How to remediate – Unpassworded ‘sync’ Account

1. Introduction

The ‘sync’ account vulnerability refers to an account on a remote host that has no password set. This poses a critical security risk as it allows attackers to gain unauthorised access and potentially escalate privileges within the system. Systems commonly affected include those using synchronisation services or accounts for data transfer, often found in file servers, Active Directory environments, and cloud-based storage solutions. A successful exploit could compromise confidentiality, integrity, and availability of data on the host.

2. Technical Explanation

The vulnerability occurs because a system administrator has not assigned a password to the ‘sync’ account. This allows an attacker with network access to authenticate as this account without providing credentials. CVE-1999-0502 describes this issue, relating to weak or missing passwords on Unix systems. An example attack involves an attacker using standard authentication tools like SSH or RDP to connect to the system directly as the ‘sync’ user, bypassing normal login procedures.

• Root cause: The account ‘sync’ lacks a password due to administrative oversight.
• Exploit mechanism: An attacker attempts to authenticate to the host using the ‘sync’ username with no password provided. Successful authentication grants access based on the account’s permissions. For example, an attacker could attempt SSH login as ‘sync’.
• Scope: Unix-like systems and potentially Windows systems where a ‘sync’ account exists without password protection are affected.

3. Detection and Assessment

Confirming vulnerability requires checking the account configuration on the host system. A quick check involves listing accounts with empty passwords, while thorough assessment includes reviewing authentication logs for successful logins as the ‘sync’ user without credentials.

• Scanning: Nessus plugin ID 34859 may identify unpassworded accounts, but results should be verified manually.
• Logs and evidence: Examine `/var/log/auth.log` or equivalent system logs for successful authentication attempts by the ‘sync’ user without a password.

cut -d: -f1 /etc/shadow | grep sync

4. Solution / Remediation Steps

The issue must be fixed by either setting a strong password for the ‘sync’ account or disabling it if no longer needed. These steps are small, testable and safe to roll back.

4.1 Preparation

• Ensure you have an alternative administrative account available for access in case of issues. A rollback plan involves restoring from backup or re-enabling the account if disabled.
• Change windows may be required depending on service impact, and approval from a senior administrator is advised.

4.2 Implementation

1. Step 1: Set a strong password for the ‘sync’ account using the `passwd sync` command on Linux systems. You will be prompted to enter a new password twice.
2. Step 2: Verify the password has been set by attempting to list shadow file entries again, confirming that there is now an encrypted password hash present.
3. Step 3: If the account is not required, disable it using `usermod -L sync` on Linux systems. This locks the account preventing logins.

4.3 Config or Code Example

Before

sync::0:0:99999:99999::/home/sync:/bin/bash

After

sync:$6$rounds=5000$salt$hashedpasswordhash::0:0:99999:99999::/home/sync:/bin/bash

4.4 Security Practices Relevant to This Vulnerability

Several security practices directly address this vulnerability type. Least privilege reduces the impact of compromise, while strong password policies enforce secure credentials. Regular account reviews identify unused or improperly configured accounts.

• Practice 1: Implement least privilege principles, granting only necessary permissions to the ‘sync’ account.
• Practice 2: Enforce a strong password policy requiring complex passwords and regular changes.

4.5 Automation (Optional)

A script can automate password setting across multiple systems. Exercise caution when automating password changes, ensuring proper error handling and logging.

#!/bin/bash
for host in $(cat /tmp/hostlist); do
  ssh $host "passwd sync" &
done

5. Verification / Validation

Confirm the fix by verifying a password is set for the ‘sync’ account and attempting to authenticate with incorrect credentials. A service smoke test ensures synchronisation functionality remains operational.

• Post-fix check: Run `cut -d: -f1 /etc/shadow | grep sync` again. The output should show an encrypted password hash instead of empty fields.
• Re-test: Repeat the initial detection method to confirm no unpassworded accounts are present.
• Monitoring: Monitor authentication logs for failed login attempts using the ‘sync’ account, which could indicate brute-force attacks.

cut -d: -f1 /etc/shadow | grep sync

6. Preventive Measures and Monitoring

Update security baselines to include password requirements for all accounts. Implement automated checks in CI pipelines to detect unpassworded accounts during system builds. A regular patch cycle ensures timely application of security updates.

• Baselines: Update your Linux hardening baseline (for example, CIS benchmark) to require passwords on all user accounts.
• Asset and patch process: Review account configurations during regular security audits or as part of the patching cycle.

7. Risks, Side Effects, and Roll Back

Setting an incorrect password may lock out services reliant on the ‘sync’ account. Disabling a required account will break functionality. Roll back involves restoring from backup or re-enabling the account.

• Roll back: Restore from backup if necessary, or re-enable the ‘sync’ account using `usermod -U sync` on Linux systems.

8. References and Resources

• Vendor advisory or bulletin: N/A
• NVD or CVE entry: CVE-1999-0502
• Product or platform documentation relevant to the fix: N/A