CVE-2026-103647 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') in hMailServer
Remote attackers can execute scripts in the webmail origin via malicious encrypted attachments, potentially taking over user accounts.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2026-103647?
CVE-2026-103647 is a high-severity cross-site scripting vulnerability in Progressive Robot Ltd hMailServer, published on 8 Oct 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official 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.
Weakness: CWE-79
Which versions are affected by CVE-2026-103647?
How do I fix CVE-2026-103647?
No fix has been published for CVE-2026-103647 yet. Limit exposure of the affected software until one is available.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-103647 being exploited?
There is no known exploitation of CVE-2026-103647 in the wild and no public exploit code.
Sources
Also known as GHSA-xr9h-mj6v-hm4m.