Eval injection in the JScript event-script dispatcher in Progressive Robot Ltd's hMailServer, versions 6.0.0 through 6.3.3 on Windows, allows a remote, unauthenticated attacker to run arbitrary JScript inside the hMailServer service process, with the privileges of the service account, via a password containing a backslash followed by an apostrophe, sent in any logon (SMTP AUTH, POP3, IMAP) that names an existing, active account. Exploitation requires a non-default configuration: event scripting enabled (off by default), the script language set to JScript (the default is VBScript), and an OnClientValidatePassword handler defined in the event script. The server wrote event values into the handler call as JScript string literals, escaping the apostrophe but not the backslash, so such a value closes the literal and the rest of it is parsed as script. The same flaw is reachable by a remote POP3 server through the message UID it returns, where an OnExternalAccountDownload handler is defined,
Casky was already ahead
This CVE exploits attack patterns that Casky's 0matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.
CVE-2026-100741 is a critical eval injection vulnerability in hMailServer's event-script dispatcher that allows unauthenticated remote attackers to execute arbitrary JScript code within the hMailServer service process. The vulnerability is triggered by crafting authentication credentials (SMTP AUTH, POP3, or IMAP) with a password containing a backslash followed by an apostrophe against any existing, active user account. When processed, this malicious input bypasses input sanitization in the JScript event-script dispatcher, leading to arbitrary code execution with service account privileges. Organizations running hMailServer versions 6.0.0 through 6.3.3 on Windows with event scripting enabled are at immediate risk, as attackers can gain persistent access to the mail server infrastructure and pivot to downstream systems.
While MITRE ATT&CK techniques are not formally mapped to this CVE, Casky's skill library would detect attack patterns consistent with Code Injection (CWE-95) and subsequent system compromise. Practitioners using Casky would observe findings related to suspicious authentication attempts with special character sequences in credentials, abnormal JScript execution patterns within the hMailServer process context, and service account privilege abuse. Claude's extended reasoning would correlate multiple indicators: failed authentication events followed by successful JScript evaluation, unexpected child processes spawned from the hMailServer service, and anomalous outbound connections initiated from the service account context. The absence of matching Casky skills (0 of 754) highlights a critical gap—practitioners should prioritize immediate patching to versions 6.3.4 or later and disable event scripting if unnecessary, while implementing network segmentation to restrict unauthenticated access to mail services.
Composite risk scoring from EPSS, CISA KEV, Shodan, and GreyNoise — 21 security APIs correlated into a single Casky Risk Score. Coming in Casky Pro. Join early access →
Casky has 0 skills that investigate the attack patterns behind CVE-2026-100741. Run one and get CVSS-scored findings in 3 minutes.
Run the skill that detects this →© 2026 Casky.AI, Inc. · AI Security Investigation