Add expire_time in security link.
http://wiki.nginx.org/HttpSecureLinkModule:
secure_link
syntax: secure_link $md5_hash[,$expire_time]
This directive specifies the MD5 hash value and the expired time of this link URL's. The $md5_hash should be encoded by modified Base64 for URL. The $expired_time is the seconds since 1970-01-01 00:00:00 UTC. If you did not add the $expire_time, then the link URL will never be expired.

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