credential: sanitize the user prompt

When asking the user interactively for credentials, we want to avoid
misleading them e.g. via control sequences that pretend that the URL
targets a trusted host when it does not.

While Git learned, over the course of the preceding commits, to disallow
URLs containing URL-encoded control characters by default, credential
helpers are still allowed to specify values very freely (apart from Line
Feed and NUL characters, anything is allowed), and this would allow,
say, a username containing control characters to be specified that would
then be displayed in the interactive terminal prompt asking the user for
the password, potentially sending those control characters directly to
the terminal. This is undesirable because control characters can be used
to mislead users to divulge secret information to untrusted sites.

To prevent such an attack vector, let's add a `git_prompt()` that forces
the displayed text to be sanitized, i.e. displaying question marks
instead of control characters.

Note: While this commit's diff changes a lot of `user@host` strings to
`user%40host`, which may look suspicious on the surface, there is a good
reason for that: this string specifies a user name, not a
<username>@<hostname> combination! In the context of t5541, the actual
combination looks like this: `user%40@127.0.0.1:5541`. Therefore, these
string replacements document a net improvement introduced by this
commit, as `user@host@127.0.0.1` could have left readers wondering where
the user name ends and where the host name begins.

Hinted-at-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config/credential.txt

@@ -14,6 +14,12 @@ credential.useHttpPath::
 	or https URL to be important. Defaults to false. See
 	linkgit:gitcredentials[7] for more information.
 
+credential.sanitizePrompt::
+	By default, user names and hosts that are shown as part of the
+	password prompt are not allowed to contain control characters (they
+	will be URL-encoded by default). Configure this setting to `false` to
+	override that behavior.
+
 credential.username::
 	If no username is set for a network authentication, use this username
 	by default. See credential.<context>.* below, and

credential.c

@@ -67,6 +67,8 @@ static int credential_config_callback(const char *var, const char *value,
 	}
 	else if (!strcmp(key, "usehttppath"))
 		c->use_http_path = git_config_bool(var, value);
+	else if (!strcmp(key, "sanitizeprompt"))
+		c->sanitize_prompt = git_config_bool(var, value);
 
 	return 0;
 }
@@ -179,7 +181,10 @@ static char *credential_ask_one(const char *what, struct credential *c,
 	struct strbuf prompt = STRBUF_INIT;
 	char *r;
 
-	credential_describe(c, &desc);
+	if (c->sanitize_prompt)
+		credential_format(c, &desc);
+	else
+		credential_describe(c, &desc);
 	if (desc.len)
 		strbuf_addf(&prompt, "%s for '%s': ", what, desc.buf);
 	else

credential.h

@@ -119,7 +119,8 @@ struct credential {
 		 configured:1,
 		 quit:1,
 		 use_http_path:1,
-		 username_from_proto:1;
+		 username_from_proto:1,
+		 sanitize_prompt:1;
 
 	char *username;
 	char *password;
@@ -132,6 +133,7 @@ struct credential {
 #define CREDENTIAL_INIT { \
 	.helpers = STRING_LIST_INIT_DUP, \
 	.password_expiry_utc = TIME_MAX, \
+	.sanitize_prompt = 1, \
 }
 
 /* Initialize a credential structure, setting all fields to empty. */

t/t0300-credentials.sh

@@ -45,6 +45,10 @@ test_expect_success 'setup helper scripts' '
 	test -z "$pexpiry" || echo password_expiry_utc=$pexpiry
 	EOF
 
+	write_script git-credential-cntrl-in-username <<-\EOF &&
+	printf "username=\\007latrix Lestrange\\n"
+	EOF
+
 	PATH="$PWD:$PATH"
 '
 
@@ -825,4 +829,20 @@ test_expect_success 'credential config with partial URLs' '
 	test_i18ngrep "skipping credential lookup for key" stderr
 '
 
