alt Hacker NewsFrom the related article on calif.io:
> We therefore control where PHP places a pointer and its associated type information. This is application-dependent RCE under both Apache mod_php and PHP-FPM.
Both articles describe how you can end up with freed memory that still has pointers into it. I can understand that this should never be possible, but it seems to me that both articles skip the very important step of how to actually turn this into RCE?
From my (admittedly limited) understanding of these things you would not only need to write arbitrary memory, but also need to be able to jump to the relevant address to execute such things. Or do they overwrite the PHP (byte)code with their own and then just let the interpreter do its thing?
Replies
Plenty of function pointers to overwrite, presumably
It's generally safe for a defender to assume that if an attacker can corrupt the heap they can achieve RCE, because there are usually so many ways to do it. They're all very specific ways that are hard to find because of all the complexity and mitigations, but there are a practically unlimited supply if someone puts in the effort to find one. But as a defender you don't need to, you should just say game over as soon as you get heap corruption, and go fix it.
Generally the attacker has to cause a sequence of heap allocations so that a certain kind of object gets allocated into the freed space that's still being pointed to, then use the old pointer to corrupt that object, then use the object. A good target is any C++ object with a vtable pointer - not sure if PHP is written in C++. If you can rewrite a vtable pointer and make it point to attacker-controlled data, you can get execution to jump to an attacker-controlled address.