With "Wifi attacker", they mean man-in-the-middle attacker.
Which is most easily done by a malicious Wifi provider.
A group of security researchers have analyzed 50 of the top Chrome extensions and another 50 chosen by random, and found out that 27 of the 100 extensions contain one or more vulnerabilities in their cores, for a total of 51 vulnerabilities :
We reviewed 100 Chrome extensions and found that 27 of the 100 extensions leak all of their privileges to a web or WiFi attacker. Bugs in extensions put users at risk by leaking private information (like passwords and history) to web and WiFi attackers. Web sites may be evil or contain malicious content from users or advertisers. Attackers on public WiFi networks (like in coffee shops and airports) can change all HTTP content. We’ll show you how you can prevent attacks on your extension using Content Security Policy.
49 of the 51 vulnerabilities found can be patched by adapting the extensions to use one of two offered Content Security Policies (CSP).
The group is urging extension developers to use CSP directives to protect their users from the potential security consequences of core extension bugs.
