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NSA Highlights Threat of the ALPACA Technique

In an advisory released on October 7th, the NSA documented the risks of using wildcard TLS certificates and the potential risks of ALPACA attacks associated with their usage. An Application Layer Protocol Content Confusion Attack, or ALPACA, essentially tricks a server that is running multiple protocols into responding to encrypted HTTPS requests via unencrypted protocols. According to the discovering researchers, the ALPACA technique “can extract session cookies and other private user data or execute arbitrary JavaScript in the context of the vulnerable web server, therefore bypassing TLS and web application security.” Although the severity of an ALPACA attack being executed was low at the time it was discovered, it was believed that there were nearly 120,000 vulnerable servers at that time. The tide has changed now as multiple companies and organizations are implementing defenses against ALPACA.


Google built defenses against this attack into the Chrome browser earlier this year. The NSA is also asking organizations to enable Application-Layer Protocol Negotiation (ALPN). ALPN is a TLS extension that prevents servers from responding to requests via non-allowed protocols, such as FTP, IMAP, or whatever the server owner decides to disallow. Further recommendations for defending against ALPACA attacks can be found in the NSA advisory here: