CVE-2025-4679: Synology Active Backup Information Disclosure

CVE-2025-4679 Overview

CVE-2025-4679 is a sensitive information disclosure vulnerability affecting Synology Active Backup for Microsoft 365. This vulnerability allows remote authenticated attackers to obtain sensitive information through unspecified attack vectors. The flaw is classified under CWE-522 (Insufficiently Protected Credentials), indicating that credential storage or handling mechanisms within the application fail to adequately protect sensitive authentication data.

Critical Impact

Authenticated attackers can exfiltrate sensitive information from backup systems, potentially compromising Microsoft 365 credentials and data protected by the backup solution.

Affected Products

• Synology Active Backup for Microsoft 365
• Synology NAS devices running Active Backup for Microsoft 365 package
• Enterprise backup environments utilizing Synology's Microsoft 365 backup solution

Discovery Timeline

• May 16, 2025 - CVE-2025-4679 published to NVD
• July 2, 2025 - Last updated in NVD database

Technical Details for CVE-2025-4679

Vulnerability Analysis

The vulnerability resides in Synology Active Backup for Microsoft 365's handling of sensitive information. According to the CWE-522 classification, the application insufficiently protects credentials, allowing authenticated users to access information they should not be authorized to view. This type of vulnerability typically occurs when sensitive data such as API tokens, authentication credentials, or backup encryption keys are stored or transmitted without adequate protection mechanisms.

The network-accessible nature of this vulnerability means that any authenticated user with network access to the Synology NAS running Active Backup for Microsoft 365 could potentially exploit this flaw. The attack requires low privileges and no user interaction, making it relatively straightforward for a malicious insider or compromised account to leverage.

Root Cause

The root cause stems from insufficiently protected credentials (CWE-522) within the Active Backup for Microsoft 365 application. This weakness typically manifests when:

• Sensitive credentials are stored in plaintext or weakly encrypted formats
• Access controls on credential storage locations are inadequate
• API endpoints expose sensitive configuration data without proper authorization checks
• Backup metadata contains unprotected authentication tokens or secrets

Security researchers at modzero documented similar issues in Synology's backup infrastructure, highlighting how backup solutions can inadvertently become security risks when credential protection is insufficient.

Attack Vector

The attack is network-based and requires the attacker to have authenticated access to the Synology system. The exploitation path involves:

1. An attacker with valid low-privilege credentials authenticates to the Synology NAS
2. The attacker accesses the Active Backup for Microsoft 365 interface or API endpoints
3. Through unspecified vectors, the attacker retrieves sensitive information that should be protected
4. The compromised information could include Microsoft 365 OAuth tokens, service account credentials, or backup encryption keys

Given the confidentiality-focused impact of this vulnerability, successful exploitation results in unauthorized access to sensitive data without affecting system integrity or availability. Technical details of the exploitation methodology are available in the modzero security report.

Detection Methods for CVE-2025-4679

Indicators of Compromise

• Unusual API requests to Active Backup for Microsoft 365 endpoints from authenticated sessions
• Abnormal access patterns to backup configuration or credential storage areas
• Unexpected data exfiltration activity from Synology NAS systems
• Log entries showing repeated access to sensitive backup settings by low-privilege accounts

Detection Strategies

• Monitor Synology DSM audit logs for anomalous access to Active Backup for Microsoft 365 settings
• Implement alerting on unusual API call patterns targeting backup credential endpoints
• Deploy network traffic analysis to detect potential data exfiltration from NAS devices
• Review authentication logs for compromised or suspicious account activity

Monitoring Recommendations

• Enable comprehensive logging for Active Backup for Microsoft 365 package activities
• Configure SIEM rules to alert on access to sensitive backup configuration endpoints
• Establish baseline access patterns for backup administration functions to identify anomalies
• Monitor for bulk data retrieval operations from backup metadata or configuration stores

How to Mitigate CVE-2025-4679

Immediate Actions Required

• Review and apply the security update from Synology as specified in their security advisory
• Audit user accounts with access to Active Backup for Microsoft 365 and remove unnecessary privileges
• Rotate Microsoft 365 credentials and OAuth tokens used by the backup solution
• Review access logs for any historical exploitation attempts

Patch Information

Synology has released security updates to address this vulnerability. Administrators should consult the Synology Security Advisory SA-25-06 for specific patch versions and update instructions. It is critical to apply the latest package update for Active Backup for Microsoft 365 through DSM Package Center.

Workarounds

• Restrict network access to the Synology NAS to trusted administrator IP addresses only
• Implement additional authentication layers such as two-factor authentication for DSM access
• Segment the Synology NAS on a separate network VLAN with strict firewall rules
• Disable remote access to the Synology DSM interface until patches can be applied
• Review and minimize the number of user accounts with access to backup administration functions

For additional technical details on the vulnerability class, refer to the modzero blog post on Synology backup security.