From 10372307c11d0128c40d730418d3776f1a959ce3 Mon Sep 17 00:00:00 2001
From: Jeff King <peff@peff.net>
Date: Wed, 11 Mar 2020 17:53:41 -0400
Subject: [PATCH] credential: avoid writing values with newlines

The credential protocol that we use to speak to helpers can't represent
values with newlines in them. This was an intentional design choice to
keep the protocol simple, since none of the values we pass should
generally have newlines.

However, if we _do_ encounter a newline in a value, we blindly transmit
it in credential_write(). Such values may break the protocol syntax, or
worse, inject new valid lines into the protocol stream.

The most likely way for a newline to end up in a credential struct is by
decoding a URL with a percent-encoded newline. However, since the bug
occurs at the moment we write the value to the protocol, we'll catch it
there. That should leave no possibility of accidentally missing a code
path that can trigger the problem.

At this level of the code we have little choice but to die(). However,
since we'd not ever expect to see this case outside of a malicious URL,
that's an acceptable outcome.

Reported-by: Felix Wilhelm <fwilhelm@google.com>
---
 credential.c           | 2 ++
 t/t0300-credentials.sh | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/credential.c b/credential.c
index 62be651..a79aff0 100644
--- a/credential.c
+++ b/credential.c
@@ -195,6 +195,8 @@ static void credential_write_item(FILE *fp, const char *key, const char *value)
 {
 	if (!value)
 		return;
+	if (strchr(value, '\n'))
+		die("credential value for %s contains newline", key);
 	fprintf(fp, "%s=%s\n", key, value);
 }
 
diff --git a/t/t0300-credentials.sh b/t/t0300-credentials.sh
index 82eaaea..26f3c3a 100755
--- a/t/t0300-credentials.sh
+++ b/t/t0300-credentials.sh
@@ -308,4 +308,10 @@ test_expect_success 'empty helper spec resets helper list' '
 	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
-- 
1.8.3.1