+BEL="$(printf '\007')"
+
+test_expect_success 'interactive prompt is sanitized' '
+	check fill cntrl-in-username <<-EOF
+	protocol=https
+	host=example.org
+	--
+	protocol=https
+	host=example.org
+	username=${BEL}latrix Lestrange
+	password=askpass-password
+	--
+	askpass: Password for ${SQ}https://%07latrix%20Lestrange@example.org${SQ}:
+	EOF
+'
+
 test_done

t/t5541-http-push-smart.sh

@@ -351,7 +351,7 @@ test_expect_success 'push over smart http with auth' '
 	git push "$HTTPD_URL"/auth/smart/test_repo.git &&
 	git --git-dir="$HTTPD_DOCUMENT_ROOT_PATH/test_repo.git" \
 		log -1 --format=%s >actual &&
-	expect_askpass both user@host &&
+	expect_askpass both user%40host &&
 	test_cmp expect actual
 '
 
@@ -363,7 +363,7 @@ test_expect_success 'push to auth-only-for-push repo' '
 	git push "$HTTPD_URL"/auth-push/smart/test_repo.git &&
 	git --git-dir="$HTTPD_DOCUMENT_ROOT_PATH/test_repo.git" \
 		log -1 --format=%s >actual &&
-	expect_askpass both user@host &&
+	expect_askpass both user%40host &&
 	test_cmp expect actual
 '
 
@@ -393,7 +393,7 @@ test_expect_success 'push into half-auth-complete requires password' '
 	git push "$HTTPD_URL/half-auth-complete/smart/half-auth.git" &&
 	git --git-dir="$HTTPD_DOCUMENT_ROOT_PATH/half-auth.git" \
 		log -1 --format=%s >actual &&
-	expect_askpass both user@host &&
+	expect_askpass both user%40host &&
 	test_cmp expect actual
 '
 

t/t5550-http-fetch-dumb.sh

@@ -90,13 +90,13 @@ test_expect_success 'http auth can use user/pass in URL' '
 test_expect_success 'http auth can use just user in URL' '
 	set_askpass wrong pass@host &&
 	git clone "$HTTPD_URL_USER/auth/dumb/repo.git" clone-auth-pass &&
-	expect_askpass pass user@host
+	expect_askpass pass user%40host
 '
 
 test_expect_success 'http auth can request both user and pass' '
 	set_askpass user@host pass@host &&
 	git clone "$HTTPD_URL/auth/dumb/repo.git" clone-auth-both &&
-	expect_askpass both user@host
+	expect_askpass both user%40host
 '
 
 test_expect_success 'http auth respects credential helper config' '
@@ -114,14 +114,14 @@ test_expect_success 'http auth can get username from config' '
 	test_config_global "credential.$HTTPD_URL.username" user@host &&
 	set_askpass wrong pass@host &&
 	git clone "$HTTPD_URL/auth/dumb/repo.git" clone-auth-user &&
-	expect_askpass pass user@host
+	expect_askpass pass user%40host
 '
 
 test_expect_success 'configured username does not override URL' '
 	test_config_global "credential.$HTTPD_URL.username" wrong &&
 	set_askpass wrong pass@host &&
 	git clone "$HTTPD_URL_USER/auth/dumb/repo.git" clone-auth-user2 &&
-	expect_askpass pass user@host
+	expect_askpass pass user%40host
 '
 
 test_expect_success 'set up repo with http submodules' '
@@ -142,7 +142,7 @@ test_expect_success 'cmdline credential config passes to submodule via clone' '
 	set_askpass wrong pass@host &&
 	git -c "credential.$HTTPD_URL.username=user@host" \
 		clone --recursive super super-clone &&
-	expect_askpass pass user@host
+	expect_askpass pass user%40host
 '
 
 test_expect_success 'cmdline credential config passes submodule via fetch' '
@@ -153,7 +153,7 @@ test_expect_success 'cmdline credential config passes submodule via fetch' '
 	git -C super-clone \
 	    -c "credential.$HTTPD_URL.username=user@host" \
 	    fetch --recurse-submodules &&
-	expect_askpass pass user@host
+	expect_askpass pass user%40host
 '
 
