[PATCH v5] remote: create fetch.credentialsInUrl config

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <derrickstolee@github.com>

Users sometimes provide a "username:password" combination in their
plaintext URLs. Since Git stores these URLs in plaintext in the
.git/config file, this is a very insecure way of storing these
credentials. Credential managers are a more secure way of storing this
information.

System administrators might want to prevent this kind of use by users on
their machines.

Create a new "fetch.credentialsInUrl" config option and teach Git to
warn or die when seeing a URL with this kind of information. The warning
anonymizes the sensitive information of the URL to be clear about the
issue.

This change currently defaults the behavior to "allow" which does
nothing with these URLs. We can consider changing this behavior to
"warn" by default if we wish. At that time, we may want to add some
advice about setting fetch.credentialsInUrl=ignore for users who still
want to follow this pattern (and not receive the warning).

An earlier version of this change injected the logic into
url_normalize() in urlmatch.c. While most code paths that parse URLs
eventually normalize the URL, that normalization does not happen early
enough in the stack to avoid attempting connections to the URL first. By
inserting a check into the remote validation, we identify the issue
before making a connection. In the old code path, this was revealed by
testing the new t5601-clone.sh test under --stress, resulting in an
instance where the return code was 13 (SIGPIPE) instead of 128 from the
die().

However, we can reuse the parsing information from url_normalize() in
order to benefit from its well-worn parsing logic. We can use the struct
url_info that is created in that method to replace the password with
"<redacted>" in our error messages. This comes with a slight downside
that the normalized URL might look slightly different from the input URL
(for instance, the normalized version adds a closing slash). This should
not hinder users figuring out what the problem is and being able to fix
the issue.

As an attempt to ensure the parsing logic did not catch any
unintentional cases, I modified this change locally to to use the "die"
option by default. Running the test suite succeeds except for the
explicit username:password URLs used in t5550-http-fetch-dumb.sh and
t5541-http-push-smart.sh. This means that all other tested URLs did not
trigger this logic.

The tests show that the proper error messages appear (or do not
appear), but also count the number of error messages. When only warning,
each process validates the remote URL and outputs a warning. This
happens twice for clone, three times for fetch, and once for push.

Co-authored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
---
    remote: create fetch.credentialsInUrl config
    
    Users can specify credentials in their URL using the
    username:password@domain format. This is potentially hazardous since the
    URL is stored in plaintext and can also appear in trace2 logs and other
    places. Add a new config option that allows warnings or failures when
    discovering URLs with this format. The default behavior does not change
    in this series, although we may want to move to the warn state by
    default in the future.
    
    This is a modified version of the patch I submitted a while ago [1].
    
    Based on the feedback, changing the behavior to fail by default was not
    a good approach. Further, the idea to stop storing the credentials in
    config and redirect them to a credential manager was already considered
    by Peff [2] but not merged.
    
    This patch does what should be the simplest thing we can do: create a
    config option that will cause the user to get a warning or a failure,
    depending on its value. The default is to ignore the setting, identical
    to the current behavior. We can talk about changing this default to
    "warn" in the future, but it would be safest to release with ignore as
    the default until we are sure that we are not going to start warning on
    false positives.
    
    This patch would be sufficient for the interested internal parties that
    want to prevent users from storing credentials this way. System
    administrators can modify system-level Git config into "die" mode to
    prevent this behavior.
    
    [1]
    https://lore.kernel.org/git/pull.945.git.1619807844627.gitgitgadget@gmail.com
    Reject passwords in URLs (April 2021).
    
    [2]
    https://lore.kernel.org/git/20190519050724.GA26179@sigill.intra.peff.net/
    Re: Git ransom campaign incident report - May 2019
    
    
    Updates in v5
    =============
    
     * Squashed in Junio's recommended changes.
     * Noticed that the redacted strbuf wasn't reset between loop
       iterations, which only matters if there are multiple URLs for the
       remote.
    
    
    Updates in v4
    =============
    
     * The warn_once() patch is dropped in favor of using a different
       location for the check (remotes_remote_get_1() instead of
       valid_remote()).
     * The parsing logic is removed in favor of using the output url_info
       from url_normalize().
     * Tests for 'fetch' and 'push' are added.
     * This requires updating the topic to be on a more-recent version of
       'master' because some change since the previous base caused 'git
       push' to output the warning only once, not twice.
    
