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CVE-2026-24423 – Unauthenticated RCE – SmarterMail before Build 100.0.9511

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Summary

CVE-2026-24423 is a critical unauthenticated remote code execution vulnerability affecting SmarterTools SmarterMail. The flaw resides in the ConnectToHub API method, which allows an unauthenticated attacker to redirect the mail server to a malicious HTTP server that returns OS commands for execution. The vulnerability has a CVSS score of 9.3 (Critical) and has been actively exploited in ransomware campaigns, leading CISA to add it to its Known Exploited Vulnerabilities catalog.

Technical details

  • Root cause: The endpoint POST /api/v1/settings/sysadmin/connect-to-hub is decorated with [AuthenticatedService(AllowAnonymous = true)], explicitly allowing anonymous access to a sensitive administrative function (CWE-306: Missing Authentication for Critical Function).
  • Trigger conditions: An attacker submits a request to the endpoint with a hubAddress parameter pointing to an attacker-controlled server.
  • Attack flow: SmarterMail issues an outbound request to the attacker’s server at /web/api/node-management/setup-initial-connection. The attacker’s server responds with a JSON payload containing a CommandMount field holding an arbitrary OS command.
  • Execution: SmarterMail passes the attacker-supplied command to CommandLine.RunCommand(), which executes it via cmd.exe on Windows or /bin/bash on Linux/Docker deployments; on Linux, commands can run with elevated privileges via sudo.
  • Attack vector: Network, no authentication or user interaction required.
  • Impact: Full remote code execution on the SmarterMail server, enabling complete compromise of confidentiality, integrity, and availability. The vulnerability has been leveraged in ransomware intrusions, reportedly including a breach of SmarterTools itself.

Affected software

  • SmarterTools SmarterMail versions prior to build 100.0.9511 (all editions where the mail server exposes the affected API)

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical) — AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  • CVSS v4.0 Base Score: 9.3 (Critical) — AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade SmarterMail to build 100.0.9511 or later, released January 15, 2026, which fixes the missing authentication check on the ConnectToHub endpoint.
  • If immediate patching is not possible:
    • Restrict network access to the SmarterMail administrative interface and API endpoints to trusted management networks only.
    • Monitor logs for requests to /api/v1/settings/sysadmin/connect-to-hub and for unexpected outbound connections initiated by the mail server.
    • Query the unauthenticated /api/v1/licensing/about endpoint to verify the current installed build number across all SmarterMail instances.
    • Treat any SmarterMail server exposed to the internet without confirmed patching as compromised and investigate for indicators of ransomware precursor activity.

References

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