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In this article, we will be looking at a new exploitation technique using the default OPcache engine from PHP 7. Using this attack vector, we can bypass certain hardening techniques that disallow the file write access in the web directory. This could be used by an attacker to execute his own malicious code in a hardened environment. OPcache is the new built-in caching engine with PHP 7.0.

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It compiles PHP scripts and sets the resulting bytecode in memory. It also offers caching in the filesystem when specifying a destination folder in your : . The hash is most likely used to ensure binary compatibility between installations. The directory is created by OPcache when caching its first file.

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As we’ll see later on, each OPcache file will also have a copy of that , overwriting the OPcache file will not execute our webshell. This restriction can be bypassed if the server running our vulnerable website is restarted. Since the memory cache will have been emptied, OPcache will use the file cache to fill the memory cache, thus executing our webshell. Under frameworks like Word Press, there are some deprecated files that are still publicly accessible (for example : registration-functions.php).

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Since these files are deprecated, they are never loaded and do not have a cached version in memory or in the filesystem. After uploading our malicious payload (), OPcache will run our binary webshell. If timestamp validation is enabled, OPcache will check the timestamp of the requested PHP source file and compare it to the timestamp header field of the cache file. If they do not match, the cache file is discarded and a new one is created. To successfully bypass this restriction, an attacker must know the timestamp of the targeted source file. That being said, under frameworks like Word Press, the timestamps for the source files are available as they remain intact upon extracting the zip or tar archive. Therefore, these timestamps will be the same across multiple versions of Word Press. Knowing the timestamp, an attacker can then modify his payload accordingly and successfully override the cache, complying with the setting. The timestamp is located 34 bytes from the beginning of the file: Here’s a quick demo showing how the attack works : As we’ve mentioned briefly, we also have a Git Hub repository.


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