    
    Updates in v3
    =============
    
     * Because of some flaky behavior around SIGPIPE, the URL check needed
       to move to be earlier in the command.
     * For this reason, I moved the cred check into remote.c's
       valid_remote() check. This changed the previous BUG() statements into
       early returns.
     * I repeated the check with the test suite to see if this parsing fails
       on any existing cases, but it is worth double-checking the parsing
       rules.
     * Documentation is more consistent about using placeholders.
     * A test for the "allow" case is now included.
     * A new patch is added that creates the warn_once() helper. This
       reduces multiple advisory warnings with the same text from being
       written by the same process.
    
    
    Updates in v2
    =============
    
     * Documentation is slightly expanded to include the fact that Git
       stores the given URL as plaintext in its config.
     * The new method has a new documentation comment that details the
       necessary preconditions.
     * "ignore" is now "allow"
     * Additional checks on colon_ptr are added.
     * Use strbuf_splice() instead of custom string-walking logic.
     * Use "" instead of asterisks.
     * Config value checks are no longer case sensitive.
    
    Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1237%2Fderrickstolee%2Fcreds-in-url-v5
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1237/derrickstolee/creds-in-url-v5
Pull-Request: https://github.com/gitgitgadget/git/pull/1237

Range-diff vs v4:

 1:  5fb038402d6 ! 1:  b744a9887e5 remote: create fetch.credentialsInUrl config
     @@ Commit message
          each process validates the remote URL and outputs a warning. This
          happens twice for clone, three times for fetch, and once for push.
      
     +    Co-authored-by: Junio C Hamano <gitster@pobox.com>
     +    Signed-off-by: Junio C Hamano <gitster@pobox.com>
          Signed-off-by: Derrick Stolee <derrickstolee@github.com>
      
       ## Documentation/config/fetch.txt ##
     @@ remote.c: const char *remote_ref_for_branch(struct branch *branch, int for_push)
      +	int i;
      +	const char *value;
      +	struct strbuf redacted = STRBUF_INIT;
     ++	int warn_not_die;
      +
     -+	if (git_config_get_string_tmp("fetch.credentialsinurl", &value) ||
     -+	    !strcmp("allow", value))
     ++	if (git_config_get_string_tmp("fetch.credentialsinurl", &value))
      +		return;
      +
     ++	if (!strcmp("warn", value))
     ++		warn_not_die = 1;
     ++	else if (!strcmp("die", value))
     ++		warn_not_die = 0;
     ++	else if (!strcmp("allow", value))
     ++		return;
     ++	else
     ++		die(_("unrecognized value fetch.credentialsInURL: '%s'"), value);
     ++
      +	for (i = 0; i < remote->url_nr; i++) {
     -+		struct url_info url_info = { NULL };
     -+		url_normalize(remote->url[i], &url_info);
     ++		struct url_info url_info = { 0 };
      +
     -+		if (!url_info.passwd_len)
     ++		if (!url_normalize(remote->url[i], &url_info) ||
     ++		    !url_info.passwd_off)
      +			goto loop_cleanup;
      +
     ++		strbuf_reset(&redacted);
      +		strbuf_add(&redacted, url_info.url, url_info.passwd_off);
      +		strbuf_addstr(&redacted, "<redacted>");
     -+		strbuf_addstr(&redacted, url_info.url + url_info.passwd_off + url_info.passwd_len);
     ++		strbuf_addstr(&redacted,
     ++			      url_info.url + url_info.passwd_off + url_info.passwd_len);
      +
     -+		if (!strcmp("warn", value))
     ++		if (warn_not_die)
      +			warning(_("URL '%s' uses plaintext credentials"), redacted.buf);
     -+		if (!strcmp("die", value))
     ++		else
      +			die(_("URL '%s' uses plaintext credentials"), redacted.buf);
      +
      +loop_cleanup:
     @@ t/t5516-fetch-push.sh: test_expect_success 'refuse to push a hidden ref, and mak
      +
      +	test_must_fail git -c fetch.credentialsInUrl=die fetch https://username:password@localhost 2>err &&
      +	grep "fatal: $message" err >warnings &&
     ++	test_line_count = 1 warnings &&
     ++
     ++	test_must_fail git -c fetch.credentialsInUrl=die fetch https://username:@localhost 2>err &&
     ++	grep "fatal: $message" err >warnings &&
      +	test_line_count = 1 warnings
      +'
      +
     @@ t/t5601-clone.sh: test_expect_success 'clone respects GIT_WORK_TREE' '
      +
      +	test_must_fail git -c fetch.credentialsInUrl=die clone https://username:password@localhost attempt3 2>err &&
      +	grep "fatal: $message" err >warnings &&
     ++	test_line_count = 1 warnings &&
     ++
     ++	test_must_fail git -c fetch.credentialsInUrl=die clone https://username:@localhost attempt3 2>err &&
     ++	grep "fatal: $message" err >warnings &&
      +	test_line_count = 1 warnings
      +'
      +


