DNS Rebinding Enables Administrator Password Brute Force

Missing Host header validation and missing throttling of failed administrator sign-ins in the REST API listener of Progressive Robot hMailServer 6.0.0 through 6.3.5 allow a remote attacker to brute-force the server administrator's password through the administrator's own browser by DNS rebinding. The listener, which is off by default and bound to the loopback when enabled, answered requests whatever their Host header named, and a failed sign-in with the administrator's password from the loopback was neither auto-banned nor delayed. A web page whose host name the attacker rebinds to 127.0.0.1, opened in a browser on the server, can therefore send authenticated requests to the listener, read the answers and try administrator passwords at full speed until one is accepted, giving the attacker full administrative control of the mail server.

Casky was already ahead

This CVE exploits attack patterns that Casky's 323matched 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.

Analysis

CVE-2026-104659 affects Progressive Robot hMailServer versions 6.0.0 through 6.3.5, exploiting a critical authentication bypass in the REST API listener. The vulnerability combines two security failures: missing Host header validation and absent rate limiting on failed administrator sign-in attempts. Even though the listener defaults to disabled and binds to localhost, an attacker can leverage DNS rebinding to redirect the administrator's own browser into making repeated authentication requests against the server, effectively performing a brute-force attack from a trusted source. This affects any organization running vulnerable hMailServer versions with the REST API listener enabled, potentially allowing complete compromise of email infrastructure.

Casky's 323 matched security skills map this attack across MITRE ATT&CK's Initial Access (TA0001) and Credential Access (TA0043) tactics, enabling practitioners to detect the underlying patterns. Extended reasoning through Claude AI correlates the CWE-346 validation failure with typical DNS rebinding indicators—such as mismatched Host headers combined with repeated failed authentication from loopback ranges—and identifies the absence of throttling mechanisms that should appear in security telemetry. Practitioners using Casky would observe findings highlighting: unusual volumes of failed admin authentication attempts originating from localhost, REST API requests with suspicious or manipulated Host headers, and the absence of rate-limiting controls on authentication endpoints. These signals surface the attack chain before credential compromise occurs, enabling rapid remediation.

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Casky Risk Score

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EPSS Probability

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Shodan Exposure

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GreyNoise Signal

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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 →

323 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-104659.

Abuse of Public-Facing API: Mobile API Abuse

mobile security · low

Run

abusing-dpapi-for-credential-access

red teaming · high

Run

abusing-shadow-credentials-for-privesc

red teaming · high

Run

MITRE ATT&CK Techniques

TA0001 TA0043

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