*This blog entry was originally published on March 23, 2015 on the original Polito Blog by Ian Duffy. It was re-posted on October 2, 2017 due to migrating to a new blog platform.
Recently I was working on a tool similar to Raven that was written by Raphael Mudge, basically that contacts a web server, pulls down some shellcode, and then executes the shellcode. My initial version of the program executed the shellcode by creating a buffer via VirtualAlloc and then passing that buffer (cast as an LPTHREAD_START_ROUTINE) to CreateThread:
char * sc = VirtualAlloc(NULL,
4096,
MEM_COMMIT,
PAGE_EXECUTE_READWRITE);
// Copy meterpreter reverse HTTPS shellcode to buffer
HANDLE hThread = CreateThread(NULL,
0,
(LPTHREAD_START_ROUTINE)sc,
NULL,
0);
The program I was writing was designed to possibly have to run for several weeks, so memory conservation and efficiency were paramount. Using Sysinternals vmmap.exe tool, I began running the tool and monitoring the amount of memory used for the stack, heap, private bytes, working set, etc. What I noticed was puzzling.
Once the meterpreter session is closed and the thread was finished, I would free the memory buffer using VirtualFree and I would close the handle for the thread. However, it seemed that the amount of memory used by the application would increase every time it successfully downloaded and executed the shellcode from the webserver. Once the thread finished, the memory used by that thread was not freed.
Figure 1 - Before Shellcode Download / Execute
Figure 2 - After Shellcode Download / Execute
Figure 3 - Shell Session Opened
Figure 4 - Shell Session Killed
Figure 5 - Memory used after Meterpreter Session was Killed
Figure 6 - After a few more Meterpreter Sessions
To see if it was my application causing the issue, I would try downloading the shellcode, copying it to the buffer, but not executing it via CreateThread. Then I would free the buffer as before. The issue did not persist. Thus it would appear that meterpreter's reverse_https shellcode is allocating buffers and the memory that it allocates is not freed. This makes sense - the first stage contacts the metasploit payload handler and downloads the follow-on stages for meterpreter - allocating memory buffers as needed. The shellcode does not appear to contain code to free the memory that it allocates - this may be a side-effect of optimizing the shellcode for size (extra instructions to free memory require larger shellcode which can be a bad thing). The problem is, this can cause memory leaks in the long-term if the program executes multiple shellcode buffers over the course of its lifetime - the application will reserve more and more memory until it runs out or crashes. This is not ideal for my program since it was supposed to function for medium- to long-term (one-week to multi-week engagements).
Thus I decided that I would need to execute the shellcode in a new process. This should have the effect of freeing any memory used by meterpreter when the process exits. Borrowing the code from inject.c in Raven, I spawn a new process, WriteProcessMem to create an executable buffer in that process, and then CreateRemoteThread on that buffer. This worked fine and there were no more memory management issues within my program. Problem solved - or so I thought.
Using the reverse_https meterpreter, I noticed that even though I had the EXITFUNC set as "process", the process in which the shellcode was executed would not exit when the merterpreter session was terminated. This appears to be related to this bug report for the Metasploit Framework. Apparently the Windows WinInet API is a tad convoluted and doesn't behave as expected when functions are called via the shellcode. When the final shellcode thread exits, for some reason the program does not get the notification and it continues to run merrily along. In the end, this behavior have the effect of leaving a bunch of zombie processes running - obviously not a desirable outcome.
In order for the process to be terminated successfully, it looks like I will have to monitor the thread execution via WaitForSingleObject and then terminate the process once the thread completes:
// Create the remote thread
HANDLE hThread = CreateRemoteThread(hProcess,
NULL,
0,
(LPTHREAD_START_ROUTINE)sc,
NULL,
0,
NULL);
// Wait for the thread to terminate
WaitForSingleObject(hThread, INFINITE);
// Clean Up
CloseHandle(hThread);
DWORD dwExitCode = 0;
GetExitCodeProcess(hProcess, &dwExitCode);
if (dwExitCode == STILL_ACTIVE) {
TerminateProcess(hProcess, 0);
}
After that, the Process exits cleanly and no more memory leak issues!
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