JavaScript injections can be used to do a number of things; You can check cookie contents, swap cookies, temporarily edit a webpage, modify web forms, and even do some malicious stuff.

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In the third episode of #AskGoogleWebmasters, Webmaster Trends Analyst John Mueller responded to whether Googlebot can detect client-side JavaScript redirects.

The code can be injected into the user's browser through JavaScript associated with a malicious advertisement distributed through a Web ad service or an IFRAME in a linkjacked site, ad, or other ...