 Documentation/config/fetch.txt | 14 ++++++++++
 remote.c                       | 48 ++++++++++++++++++++++++++++++++++
 t/t5516-fetch-push.sh          | 32 +++++++++++++++++++++++
 t/t5601-clone.sh               | 23 ++++++++++++++++
 4 files changed, 117 insertions(+)

diff --git a/Documentation/config/fetch.txt b/Documentation/config/fetch.txt
index cd65d236b43..0db7fe85bb8 100644
--- a/Documentation/config/fetch.txt
+++ b/Documentation/config/fetch.txt
@@ -96,3 +96,17 @@ fetch.writeCommitGraph::
 	merge and the write may take longer. Having an updated commit-graph
 	file helps performance of many Git commands, including `git merge-base`,
 	`git push -f`, and `git log --graph`. Defaults to false.
+
+fetch.credentialsInUrl::
+	A URL can contain plaintext credentials in the form
+	`<protocol>://<user>:<password>@<domain>/<path>`. Using such URLs
+	is not recommended as it exposes the password in multiple ways,
+	including Git storing the URL as plaintext in the repository config.
+	The `fetch.credentialsInUrl` option provides instruction for how Git
+	should react to seeing such a URL, with these values:
++
+* `allow` (default): Git will proceed with its activity without warning.
+* `warn`: Git will write a warning message to `stderr` when parsing a URL
+  with a plaintext credential.
+* `die`: Git will write a failure message to `stderr` when parsing a URL
+  with a plaintext credential.
diff --git a/remote.c b/remote.c
index 930fdc9c2f6..2b6a8b3df7d 100644
--- a/remote.c
+++ b/remote.c
@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "config.h"
 #include "remote.h"
+#include "urlmatch.h"
 #include "refs.h"
 #include "refspec.h"
 #include "object-store.h"
@@ -614,6 +615,50 @@ const char *remote_ref_for_branch(struct branch *branch, int for_push)
 	return NULL;
 }
 
+static void validate_remote_url(struct remote *remote)
+{
+	int i;
+	const char *value;
+	struct strbuf redacted = STRBUF_INIT;
+	int warn_not_die;
+
+	if (git_config_get_string_tmp("fetch.credentialsinurl", &value))
+		return;
+
+	if (!strcmp("warn", value))
+		warn_not_die = 1;
+	else if (!strcmp("die", value))
+		warn_not_die = 0;
+	else if (!strcmp("allow", value))
+		return;
+	else
+		die(_("unrecognized value fetch.credentialsInURL: '%s'"), value);
+
+	for (i = 0; i < remote->url_nr; i++) {
+		struct url_info url_info = { 0 };
+
+		if (!url_normalize(remote->url[i], &url_info) ||
+		    !url_info.passwd_off)
+			goto loop_cleanup;
+
+		strbuf_reset(&redacted);
+		strbuf_add(&redacted, url_info.url, url_info.passwd_off);
+		strbuf_addstr(&redacted, "<redacted>");
+		strbuf_addstr(&redacted,
+			      url_info.url + url_info.passwd_off + url_info.passwd_len);
+
+		if (warn_not_die)
+			warning(_("URL '%s' uses plaintext credentials"), redacted.buf);
+		else
+			die(_("URL '%s' uses plaintext credentials"), redacted.buf);
+
+loop_cleanup:
+		free(url_info.url);
+	}
+
+	strbuf_release(&redacted);
+}
+
 static struct remote *
 remotes_remote_get_1(struct remote_state *remote_state, const char *name,
 		     const char *(*get_default)(struct remote_state *,
@@ -639,6 +684,9 @@ remotes_remote_get_1(struct remote_state *remote_state, const char *name,
 		add_url_alias(remote_state, ret, name);
 	if (!valid_remote(ret))
 		return NULL;
+
+	validate_remote_url(ret);
+
 	return ret;
 }
 
