diff --git a/challenge/http01/domain_matcher.go b/challenge/http01/domain_matcher.go
index 058d1a314..d6eb28380 100644
--- a/challenge/http01/domain_matcher.go
+++ b/challenge/http01/domain_matcher.go
@@ -200,5 +200,6 @@ func matchDomain(src, domain string) bool {
 		domain = "[" + domain + "]"
 	}
 
-	return strings.HasPrefix(src, domain)
+	// Case-insensitive prefix (domain) match.
+	return len(src) >= len(domain) && strings.EqualFold(src[:len(domain)], domain)
 }