# UNDF: UNDF-2026-000000204
From 91fd961 Mon Sep 17 00:00:00 2001
Subject: [CWE-407] ssl_ncp: fix O(n²) cipher negotiation in ncp_get_best_cipher

ncp_get_best_cipher() iterated the server cipher list (outer strsep
loop) and for each token called tls_item_in_cipher_list(), which
allocates a copy of the peer list, walks it with strtok, and frees
it — O(m) work plus a malloc+free per outer iteration.

Total cost: O(n*m) comparisons + O(n) heap allocations per TLS
handshake, where n = len(server_list), m = len(peer_ncp_list).

Same root cause affects p2p_ncp_get_common_cipher() (ssl_ncp.c:388)
and dco_check_option_conflict() (dco.c:468).

Fix for ncp_get_best_cipher: split peer_ncp_list once into a small
stack-allocated array before the outer loop.  Inner membership test
becomes a straight array scan — still O(m) but with no heap
allocation and cache-hot data.  With typical lists of 3–8 ciphers
this is effectively O(1).

The same pattern should be applied to the other two call sites.

--- a/src/openvpn/ssl_ncp.c
+++ b/src/openvpn/ssl_ncp.c
@@ -246,6 +246,10 @@ ncp_get_best_cipher(const char *server_list, const char *peer_info,
                     const char *remote_cipher, struct gc_arena *gc)
 {
+#define NCP_MAX_CIPHERS 32
+    const char *peer_arr[NCP_MAX_CIPHERS];
+    int peer_count = 0;
+
     struct gc_arena gc_tmp = gc_new();

     const char *peer_ncp_list = tls_peer_ncp_list(peer_info, &gc_tmp);
@@ -255,13 +259,31 @@ ncp_get_best_cipher(const char *server_list, const char *peer_info,
         remote_cipher = "";
     }

+    /* Split peer_ncp_list once into a fixed array — O(m) one-time cost,
+     * avoids O(n) repeated malloc+strtok inside the outer loop. */
+    {
+        char *tmp = string_alloc(peer_ncp_list, &gc_tmp);
+        char *tok = strtok(tmp, ":");
+        while (tok && peer_count < NCP_MAX_CIPHERS) {
+            peer_arr[peer_count++] = tok;
+            tok = strtok(NULL, ":");
+        }
+    }
+
     char *tmp_ciphers = string_alloc(server_list, &gc_tmp);

     const char *token;
     while ((token = strsep(&tmp_ciphers, ":")))
     {
-        if (tls_item_in_cipher_list(token, peer_ncp_list) || streq(token, remote_cipher))
-        {
+        int found = streq(token, remote_cipher);
+        if (!found) {
+            for (int pi = 0; pi < peer_count && !found; pi++)
+                found = (strcmp(token, peer_arr[pi]) == 0);
+        }
+        if (found)
             break;
-        }
     }

     char *ret = NULL;