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Message-ID: <20250709093238.7a9d50d9@hboeck.de>
Date: Wed, 9 Jul 2025 09:32:38 +0200
From: Hanno Böck <hanno@...eck.de>
To: oss-security@...ts.openwall.com
Subject: Opossum attack / Opportunistic HTTP (RFC 2817) insecure
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/
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