CVE-2026-100741: hMailServer JScript RCE

How to Detect CVE-2026-100741

Start with configuration and asset discovery. Identify Windows systems running hMailServer 6.0.0 through 6.3.3, then determine:

• Whether event scripting is enabled.
• Whether the language is JScript.
• Whether OnClientValidatePassword, OnExternalAccountDownload, or OnDeliveryFailed handlers are present.
• Whether SMTP, POP3, or IMAP services are externally reachable.

Do not rely on the presence of a mail service alone because the vulnerable execution path depends on configuration.

Review hMailServer logs for:

• Unusual authentication activity involving existing accounts.
• Repeated failures with malformed-looking password values.
• Unexpected POP3 retrieval behavior.
• SMTP delivery failures that coincide with suspicious source addresses.

Exact log fields and formatting can vary by hMailServer configuration. Preserve original log files and validate suspected patterns against a known-good installation.

For broader guidance on recognizing suspicious application behavior, see the FAQ on detecting stored XSS in admin panels. The same principles—reviewing unexpected input flow, server-side execution, and correlated logs—can support this investigation.

Technical Detection Notes

A practical first-pass search on a Windows host can locate likely event-script configuration files and references to relevant handlers:

$paths = @(
  'C:\Program Files\hMailServer',
  'C:\Program Files (x86)\hMailServer'
)

Get-ChildItem -Path $paths -Recurse -File -ErrorAction SilentlyContinue |
  Select-String -Pattern 'OnClientValidatePassword|OnExternalAccountDownload|OnDeliveryFailed' |
  Select-Object Path, LineNumber, Line

The following process-creation query is a detection aid rather than a hMailServer-specific signature. Investigate unexpected child processes created by the hMailServer service, especially scripting engines, command shells, PowerShell, or utilities that create or access COM-backed functionality:

DeviceProcessEvents
| where InitiatingProcessFileName in~ ("hMailServer.exe", "hmailserver.exe")
| where FileName in~ (
    "cmd.exe", "powershell.exe", "pwsh.exe", "wscript.exe",
    "cscript.exe", "mshta.exe", "rundll32.exe"
)
| project Timestamp, DeviceName, InitiatingProcessFileName,
          FileName, ProcessCommandLine, AccountName

For environments without endpoint telemetry, enable or review Windows process-creation auditing and correlate suspicious child-process events with hMailServer authentication, POP3, or SMTP logs.

A matching sequence of crafted authentication failures followed by a new child process should be escalated as a possible compromise, not treated as ordinary mail noise.

Mitigation and Patching

Upgrade hMailServer to 6.3.4 or later, subject to confirmation from the vendor release documentation. The referenced release is the apparent fixed version for the affected range through 6.3.3.

Before upgrading, back up:

• The hMailServer database.
• Configuration files.
• Event scripts.
• Certificates.
• Mail stores.

Test the release in a representative environment if the server supports production mail flow.

Because the supplied material does not provide verified vendor command-line installer syntax, use the official hMailServer 6.3.4 installer or an approved software-distribution workflow instead of relying on an unverified silent-install command.

Record the installed version after the change and confirm that event scripts and mail-flow integrations still operate as intended.

If immediate upgrading is not possible:

1. Disable event scripting where operationally feasible.
2. Do not select JScript for event processing.
3. Review and remove unnecessary definitions for OnClientValidatePassword, OnExternalAccountDownload, and OnDeliveryFailed.
4. Restrict access to exposed mail protocols.
5. Continue toward full patching.

These actions reduce exposure but should be treated as temporary risk reduction, not a substitute for upgrading.

Credential and Access Controls

Review the hMailServer service account and reduce its permissions to the minimum required for mail processing. Rotate credentials that may have been accessible to the service account if compromise is suspected.

Use unique, strong credentials for administrator and service accounts, stored in an approved enterprise password manager such as Try 1Password →. Credential management does not fix the vulnerability, but it can limit the impact of service-account compromise and reduce password reuse.

Network Containment

A concrete network-containment option is to restrict inbound access to mail protocols to approved relay hosts, VPN ranges, or trusted network segments.

The following Windows Firewall example blocks inbound SMTP, POP3, and IMAP traffic. Adapt it carefully because it can interrupt legitimate mail delivery:

New-NetFirewallRule `
  -DisplayName "Temporary hMailServer inbound containment" `
  -Direction Inbound `
  -Action Block `
  -Protocol TCP `
  -LocalPort 25,110,143 `
  -Profile Domain,Private,Public

A less disruptive approach is to permit only approved source ranges with explicit allow rules and enforce restrictions at the perimeter firewall or mail gateway.

Do not expose administrative interfaces or database services while investigating the host.

Verification After Remediation

After containment or patching, restart the hMailServer service through the organization’s normal change process and verify the running binary version:

Get-Service -Name hMailServer
Get-Process -Name hMailServer -FileVersionInfo |
  Select-Object Path, FileVersion, ProductVersion

Confirm that:

• The installed version is 6.3.4 or later.
• JScript event scripting is disabled unless there is a documented business requirement.
• Unnecessary event handlers have been removed.
• Mail flow operates normally.
• No unexpected child processes are created by hMailServer.
• Firewall and gateway restrictions match the approved architecture.

If compromise is suspected, isolate the host, preserve hMailServer and Windows event logs, collect process and network telemetry, rotate credentials that may have been accessible to the service account, and investigate persistence mechanisms.

Patch installation alone does not remove attacker-created accounts, scheduled tasks, web shells, modified scripts, or other post-exploitation artifacts. Organizations should document any temporary mitigations as compensating controls and track them through to permanent remediation. See the compensating control glossary entry for related risk-management terminology.

References

The NVD record for CVE-2026-100741 is available at:

• NVD CVE-2026-100741 record

The vendor-referenced corrective commit is:

• hMailServer vendor commit 5a3de9001d6c5e833662ada8e44ce10d42d76b1a

The apparent remediation release is:

• hMailServer 6.3.4 release page

CISA’s Known Exploited Vulnerabilities catalog is available at:

• CISA KEV catalog

The available research establishes the affected range, root cause, configuration prerequisites, and apparent remediation release. It does not establish:

• The complete CVSS vector.
• The exact post-6.2.25 COM-object behavior.
• Confirmed exploitation in the wild.
• A verified public proof of concept.

Defenders should treat those items as unknown rather than infer them from the severity score. The first actions are to inventory hMailServer 6.0.0 through 6.3.3, verify event-scripting configuration, restrict exposed services where necessary, and upgrade affected systems to 6.3.4 or later.

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