This is an interesting new attack on some TLS configurations: https://opossum-attack.com/
It exposes the fact that some protocols support an opportunistic TLS variant that has a slightly different underlying protocol than the implicit TLS variant. The practical impact this has on servers supporting opportunistic HTTP (RFC 2817) is that a man-in-the-middle attacker can serve a different file from a server to an HTTPS request. It may impact other protocols that support STARTTLS, but one would have to find an exploitable protocol difference. For any software implementing Opportunistic HTTP / RFC 2817, particularly on the server side, it is advisable to completely remove that support to prevent this attack. Opportunistic HTTP / RFC 2817 is not widely used or supported. It can be enabled in Apache httpd, according to the Opossum web page, Apache plans deprecation of that feature (CVE-2025-49812). The web page also mentions Icecast and CUPS as "Patch in progress", and Cyrus IMAPD disabled STARTTLS by default in response to this research. This shows again that "upgrading" an unencrypted connection to TLS can be the cause of surprising security issues. (I've been involved in some research a few years ago showing security issues in STARTTLS for e-mail, which is a closely related issue: https://nostarttls.secvuln.info/ ) -- Hanno Böck - Independent security researcher https://itsec.hboeck.de/ https://badkeys.info/
