[PATCH v1] tapctl: prevent binary planting with netsh

From: Heiko Hund <[email protected]>

Provide a application name to CreateProcess(), so that it doesn't try to
locate an executable - somewhere. Instead construct the full path to
netsh.exe in the system dir and pass that to the function instead of NULL.

Discovered and reported by BreachX Zero Day Labs, using Typhon AI Mil v2.
Contributing Researcher: Vivek Parikh.

Reported-by: Vivek Parikh <[email protected]>
Tested-by: Vivek Parikh <[email protected]>
Github: OpenVPN/openvpn-private-issues#164
CVE: 2026-84226
Change-Id: I70282985a7e8e46add92b2277674cbca6bce39c1
Signed-off-by: Heiko Hund <[email protected]>
Acked-by: Razvan Cojocaru <[email protected]>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1890
---

This change was reviewed on Gerrit and approved by at least one
developer. I request to merge it to master.

Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1890
This mail reflects revision 1 of this Change.

Acked-by according to Gerrit (reflected above):
Razvan Cojocaru <[email protected]>

        
diff --git a/src/tapctl/tap.c b/src/tapctl/tap.c
index 1d94988..769df6c 100644
--- a/src/tapctl/tap.c
+++ b/src/tapctl/tap.c
@@ -921,32 +921,38 @@
                                     bEnable ? enable_device : disable_device, pbRebootRequired);
 }
 
-/* stripped version of ExecCommand in interactive.c */
 static DWORD
-ExecCommand(const WCHAR *cmdline)
+ExecNetsh(PWSTR cmdline)
 {
     DWORD exit_code;
     STARTUPINFOW si;
     PROCESS_INFORMATION pi;
     DWORD proc_flags = CREATE_NO_WINDOW | CREATE_UNICODE_ENVIRONMENT;
-    WCHAR *cmdline_dup = NULL;
 
     ZeroMemory(&si, sizeof(si));
     ZeroMemory(&pi, sizeof(pi));
-
     si.cb = sizeof(si);
 
-    /* CreateProcess needs a modifiable cmdline: make a copy */
-    cmdline_dup = _wcsdup(cmdline);
-    if (cmdline_dup
-        && CreateProcessW(NULL, cmdline_dup, NULL, NULL, FALSE, proc_flags, NULL, NULL, &si, &pi))
+    WCHAR appName[MAX_PATH];
+    WCHAR netsh_exe[] = L"\\netsh.exe";
+    UINT sysdir_len = GetSystemDirectoryW(appName, _countof(appName));
+    if (sysdir_len == 0)
+    {
+        wcscpy_s(appName, _countof(appName), L"C:\\Windows\\system32");
+    }
+    else if (sysdir_len + _countof(netsh_exe) > _countof(appName))
+    {
+        return ERROR_INSUFFICIENT_BUFFER;
+    }
+    wcscat_s(appName, _countof(appName), netsh_exe);
+
+    if (CreateProcessW(appName, cmdline, NULL, NULL, FALSE, proc_flags, NULL, NULL, &si, &pi))
     {
         WaitForSingleObject(pi.hProcess, INFINITE);
         if (!GetExitCodeProcess(pi.hProcess, &exit_code))
         {
             exit_code = GetLastError();
         }
-
         CloseHandle(pi.hProcess);
         CloseHandle(pi.hThread);
     }
@@ -955,7 +961,6 @@
         exit_code = GetLastError();
     }
 
-    free(cmdline_dup);
     return exit_code;
 }
 
@@ -1013,7 +1018,7 @@
 
     free(szOldName);
 
-    dwResult = ExecCommand(szCmdLine);
+    dwResult = ExecNetsh(szCmdLine);
     free(szCmdLine);
 
     if (dwResult != ERROR_SUCCESS)