From f99e60c33948432648babd29911075295398862e Mon Sep 17 00:00:00 2001
From: saadgodikat <saadgodikat601@gmail.com>
Date: Fri, 25 Sep 2026 21:30:16 +0530
Subject: [PATCH] Fix DER certificate output on Windows

Fixes #10152

Signed-off-by: saadgodikat <saadgodikat601@gmail.com>
---
 programs/x509/cert_write.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index 5d9a306ee7..947d73f54a 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -238,7 +238,11 @@ static int write_certificate(mbedtls_x509write_cert *crt, const char *output_fil
         output_start = output_buf;
     }
 
-    if ((f = fopen(output_file, "w")) == NULL) {
+    /*
+     * DER output is binary data. Use binary mode on Windows to prevent
+     * text-mode newline translation.
+     */
+    if ((f = fopen(output_file, opt.format == FORMAT_DER ? "wb" : "w")) == NULL) {
         return -1;
     }