diff -ur b/credential.c a/credential.c

--- b/credential.c	2020-04-14 14:15:41.637223958 +0200

+++ a/credential.c	2020-04-14 14:59:03.325862182 +0200

@@ -190,6 +190,8 @@

 {

 	if (!value)

 		return;

+	if (strchr(value, '\n'))

+		die("credential value for %s contains newline", key);

 	fprintf(fp, "%s=%s\n", key, value);

 }

diff -ur b/t/t0300-credentials.sh a/t/t0300-credentials.sh

--- b/t/t0300-credentials.sh	2020-04-14 14:15:41.569223126 +0200

+++ a/t/t0300-credentials.sh	2020-04-14 15:00:01.331571252 +0200

@@ -289,4 +289,10 @@

 	EOF

 '

+test_expect_success 'url parser rejects embedded newlines' '

+	test_must_fail git credential fill <<-\EOF

+	url=https://one.example.com?%0ahost=two.example.com/

+	EOF

+'

+

 test_done