 test_expect_success 'cmdline credential config passes submodule update' '
@@ -170,7 +170,7 @@ test_expect_success 'cmdline credential config passes submodule update' '
 	git -C super-clone \
 	    -c "credential.$HTTPD_URL.username=user@host" \
 	    submodule update &&
-	expect_askpass pass user@host
+	expect_askpass pass user%40host
 '
 
 test_expect_success 'fetch changes via http' '

t/t5551-http-fetch-smart.sh

@@ -181,7 +181,7 @@ test_expect_success 'clone from password-protected repository' '
 	echo two >expect &&
 	set_askpass user@host pass@host &&
 	git clone --bare "$HTTPD_URL/auth/smart/repo.git" smart-auth &&
-	expect_askpass both user@host &&
+	expect_askpass both user%40host &&
 	git --git-dir=smart-auth log -1 --format=%s >actual &&
 	test_cmp expect actual
 '
@@ -199,7 +199,7 @@ test_expect_success 'clone from auth-only-for-objects repository' '
 	echo two >expect &&
 	set_askpass user@host pass@host &&
 	git clone --bare "$HTTPD_URL/auth-fetch/smart/repo.git" half-auth &&
-	expect_askpass both user@host &&
+	expect_askpass both user%40host &&
 	git --git-dir=half-auth log -1 --format=%s >actual &&
 	test_cmp expect actual
 '
@@ -224,14 +224,14 @@ test_expect_success 'redirects send auth to new location' '
 	set_askpass user@host pass@host &&
 	git -c credential.useHttpPath=true \
 	  clone $HTTPD_URL/smart-redir-auth/repo.git repo-redir-auth &&
-	expect_askpass both user@host auth/smart/repo.git
+	expect_askpass both user%40host auth/smart/repo.git
 '
 
 test_expect_success 'GIT_TRACE_CURL redacts auth details' '
 	rm -rf redact-auth trace &&
 	set_askpass user@host pass@host &&
 	GIT_TRACE_CURL="$(pwd)/trace" git clone --bare "$HTTPD_URL/auth/smart/repo.git" redact-auth &&
-	expect_askpass both user@host &&
+	expect_askpass both user%40host &&
 
 	# Ensure that there is no "Basic" followed by a base64 string, but that
 	# the auth details are redacted
@@ -243,7 +243,7 @@ test_expect_success 'GIT_CURL_VERBOSE redacts auth details' '
 	rm -rf redact-auth trace &&
 	set_askpass user@host pass@host &&
 	GIT_CURL_VERBOSE=1 git clone --bare "$HTTPD_URL/auth/smart/repo.git" redact-auth 2>trace &&
-	expect_askpass both user@host &&
+	expect_askpass both user%40host &&
 
 	# Ensure that there is no "Basic" followed by a base64 string, but that
 	# the auth details are redacted
@@ -256,7 +256,7 @@ test_expect_success 'GIT_TRACE_CURL does not redact auth details if GIT_TRACE_RE
 	set_askpass user@host pass@host &&
 	GIT_TRACE_REDACT=0 GIT_TRACE_CURL="$(pwd)/trace" \
 		git clone --bare "$HTTPD_URL/auth/smart/repo.git" redact-auth &&
-	expect_askpass both user@host &&
+	expect_askpass both user%40host &&
 
 	grep -i "Authorization: Basic [0-9a-zA-Z+/]" trace
 '
@@ -568,7 +568,7 @@ test_expect_success 'http auth remembers successful credentials' '
 	# the first request prompts the user...
 	set_askpass user@host pass@host &&
 	git ls-remote "$HTTPD_URL/auth/smart/repo.git" >/dev/null &&
-	expect_askpass both user@host &&
+	expect_askpass both user%40host &&
 
 	# ...and the second one uses the stored value rather than
 	# prompting the user.
@@ -599,7 +599,7 @@ test_expect_success 'http auth forgets bogus credentials' '
 	# us to prompt the user again.
 	set_askpass user@host pass@host &&
 	git ls-remote "$HTTPD_URL/auth/smart/repo.git" >/dev/null &&
-	expect_askpass both user@host
+	expect_askpass both user%40host
 '
 
 test_expect_success 'client falls back from v2 to v0 to match server' '