fetchhead: strip credentials from remote URL

If fetching from an anonymous remote via its URL, then the URL gets
written into the FETCH_HEAD reference. This is mainly done to give
valuable context to some commands, like for example git-merge(1), which
will put the URL into the generated MERGE_MSG. As a result, what gets
written into FETCH_HEAD may become public in some cases. This is
especially important considering that URLs may contain credentials, e.g.
when cloning 'https://foo:bar@example.com/repo' we persist the complete
URL into FETCH_HEAD and put it without any kind of sanitization into the
MERGE_MSG. This is obviously bad, as your login data has now just leaked
as soon as you do git-push(1).

When writing the URL into FETCH_HEAD, upstream git does strip
credentials first. Let's do the same by trying to parse the remote URL
as a "real" URL, removing any credentials and then re-formatting the
URL. In case this fails, e.g. when it's a file path or not a valid URL,
we just fall back to using the URL as-is without any sanitization. Add
tests to verify our behaviour.

src/fetchhead.c

@@ -14,6 +14,7 @@
 #include "futils.h"
 #include "filebuf.h"
 #include "refs.h"
+#include "net.h"
 #include "repository.h"
 
 int git_fetchhead_ref_cmp(const void *a, const void *b)
@@ -36,6 +37,33 @@ int git_fetchhead_ref_cmp(const void *a, const void *b)
 	return 0;
 }
 
+static char *sanitized_remote_url(const char *remote_url)
+{
+	git_net_url url = GIT_NET_URL_INIT;
+	char *sanitized = NULL;
+	int error;
+
+	if (git_net_url_parse(&url, remote_url) == 0) {
+		git_buf buf = GIT_BUF_INIT;
+
+		git__free(url.username);
+		git__free(url.password);
+		url.username = url.password = NULL;
+
+		if ((error = git_net_url_fmt(&buf, &url)) < 0)
+			goto fallback;
+
+		sanitized = git_buf_detach(&buf);
+	}
+
+fallback:
+	if (!sanitized)
+		sanitized = git__strdup(remote_url);
+
+	git_net_url_dispose(&url);
+	return sanitized;
+}
+
 int git_fetchhead_ref_create(
 	git_fetchhead_ref **out,
 	git_oid *oid,
@@ -57,11 +85,15 @@ int git_fetchhead_ref_create(
 	git_oid_cpy(&fetchhead_ref->oid, oid);
 	fetchhead_ref->is_merge = is_merge;
 
-	if (ref_name)
+	if (ref_name) {
 		fetchhead_ref->ref_name = git__strdup(ref_name);
+		GIT_ERROR_CHECK_ALLOC(fetchhead_ref->ref_name);
+	}
 
-	if (remote_url)
-		fetchhead_ref->remote_url = git__strdup(remote_url);
+	if (remote_url) {
+		fetchhead_ref->remote_url = sanitized_remote_url(remote_url);
+		GIT_ERROR_CHECK_ALLOC(fetchhead_ref->remote_url);
+	}
 
 	*out = fetchhead_ref;
 

tests/fetchhead/nonetwork.c

@@ -108,7 +108,7 @@ void test_fetchhead_nonetwork__write(void)
 typedef struct {
 	git_vector *fetchhead_vector;
 	size_t idx;
-} fetchhead_ref_cb_data; 
+} fetchhead_ref_cb_data;
 
 static int fetchhead_ref_cb(const char *name, const char *url,
 	const git_oid *oid, unsigned int is_merge, void *payload)
@@ -493,3 +493,21 @@ void test_fetchhead_nonetwork__create_with_multiple_refspecs(void)
 	git_remote_free(remote);
 	git_buf_dispose(&path);
 }
+
+void test_fetchhead_nonetwork__credentials_are_stripped(void)
+{
+	git_fetchhead_ref *ref;
+	git_oid oid;
+
+	cl_git_pass(git_oid_fromstr(&oid, "49322bb17d3acc9146f98c97d078513228bbf3c0"));
+	cl_git_pass(git_fetchhead_ref_create(&ref, &oid, 0,
+		"refs/tags/commit_tree", "http://foo:bar@github.com/libgit2/TestGitRepository"));
+	cl_assert_equal_s(ref->remote_url, "http://github.com/libgit2/TestGitRepository");
+	git_fetchhead_ref_free(ref);
+
+	cl_git_pass(git_oid_fromstr(&oid, "49322bb17d3acc9146f98c97d078513228bbf3c0"));
+	cl_git_pass(git_fetchhead_ref_create(&ref, &oid, 0,
+		"refs/tags/commit_tree", "https://foo:bar@github.com/libgit2/TestGitRepository"));
+	cl_assert_equal_s(ref->remote_url, "https://github.com/libgit2/TestGitRepository");
+	git_fetchhead_ref_free(ref);
+}

tests/online/fetchhead.c

@@ -154,3 +154,20 @@ void test_online_fetchhead__colon_only_dst_refspec_creates_no_branch(void)
 
 	cl_assert_equal_i(refs, count_references());
 }
+
+void test_online_fetchhead__creds_get_stripped(void)
+{
+	git_buf buf = GIT_BUF_INIT;
+	git_remote *remote;
+
+	cl_git_pass(git_repository_init(&g_repo, "./foo", 0));
+	cl_git_pass(git_remote_create_anonymous(&remote, g_repo, "https://foo:bar@github.com/libgit2/TestGitRepository"));
+	cl_git_pass(git_remote_fetch(remote, NULL, NULL, NULL));
+
+	cl_git_pass(git_futils_readbuffer(&buf, "./foo/.git/FETCH_HEAD"));
+	cl_assert_equal_s(buf.ptr,
+		"49322bb17d3acc9146f98c97d078513228bbf3c0\t\thttps://github.com/libgit2/TestGitRepository\n");
+
+	git_remote_free(remote);
+	git_buf_dispose(&buf);
+}