Prior to this we substring matched and allowed a leading .

on a SAN DNSname constraint. This is not correct, as with
a DNSname constraint, it may exacly match or match zero or
more additional components on the front of the candidte to
match.

Spotted by Haruto Kimura <hkimura2026@gmail.com>

ok tb@ kenjiro@

2 files changed, 26 insertions, 5 deletions

diff --git a/src/lib/libcrypto/x509/x509_constraints.c b/src/lib/libcrypto/x509/x509_constraints.c
index 0773d2ba71..c4f32c9cfc 100644
--- a/src/lib/libcrypto/x509/x509_constraints.c
+++ b/src/lib/libcrypto/x509/x509_constraints.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: x509_constraints.c,v 1.32 2023/09/29 15:53:59 beck Exp $ */
+/* $OpenBSD: x509_constraints.c,v 1.33 2026/04/13 17:04:23 beck Exp $ */
 /*
  * Copyright (c) 2020 Bob Beck <beck@openbsd.org>
  *
@@ -578,11 +578,30 @@ x509_constraints_sandns(char *sandns, size_t dlen, char *constraint, size_t len)
         if (len == 0)
                 return 1; /* an empty constraint matches everything */
 
-        /* match the end of the domain */
         if (dlen < len)
                 return 0;
-        suffix = sandns + (dlen - len);
-        return (strncasecmp(suffix, constraint, len) == 0);
+
+        if (dlen == len)
+                return (strncasecmp(sandns, constraint, len) == 0);
+
+        /* Support a constraint with a leading "." */
+        if (constraint[0] == '.') {
+                constraint++;
+                len--;
+        }
+
+        /*
+         * Otherwise we must have at least one extra component
+         * to match, so there must be more than just a leading .
+         */
+        if (dlen - len > 1) {
+                suffix = sandns + (dlen - len);
+                if (suffix[-1] != '.')
+                        return 0;
+                return (strncasecmp(suffix, constraint, len) == 0);
+        }
+
+        return 0;
 }
 
 /*
diff --git a/src/lib/libcrypto/x509/x509_internal.h b/src/lib/libcrypto/x509/x509_internal.h
index 9b9980ece5..e933cd9f2d 100644
--- a/src/lib/libcrypto/x509/x509_internal.h
+++ b/src/lib/libcrypto/x509/x509_internal.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: x509_internal.h,v 1.28 2024/05/19 07:12:50 jsg Exp $ */
+/* $OpenBSD: x509_internal.h,v 1.29 2026/04/13 17:04:23 beck Exp $ */
 /*
  * Copyright (c) 2020 Bob Beck <beck@openbsd.org>
  *
@@ -116,6 +116,8 @@ int x509_constraints_valid_host(CBS *cbs, int permit_ip);
 int x509_constraints_valid_sandns(CBS *cbs);
 int x509_constraints_domain(char *domain, size_t dlen, char *constraint,
     size_t len);
+int x509_constraints_sandns(char *domain, size_t dlen, char *constraint,
+    size_t len);
 int x509_constraints_parse_mailbox(CBS *candidate,
     struct x509_constraints_name *name);
 int x509_constraints_valid_domain_constraint(CBS *cbs);