CVE-2018-9336: Double Free (CWE-415)
value: openvpnserv.exe (aka the interactive service helper) in OpenVPN 2.4.x before 2.4.6 allows a local attacker to cause a double-free of memory by sending a malformed request to the interactive service. This could cause a denial-of-service through memory corruption or possibly have unspecified other impact including privilege escalation.
CWE description: The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations.
CVSS v3 base score: 7.8
Published: 2018-05-01T18:29Z
Repository: https://github.com/OpenVPN/openvpn
Language: C
Files changed: src/openvpnserv/interactive.c
Commit date: 2018-04-19 17:08:54 +0200
Commit message: Fix potential double-free() in Interactive Service (CVE-2018-9336)

Malformed input data on the service pipe towards the OpenVPN interactive
service (normally used by the OpenVPN GUI to request openvpn instances
from the service) can result in a double free() in the error handling code.

This usually only leads to a process crash (DoS by an unprivileged local
account) but since it could possibly lead to memory corruption if
happening while multiple other threads are active at the same time,
CVE-2018-9336 has been assigned to acknowledge this risk.

Fix by ensuring that sud->directory is set to NULL in GetStartUpData()
for all error cases (thus not being free()ed in FreeStartupData()).

Rewrite control flow to use explicit error label for error exit.

Discovered and reported by Jacob Baines <jbaines@tenable.com>.

CVE: 2018-9336

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20180414072617.25075-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/search?l=mid&q=20180414072617.25075-1-gert@greenie.muc.de

Signed-off-by: Gert Doering <gert@greenie.muc.de>

Diff: diff --git a/src/openvpnserv/interactive.c b/src/openvpnserv/interactive.c
index fbc32f90..861f5e70 100644
--- a/src/openvpnserv/interactive.c
+++ b/src/openvpnserv/interactive.c
@@ -451,80 +451,79 @@ out:
 
 static BOOL
 GetStartupData(HANDLE pipe, STARTUP_DATA *sud)
 {
     size_t size, len;
-    BOOL ret = FALSE;
     WCHAR *data = NULL;
     DWORD bytes, read;
 
     bytes = PeekNamedPipeAsync(pipe, 1, &exit_event);
     if (bytes == 0)
     {
         MsgToEventLog(M_SYSERR, TEXT("PeekNamedPipeAsync failed"));
         ReturnLastError(pipe, L"PeekNamedPipeAsync");
-        goto out;
+        goto err;
     }
 
     size = bytes / sizeof(*data);
     if (size == 0)
     {
         MsgToEventLog(M_SYSERR, TEXT("malformed startup data: 1 byte received"));
         ReturnError(pipe, ERROR_STARTUP_DATA, L"GetStartupData", 1, &exit_event);
-        goto out;
+        goto err;
     }
 
     data = malloc(bytes);
     if (data == NULL)
     {
         MsgToEventLog(M_SYSERR, TEXT("malloc failed"));
         ReturnLastError(pipe, L"malloc");
-        goto out;
+        goto err;
     }
 
     read = ReadPipeAsync(pipe, data, bytes, 1, &exit_event);
     if (bytes != read)
     {
         MsgToEventLog(M_SYSERR, TEXT("ReadPipeAsync failed"));
         ReturnLastError(pipe, L"ReadPipeAsync");
-        goto out;
+        goto err;
     }
 
     if (data[size - 1] != 0)
     {
         MsgToEventLog(M_ERR, TEXT("Startup data is not NULL terminated"));
         ReturnError(pipe, ERROR_STARTUP_DATA, L"GetStartupData", 1, &exit_event);
-        goto out;
+        goto err;
     }
 
     sud->directory = data;
     len = wcslen(sud->directory) + 1;
     size -= len;
     if (size <= 0)
     {
         MsgToEventLog(M_ERR, TEXT("Startup data ends at working directory"));
         ReturnError(pipe, ERROR_STARTUP_DATA, L"GetStartupData", 1, &exit_event);
-        goto out;
+        goto err;
     }
 
     sud->options = sud->directory + len;
     len = wcslen(sud->options) + 1;
     size -= len;
     if (size <= 0)
     {
         MsgToEventLog(M_ERR, TEXT("Startup data ends at command line options"));
         ReturnError(pipe, ERROR_STARTUP_DATA, L"GetStartupData", 1, &exit_event);
-        goto out;
+        goto err;
     }
 
     sud->std_input = sud->options + len;
-    data = NULL; /* don't free data */
-    ret = TRUE;
+    return TRUE;
 
-out:
+err:
+    sud->directory = NULL;		/* caller must not free() */
     free(data);
-    return ret;
+    return FALSE;
 }
 
 
 static VOID
 FreeStartupData(STARTUP_DATA *sud)

Vulnerable code:     BOOL ret = FALSE;
        goto out;
        goto out;
        goto out;
        goto out;
        goto out;
        goto out;
        goto out;
    data = NULL; /* don't free data */
    ret = TRUE;
out:
    return ret;
Fixed code:         goto err;
        goto err;
        goto err;
        goto err;
        goto err;
        goto err;
        goto err;
    return TRUE;
err:
    sud->directory = NULL;		/* caller must not free() */
    return FALSE;