[S] Change in openvpn[master]: tapctl: prevent binary planting with netsh

Attention is currently required from: d12fk, plaisthos.

Hello plaisthos,

I'd like you to do a code review.
Please visit

    http://gerrit.openvpn.net/c/openvpn/+/1890?usp=email

to review the following change.


Change subject: tapctl: prevent binary planting with netsh
......................................................................

tapctl: prevent binary planting with netsh

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]>
---
M src/tapctl/tap.c
1 file changed, 16 insertions(+), 11 deletions(-)



  git pull ssh://gerrit.openvpn.net:29418/openvpn refs/changes/90/1890/1

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)

-- 
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Gerrit-MessageType: newchange
Gerrit-Project: openvpn
Gerrit-Branch: master
Gerrit-Change-Id: I70282985a7e8e46add92b2277674cbca6bce39c1
Gerrit-Change-Number: 1890
Gerrit-PatchSet: 1
Gerrit-Owner: cron2 <[email protected]>
Gerrit-Reviewer: d12fk <[email protected]>
Gerrit-Reviewer: plaisthos <[email protected]>
Gerrit-CC: openvpn-devel <[email protected]>
Gerrit-Attention: plaisthos <[email protected]>
Gerrit-Attention: d12fk <[email protected]>

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