CVE-2026-25067 Overview
SmarterTools SmarterMail versions prior to build 9518 contain an unauthenticated path coercion vulnerability in the background-of-the-day preview endpoint. The application base64-decodes attacker-supplied input and uses it as a filesystem path without validation. On Windows systems, this allows UNC paths to be resolved, causing the SmarterMail service to initiate outbound SMB authentication attempts to attacker-controlled hosts. This can be abused for credential coercion, NTLM relay attacks, and unauthorized network authentication.
Critical Impact
This vulnerability enables unauthenticated attackers to coerce the SmarterMail service into performing NTLM authentication to attacker-controlled servers, potentially leading to credential theft and NTLM relay attacks against the organization's Windows infrastructure.
Affected Products
- SmarterTools SmarterMail versions prior to build 9518
Discovery Timeline
- 2026-01-29 - CVE CVE-2026-25067 published to NVD
- 2026-01-29 - Last updated in NVD database
Technical Details for CVE-2026-25067
Vulnerability Analysis
This vulnerability (CWE-706: Use of Incorrectly-Resolved Name or Reference) exists in the background-of-the-day preview functionality within SmarterMail. The endpoint accepts user-supplied input that is base64-decoded and directly used to construct a filesystem path without proper validation or sanitization.
The core issue lies in the application's failure to validate that the decoded path input refers to a legitimate local file resource. When an attacker provides a base64-encoded UNC path (such as \\attacker-server\share), the Windows operating system automatically attempts to authenticate to the remote SMB server using the service account credentials.
This path coercion attack is particularly dangerous because it requires no authentication to exploit. An attacker can remotely trigger outbound SMB connections from the vulnerable server, capturing NTLMv2 hashes or relaying the authentication to other services within the network.
Root Cause
The root cause is improper input validation on the background-of-the-day preview endpoint. The application accepts base64-encoded data intended to represent a file path, decodes it, and passes it directly to filesystem operations without checking whether the path is a valid local resource or a potentially malicious UNC path. Windows automatically handles UNC paths by initiating SMB connections, which exposes the service account's NTLM credentials to remote attackers.
Attack Vector
The attack is network-based and requires no authentication or user interaction. An attacker can craft a malicious HTTP request to the vulnerable endpoint containing a base64-encoded UNC path pointing to an attacker-controlled SMB server. When the SmarterMail service processes this request, it attempts to access the UNC path, triggering an outbound SMB authentication attempt.
The captured NTLM credentials can be:
- Cracked offline to recover plaintext passwords
- Relayed to other services that accept NTLM authentication (NTLM relay attacks)
- Used for credential stuffing against other systems
Detection Methods for CVE-2026-25067
Indicators of Compromise
- Unusual outbound SMB (TCP port 445) connections from the SmarterMail server to external IP addresses
- Unexpected authentication attempts logged from the SmarterMail service account
- HTTP requests to the background-of-the-day preview endpoint containing base64-encoded UNC paths
- Network traffic showing SMB session negotiation to non-corporate IP addresses
Detection Strategies
- Monitor outbound connections on ports 445 and 139 from servers running SmarterMail
- Implement network segmentation rules to block direct SMB connections from web-facing servers to the internet
- Review web server access logs for suspicious requests to the background-of-the-day preview endpoint with unusual base64 payloads
- Deploy Windows Event Log monitoring for NTLM authentication events (Event ID 4624) from the SmarterMail service account to unexpected destinations
Monitoring Recommendations
- Configure firewall alerts for outbound SMB traffic from SmarterMail servers
- Enable enhanced Windows authentication logging to track service account NTLM usage
- Implement network detection rules for SMB traffic containing the SmarterMail service context
How to Mitigate CVE-2026-25067
Immediate Actions Required
- Upgrade SmarterTools SmarterMail to build 9518 or later immediately
- Block outbound SMB traffic (ports 445 and 139) from SmarterMail servers at the network firewall
- Review Windows Event Logs for any evidence of prior exploitation
- Consider enabling SMB signing and Extended Protection for Authentication (EPA) to limit NTLM relay attack effectiveness
Patch Information
SmarterTools has addressed this vulnerability in SmarterMail build 9518. Administrators should upgrade to this version or later as soon as possible. For detailed information about the update, refer to the SmarterTools Release Notes or the VulnCheck Security Advisory.
Workarounds
- Block outbound SMB traffic from SmarterMail servers using host-based or network firewalls
- Implement network segmentation to prevent the SmarterMail server from directly connecting to external networks on SMB ports
- Consider running the SmarterMail service under an account with minimal domain privileges to limit the impact of credential compromise
# Block outbound SMB traffic using Windows Firewall
netsh advfirewall firewall add rule name="Block Outbound SMB" dir=out action=block protocol=tcp remoteport=445
netsh advfirewall firewall add rule name="Block Outbound NetBIOS" dir=out action=block protocol=tcp remoteport=139
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
