From a Wired article:
But hidden within another document leaked by Snowden was a slide that provided a few hints about detecting Quantum Insert attacks, which prompted the Fox-IT researchers to test a method that ultimately proved to be successful. They set up a controlled environment and launched a number of Quantum Insert attacks against their own machines to analyze the packets and devise a detection method.
According to the Snowden document, the secret lies in analyzing the first content-carrying packets that come back to a browser in response to its GET request. One of the packets will contain content for the rogue page; the other will be content for the legitimate site sent from a legitimate server. Both packets, however, will have the same sequence number. That, it turns out, is a dead giveaway.
Here’s why: When your browser sends a GET request to pull up a web page, it sends out a packet containing a variety of information, including the source and destination IP address of the browser as well as so-called sequence and acknowledge numbers, or ACK numbers. The responding server sends back a response in the form of a series of packets, each with the same ACK number as well as a sequential number so that the series of packets can be reconstructed by the browser as each packet arrives to render the web page.
But when the NSA or another attacker launches a Quantum Insert attack, the victim’s machine receives duplicate TCP packets with the same sequence number but with a different payload. “The first TCP packet will be the ‘inserted’ one while the other is from the real server, but will be ignored by the [browser],” the researchers note in their blog post. “Of course it could also be the other way around; if the QI failed because it lost the race with the real server response.”
Although it’s possible that in some cases a browser will receive two packets with the same sequence number from a legitimate server, they will still contain the same general content; a Quantum Insert packet, however, will have content with significant differences.
It’s important we develop defenses against these attacks, because everyone is using them.
EDITED TO ADD (5/14): Detection for QI was recently released for Bro, Snort and Suricata.