Do not require that unknown EXT_INFO extension values not contain

\0 characters. This would cause fatal connection errors if an
implementation sent e.g. string-encoded sub-values inside a value.

Reported by Denis Bider; ok markus@

diff --git a/usr.bin/ssh/kex.c b/usr.bin/ssh/kex.c
index 13d0d23..5082ebd 100644 (file)
--- a/usr.bin/ssh/kex.c
+++ b/usr.bin/ssh/kex.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.c,v 1.133 2017/05/30 14:23:52 markus Exp $ */
+/* $OpenBSD: kex.c,v 1.134 2017/06/13 12:13:59 djm Exp $ */
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
  *
@@ -366,7 +366,9 @@ kex_input_ext_info(int type, u_int32_t seq, struct ssh *ssh)
 {
        struct kex *kex = ssh->kex;
        u_int32_t i, ninfo;
-       char *name, *val, *found;
+       char *name, *found;
+       u_char *val;
+       size_t vlen;
        int r;
 
        debug("SSH2_MSG_EXT_INFO received");
@@ -376,12 +378,17 @@ kex_input_ext_info(int type, u_int32_t seq, struct ssh *ssh)
        for (i = 0; i < ninfo; i++) {
                if ((r = sshpkt_get_cstring(ssh, &name, NULL)) != 0)
                        return r;
-               if ((r = sshpkt_get_cstring(ssh, &val, NULL)) != 0) {
+               if ((r = sshpkt_get_string(ssh, &val, &vlen)) != 0) {
                        free(name);
                        return r;
                }
-               debug("%s: %s=<%s>", __func__, name, val);
                if (strcmp(name, "server-sig-algs") == 0) {
+                       /* Ensure no \0 lurking in value */
+                       if (memchr(val, '\0', vlen) != NULL) {
+                               error("%s: nul byte in %s", __func__, name);
+                               return SSH_ERR_INVALID_FORMAT;
+                       }
+                       debug("%s: %s=<%s>", __func__, name, val);
                        found = match_list("rsa-sha2-256", val, NULL);
                        if (found) {
                                kex->rsa_sha2 = 256;
@@ -392,7 +399,8 @@ kex_input_ext_info(int type, u_int32_t seq, struct ssh *ssh)
                                kex->rsa_sha2 = 512;
                                free(found);
                        }
-               }
+               } else
+                       debug("%s: %s (unrecognised)", __func__, name);
                free(name);
                free(val);
        }