Monday, August 31, 2009

Escalating from PHP Hardend Environment

There are number of PHP threats and vulnerabilities which have been reported during the past few years. These include, file inclusion attacks, remote file upload vulnerability, insecure function injection (eval,create_function,preg_replace), etc. Executing malicious shellcode over vulnerable web servers is still easier but it is quiet challenging when "post exploitation" topic is highlighted.

Today many of PHP-based web servers are hardened by default and running with low privileges. Thus, it is extremely challenging for the attacker to gain full control over the server. Let's take a brief overview on common type of protection schemes used to hardened PHP environment:

1. Limit the PHP code (i.e. control each input/output)
2. Limit the PHP interpreter
3. Harden the code against buffer overflow + memory corruption
4. Limit the possibility of arbitrary code execution
5. Non-writable filesystem
6. safe_mode (disable access to configuration settings, limit access to files/directories, limit environmental variables)
7. disable_function/disable_classes (remove un-necessary functions and classes)
8. Use memory manager (malloc/mmap) to apply safe_unlink feature and three canaries (metadata,buffer(before/after)
9. Kernel-level protection with ASLR (address space layout randomization), mprotect(), Apparmor, SELinux, GRSecurity

Now take some highlights on PHP vulnerabilities and exploitable condition:

1. Caller of the PHP application can force parameter to be passed by reference

function increase($a)
$z = 7;
// pass $z as a reference
echo $z,"\n";

This happens because we are unable to disabled the internal "allow_call_time_pass_by_reference" function.

2. executor_globals() to find the interesting target, it contains list of functions/ini entries/jmp_buf but the memory position is unknown and
it changes the structure with every single PHP version.

3. To execute the user choice of code, function dl() comes in handy but it requires:
-platform independent library
-a writable directory
-enable_dl should be activated
-setting extension_dir to the shared library directory

4. Attacking under x86 linux platform:
-PHP array leaks the pDestructor pointer which points to PHP code segment
-scan until we find ELF header in memory
-once ELF header discovered, we can also find imported functions
-select the function which have been imported from libc (memcpy)
-from there we can look any function within libc and access their addresses
-address to shellcode can be written and executed
-copying shellcode into the writable text-segment and execute it