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Abusing WebVTT and CORS for fun and profit

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WebVTT is a way html5 developers can display and cue text as subtitles for video formats. The grammar for WebVTT is pretty simple and as we know browsers are always willing to forgive any "weird" looking grammar in an effort to provide best effort experience for users. This post looks at ways to take advantage of WebVTT in some attack contexts in order to extract information or perform general DOM abuse.

Introduction
Video tags can make use of subtitle files, as follows:

WebVTT (subtitle) files need to follow this format:


The file merely describes cues, allows you to number them and associate a duration and display time for them. Display timestamps specify hours (hh), minutes (mm) , seconds (s) and milliseconds (ttt). According to my basically inspection of the grammar, most browser require you to respect the placeholders (significant figures) if you specify the magnitude. For instance, if you want to indicate hours you need to use both place holders, same goes for others. …

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