[PATCH v1] openvpnserv: detect sibling dirs in CheckConfigPath

From: Heiko Hund <[email protected]>

If the config_dir value in the registry has no trailing backslash the
check doesn't actually guarantee that a file is located within
config_dir, because a sibling dir with the same prefix, e.g. 'config'
and 'config-evil' will match and produce a positive verdict.

By also checking that there is a path separator after config_dir
prevents this attack.

Reported-By: Harshit Varu <[email protected]>
Tested-By: Harshit Varu <[email protected]>
CVE: 2026-81830
Github: OpenVPN/openvpn-private-issues#166
Change-Id: Ica5d43989b441d4377a3908f811a2953b7a9d45a
Signed-off-by: Heiko Hund <[email protected]>
Acked-by: Razvan Cojocaru <[email protected]>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1883
---

This change was reviewed on Gerrit and approved by at least one
developer. I request to merge it to release/2.6.

Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1883
This mail reflects revision 1 of this Change.

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

        
diff --git a/src/openvpnserv/validate.c b/src/openvpnserv/validate.c
index 770a7a0..b0fb6b85 100644
--- a/src/openvpnserv/validate.c
+++ b/src/openvpnserv/validate.c
@@ -56,7 +56,9 @@
 
 /*
  * Check workdir\fname is inside config_dir
- * The logic here is simple: we may reject some valid paths if ..\ is in any of the strings
+ * The logic here is simple:
+ *      we may reject some valid paths if ".." is in the filename
+ *      or if there's no "\" after the config directory
  */
 static BOOL
 CheckConfigPath(const WCHAR *workdir, const WCHAR *fname, const settings_t *s)
@@ -82,9 +84,18 @@
     }
 
     config_dir = s->config_dir;
+    size_t config_dir_len = wcslen(config_dir);
 
-    if (wcsncmp(config_dir, config_file, wcslen(config_dir)) == 0
-        && wcsstr(config_file + wcslen(config_dir), L"..") == NULL)
+    /* check for a path separator after config_dir */
+    if (config_dir_len && config_dir_len < wcslen(config_file)
+        && config_dir[config_dir_len - 1] != L'\\'
+        && config_file[config_dir_len] != L'\\')
+    {
+        return FALSE;
+    }
+
+    if (wcsncmp(config_dir, config_file, config_dir_len) == 0
+        && wcsstr(config_file + config_dir_len, L"..") == NULL)
     {
         return TRUE;
     }