1 files changed, 47 insertions, 12 deletions

diff --git a/src/lib/libtls/tls_verify.c b/src/lib/libtls/tls_verify.c
index 0cb86f686d..420e278c99 100644
--- a/src/lib/libtls/tls_verify.c
+++ b/src/lib/libtls/tls_verify.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: tls_verify.c,v 1.23 2023/05/11 07:35:27 tb Exp $ */
+/* $OpenBSD: tls_verify.c,v 1.24 2023/05/28 09:02:01 beck Exp $ */
 /*
  * Copyright (c) 2014 Jeremie Courreges-Anglas <jca@openbsd.org>
  *
@@ -205,10 +205,13 @@ static int
 tls_check_common_name(struct tls *ctx, X509 *cert, const char *name,
     int *cn_match)
 {
+        unsigned char *utf8_bytes = NULL;
         X509_NAME *subject_name;
         char *common_name = NULL;
         union tls_addr addrbuf;
         int common_name_len;
+        ASN1_STRING *data;
+        int lastpos = -1;
         int rv = -1;
 
         *cn_match = 0;
@@ -217,29 +220,60 @@ tls_check_common_name(struct tls *ctx, X509 *cert, const char *name,
         if (subject_name == NULL)
                 goto done;
 
-        common_name_len = X509_NAME_get_text_by_NID(subject_name,
-            NID_commonName, NULL, 0);
-        if (common_name_len < 0)
+        lastpos = X509_NAME_get_index_by_NID(subject_name,
+            NID_commonName, lastpos);
+        if (lastpos == -1)
                 goto done;
+        if (X509_NAME_get_index_by_NID(subject_name, NID_commonName, lastpos)
+            != -1) {
+                /*
+                 * Having multiple CN's is possible, and even happened back in
+                 * the glory days of mullets and Hammer pants. In anything like
+                 * a modern TLS cert, CN is as close to deprecated as it gets,
+                 * and having more than one is bad. We therefore fail if we have
+                 * more than one CN fed to us in the subject, treating the
+                 * certificate as hostile.
+                 */
+                tls_set_errorx(ctx, "error verifying name '%s': "
+                    "Certificate subject contains mutiple Common Name fields, "
+                    "probably a malicious or malformed certificate", name);
+                goto err;
+        }
 
-        common_name = calloc(common_name_len + 1, 1);
-        if (common_name == NULL) {
-                tls_set_error(ctx, "out of memory");
+        data = X509_NAME_ENTRY_get_data(X509_NAME_get_entry(subject_name,
+            lastpos));
+        /*
+         * Fail if we cannot encode as UTF-8, or if the UTF-8 encoding of the
+         * string contains a 0 byte. We treat any certificate with such data
+         * in the CN as hostile and fail.
+         */
+        if ((common_name_len = ASN1_STRING_to_UTF8(&utf8_bytes, data)) < 0) {
+                tls_set_errorx(ctx, "error verifying name '%s': "
+                    "Common Name field cannot be encoded as a UTF-8 string, "
+                    "probably a malicious certificate", name);
                 goto err;
         }
 
-        X509_NAME_get_text_by_NID(subject_name, NID_commonName, common_name,
-            common_name_len + 1);
+        if (common_name_len < 1 || common_name_len > 64) {
+                tls_set_errorx(ctx, "error verifying name '%s': "
+                    "Common Name field has invalid length, "
+                    "probably a malicious certificate", name);
+                goto err;
+        }
 
-        /* NUL bytes in CN? */
-        if (common_name_len < 0 ||
-            (size_t)common_name_len != strlen(common_name)) {
+        if (memchr(utf8_bytes, 0, common_name_len) != NULL) {
                 tls_set_errorx(ctx, "error verifying name '%s': "
                     "NUL byte in Common Name field, "
                     "probably a malicious certificate", name);
                 goto err;
         }
 
+        common_name = strndup(utf8_bytes, common_name_len);
+        if (common_name == NULL) {
+                tls_set_error(ctx, "out of memory");
+                goto err;
+        }
+
         /*
          * We don't want to attempt wildcard matching against IP addresses,
          * so perform a simple comparison here.
@@ -258,6 +292,7 @@ tls_check_common_name(struct tls *ctx, X509 *cert, const char *name,
         rv = 0;
 
  err:
+        free(utf8_bytes);
         free(common_name);
         return rv;
 }