Clear it in 3 steps
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Start the environment
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Investigate the target
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Submit the flag
When an Nginx server has directory index listing (autoindex) enabled, anyone can browse the full file listing of a directory that was meant to stay private. Explore the exposed directory.
Guess likely static file directory names (e.g. /files/, /uploads/)
If directory indexing is enabled, the full file list is shown
Look for a file containing the flag in that listing