From ada02a13b556ae2e97d09581b61c2d17c803a7ed Mon Sep 17 00:00:00 2001
From: Yugo Horie <u5.horie@gmail.com>
Date: Thu, 23 Feb 2023 08:09:50 +0900
Subject: [PATCH] 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().
---
 src/core/ngx_string.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index 98f270aca..f8f738472 100644
--- 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;