Core: stricter UTF-8 handling in ngx_utf8_decode(). An UTF-8 octet sequence cannot start with a 11111xxx byte (above 0xf8), see https://datatracker.ietf.org/doc/html/rfc3629#section-3. Previously, such bytes were accepted by ngx_utf8_decode() and misinterpreted as 11110xxx bytes (as in a 4-byte sequence). While unlikely, this can potentially cause issues. Fix is to explicitly reject such bytes in ngx_utf8_decode().

--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -1364,7 +1364,12 @@ ngx_utf8_decode(u_char **p, size_t n)
 
     u = **p;
 
-    if (u >= 0xf0) {
+    if (u >= 0xf8) {
+
+        (*p)++;
+        return 0xffffffff;
+
+    } else if (u >= 0xf0) {
 
         u &= 0x07;
         valid = 0xffff;