Description

Cross-site scripting in the webmail of Progressive Robot hMailServer 6.3.2 through 6.3.5 allows a remote attacker who can send a user an encrypted message to run script in the webmail's origin with that user's session. When the webmail decrypted an S/MIME message (from 6.3.2) or an OpenPGP message (from 6.3.4) in the browser, it offered each decrypted attachment as a blob URL of the media type the message declared for it. A click saved the file, but if the user opened the attachment in a new tab, a part declared as text/html was rendered as a document of the webmail's origin and its script could read the mailbox, send mail and change the account through the REST API. The webmail is served only when the REST API is enabled, which it is not by default.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "8.0",
  "pubDate": "2026-10-08T11:16:42.950Z",
  "pubdate": "2026-10-08T11:16:42.950Z",
  "executiveSummary": "A Cross-site scripting (XSS) vulnerability exists within the Progressive Robot hMailServer webmail component, affecting versions 6.3.2 through 6.3.5. The vulnerability arises from improper handling of decrypted S/MIME and OpenPGP message attachments, which allows arbitrary scripts to execute within the security context of the webmail origin.\nBy sending a specially crafted encrypted message, a remote, unauthenticated attacker can induce a victim into opening an attachment in a new browser tab. If the attachment is declared as text/html, the browser renders the content directly within the webmail's origin. This permits the execution of malicious scripts that inherit the victim's active session privileges.\nThe attack vector requires the target's webmail to have the REST API enabled, a feature not active by default. Successful exploitation grants the attacker full access to the victim's mailbox, the capability to send outgoing mail, and the ability to modify account settings via the REST API, resulting in a total compromise of the user's webmail account.",
  "technicalDetails": "The vulnerability originates from the insecure handling of decrypted message attachments within the hMailServer webmail interface. In versions 6.3.2 through 6.3.5, the application processes encrypted S/MIME (starting from 6.3.2) and OpenPGP (starting from 6.3.4) messages by decrypting attachments and surfacing them as blob URLs. The system preserves the original media type declared within the MIME headers for these blob objects.\nThe attack flow begins when an attacker sends an encrypted message containing an attachment explicitly declared as text/html. When the victim accesses the webmail and decrypts this message, the interface provides a link to the attachment. The critical flaw occurs when the victim chooses to open the attachment in a new browser tab. Because the blob URL is served directly by the webmail origin, the browser treats the rendering window as part of the same-origin policy (SOP) domain as the webmail application itself.\nOnce the browser navigates to the malicious blob URL, the embedded HTML is parsed and executed in the context of the webmail application. Since the victim is already authenticated, the malicious script executes with the full privileges of the user's session. The payload can then interact directly with the webmail's REST API, which is accessible to the origin. This allows the attacker to perform unauthorized operations, including reading the contents of the victim's mailbox, exfiltrating sensitive communications, sending spoofed emails on behalf of the user, or modifying account credentials and settings.\nExploitation is contingent upon the REST API being enabled in the hMailServer configuration. While this feature is disabled by default, environments that have enabled the REST API for integration purposes are at high risk. The lack of proper Content-Security-Policy (CSP) headers or restrictive blob URL handling fails to prevent the execution of untrusted scripts sourced from these attachments, thereby facilitating the cross-site scripting impact."
}