ntlm: Clarify details on NTLM phase 3 decoding

The code was not very clear if we accept the base64 decode if the
NTLM challenge was truncated or not.  Move the related code lines
closer to where buf is first used and comment that we are not concerned
about any truncation.

If the decoded result is truncated, the NTLM server side will reject
our new response to the challenge as it will be incorrect.  The
buffer size is fixed and known to be in a cleared state before the
decode starts.

Resolves: TOB-OVPN-14
Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230802113149.36497-1-dazo+openvpn@eurephia.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26919.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit f19391139836aa07312cf5b3ebbd00941d22ddc7)

diff --git a/src/openvpn/ntlm.c b/src/openvpn/ntlm.c
index 20527d442a7f8d53cee8572a93ba33ab35c58fff..d666b3c580f90bf6421df73f53a09b73ece84149 100644 (file)
--- a/src/openvpn/ntlm.c
+++ b/src/openvpn/ntlm.c
@@ -209,7 +209,6 @@ ntlm_phase_3(const struct http_proxy_info *p, const char *phase_2,
      */
 
     char pwbuf[sizeof(p->up.password) * 2]; /* for unicode password */
-    uint8_t buf2[128]; /* decoded reply from proxy */
     uint8_t phase3[464];
 
     uint8_t md4_hash[MD4_DIGEST_LENGTH + 5];
@@ -232,8 +231,6 @@ ntlm_phase_3(const struct http_proxy_info *p, const char *phase_2,
 
     bool ntlmv2_enabled = (p->auth_method == HTTP_AUTH_NTLM2);
 
-    CLEAR(buf2);
-
     ASSERT(strlen(p->up.username) > 0);
     ASSERT(strlen(p->up.password) > 0);
 
@@ -266,6 +263,12 @@ ntlm_phase_3(const struct http_proxy_info *p, const char *phase_2,
     /* pad to 21 bytes */
     memset(md4_hash + MD4_DIGEST_LENGTH, 0, 5);
 
+    /* If the decoded challenge is shorter than required by the protocol,
+     * the missing bytes will be NULL, as buf2 is known to be zeroed
+     * when this decode happens.
+     */
+    uint8_t buf2[128]; /* decoded reply from proxy */
+    CLEAR(buf2);
     ret_val = openvpn_base64_decode(phase_2, buf2, -1);
     if (ret_val < 0)
     {