The W3 Total Cache WordPress plugin through version 2.9.3 skips its entire output buffering and processing pipeline whenever an incoming request's User-Agent header contains the string "W3 Total Cache", without authenticating the caller. On sites that use developer-placed dynamic fragment tags, the raw mfunc/mclude HTML comments - which embed the per-site W3TC_DYNAMIC_SECURITY token - are therefore rendered directly into the page source instead of being processed and stripped, letting an unauthenticated attacker harvest the token by comparing a normal response against one sent with the magic User-Agent.
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