap_id_mapping and implementing the other strategies outlined above are critical steps in securing systems against this threat. Regular security audits, monitoring, and adherence to the principle of least privilege are essential for maintaining a robust security posture. By taking these measures, organizations can significantly reduce their risk and protect their systems and data from potential attacks. This comprehensive analysis underscores the importance of careful configuration and continuous monitoring in environments utilizing SSSD for authentication and authorization.