1 files changed, 11 insertions, 6 deletions

diff --git a/src/lib/libtls/tls_verify.c b/src/lib/libtls/tls_verify.c
index acc034d9c1..a0c39b9dd4 100644
--- a/src/lib/libtls/tls_verify.c
+++ b/src/lib/libtls/tls_verify.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: tls_verify.c,v 1.25 2023/05/28 09:06:34 beck Exp $ */
+/* $OpenBSD: tls_verify.c,v 1.26 2023/05/29 14:12:36 beck Exp $ */
 /*
  * Copyright (c) 2014 Jeremie Courreges-Anglas <jca@openbsd.org>
  *
@@ -224,6 +224,8 @@ tls_check_common_name(struct tls *ctx, X509 *cert, const char *name,
             NID_commonName, lastpos);
         if (lastpos == -1)
                 goto done;
+        if (lastpos < 0)
+                goto err;
         if (X509_NAME_get_index_by_NID(subject_name, NID_commonName, lastpos)
             != -1) {
                 /*
@@ -243,9 +245,7 @@ tls_check_common_name(struct tls *ctx, X509 *cert, const char *name,
         data = X509_NAME_ENTRY_get_data(X509_NAME_get_entry(subject_name,
             lastpos));
         /*
-         * Fail if we cannot encode as UTF-8, if the CN is of invalid length, 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.
+         * Fail if we cannot encode the CN bytes as UTF-8.
          */
         if ((common_name_len = ASN1_STRING_to_UTF8(&utf8_bytes, data)) < 0) {
                 tls_set_errorx(ctx, "error verifying name '%s': "
@@ -253,14 +253,19 @@ tls_check_common_name(struct tls *ctx, X509 *cert, const char *name,
                     "probably a malicious certificate", name);
                 goto err;
         }
-
+        /*
+         * Fail if the CN is of invalid length. RFC 5280 specifies that a CN
+         * must be between 1 and 64 bytes long.
+         */
         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;
         }
-
+        /*
+         * Fail if the resulting text contains a NUL byte.
+         */
         if (memchr(utf8_bytes, 0, common_name_len) != NULL) {
                 tls_set_errorx(ctx, "error verifying name '%s': "
                     "NUL byte in Common Name field, "