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No more risk than any other file which might contain code samples or naughty words.
Splunk wont ever execute them, and will process all inputs a plain text - the only risk is the content of the files, and whether you are happy to index the content of them (passwords, keys etc)
When specifically monitoring source code files, I was thinking that [fschange] instead of [monitor] might be a good idea. However, now I am reading that fschange is deprecated. What are your thoughts?
If your just looking to index the files when they change, you can use a normal monitor statement, and set
CHECK_METHOD = entire_md5 in props.conf which will trigger Splunk to reindex the whole file each time it changes.