29 Jun 2021
Bulletproof TLS Newsletter is a free periodic newsletter bringing you commentary and news surrounding SSL/TLS and Internet PKI, designed to keep you informed about the latest developments in this space. Maintained by Hanno Böck.
A team of security researchers has shown vulnerabilities that affect the interaction of different TLS-protected protocols if they are operated with the same host name. They named the attacks ALPACA, and a paper will be published at the upcoming USENIX conference. The underlying issue is that traditionally TLS contains no protection against cross-protocol attacks.
The most impactful attacks the researchers were able to show affect the interaction of browser HTTPS connections and FTP. When an attacker has a user account on an FTP server operated on the same host as an HTTPS server, it’s possible to let the browser generate requests that the FTP server will interpret as valid commands. This allows triggering an upload that will contain the browser’s session cookies or a download that leads to a cross-site scripting vulnerability. The attacks on FTP are an extension of a previous attack discovered by Jann Horn in 2015.
Further attacks affect the interaction between HTTPS and email protocols (IMAP, SMTP, POP3). But these attacks are less practical and only work in older browsers, like Internet Explorer and old versions of Edge from before the switch to the Chrome engine. The reason is that these older browsers allow HTTP responses without valid HTTP headers, something that was allowed in HTTP/0.9.
The underlying issue of ALPACA is that a TLS-protected connection with one protocol will be accepted by a server serving another protocol. This can be prevented with Application-Layer Protocol Negotiation (ALPN).
ALPN was originally introduced as a mechanism for SPDY and later HTTP/2 in order to allow multiplexing different protocols on the same port (e.g., HTTP/1.1 and HTTP/2). When implemented correctly on both the server and the client sides, ALPN can prevent cross-protocol attacks like ALPACA. According to the ALPN RFC, a server is expected to reject a connection when it uses ALPN and does not advertise a supported protocol.
But it turns out that many servers do not implement ALPN in this strict way and will answer with the standard protocol if a wrong protocol has been advertised by the client. Changing to a stricter implementation of the ALPN protocol therefore is a way to prevent future attacks like ALPACA.
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