diff --git a/t/t5516-fetch-push.sh b/t/t5516-fetch-push.sh
index 4dfb080433e..dc6cb3cbf5d 100755
--- a/t/t5516-fetch-push.sh
+++ b/t/t5516-fetch-push.sh
@@ -12,6 +12,7 @@ This test checks the following functionality:
 * --porcelain output format
 * hiderefs
 * reflogs
+* URL validation
 '
 
 GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
@@ -1813,4 +1814,35 @@ test_expect_success 'refuse to push a hidden ref, and make sure do not pollute t
 	test_dir_is_empty testrepo/.git/objects/pack
 '
 
+test_expect_success 'fetch warns or fails when using username:password' '
+	message="URL '\''https://username:<redacted>@localhost/'\'' uses plaintext credentials" &&
+	test_must_fail git -c fetch.credentialsInUrl=allow fetch https://username:password@localhost 2>err &&
+	! grep "$message" err &&
+
+	test_must_fail git -c fetch.credentialsInUrl=warn fetch https://username:password@localhost 2>err &&
+	grep "warning: $message" err >warnings &&
+	test_line_count = 3 warnings &&
+
+	test_must_fail git -c fetch.credentialsInUrl=die fetch https://username:password@localhost 2>err &&
+	grep "fatal: $message" err >warnings &&
+	test_line_count = 1 warnings &&
+
+	test_must_fail git -c fetch.credentialsInUrl=die fetch https://username:@localhost 2>err &&
+	grep "fatal: $message" err >warnings &&
+	test_line_count = 1 warnings
+'
+
+
+test_expect_success 'push warns or fails when using username:password' '
+	message="URL '\''https://username:<redacted>@localhost/'\'' uses plaintext credentials" &&
+	test_must_fail git -c fetch.credentialsInUrl=allow push https://username:password@localhost 2>err &&
+	! grep "$message" err &&
+
+	test_must_fail git -c fetch.credentialsInUrl=warn push https://username:password@localhost 2>err &&
+	grep "warning: $message" err >warnings &&
+	test_must_fail git -c fetch.credentialsInUrl=die push https://username:password@localhost 2>err &&
+	grep "fatal: $message" err >warnings &&
+	test_line_count = 1 warnings
+'
+
 test_done
diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
index 4a61f2c901e..d2f046b4b92 100755
--- a/t/t5601-clone.sh
+++ b/t/t5601-clone.sh
@@ -71,6 +71,29 @@ test_expect_success 'clone respects GIT_WORK_TREE' '
 
 '
 
+test_expect_success 'clone warns or fails when using username:password' '
+	message="URL '\''https://username:<redacted>@localhost/'\'' uses plaintext credentials" &&
+	test_must_fail git -c fetch.credentialsInUrl=allow clone https://username:password@localhost attempt1 2>err &&
+	! grep "$message" err &&
+
+	test_must_fail git -c fetch.credentialsInUrl=warn clone https://username:password@localhost attempt2 2>err &&
+	grep "warning: $message" err >warnings &&
+	test_line_count = 2 warnings &&
+
+	test_must_fail git -c fetch.credentialsInUrl=die clone https://username:password@localhost attempt3 2>err &&
+	grep "fatal: $message" err >warnings &&
+	test_line_count = 1 warnings &&
+
+	test_must_fail git -c fetch.credentialsInUrl=die clone https://username:@localhost attempt3 2>err &&
+	grep "fatal: $message" err >warnings &&
+	test_line_count = 1 warnings
+'
+
+test_expect_success 'clone does not detect username:password when it is https://username@domain:port/' '
+	test_must_fail git -c fetch.credentialsInUrl=warn clone https://username@localhost:8080 attempt3 2>err &&
+	! grep "uses plaintext credentials" err
+'
+
 test_expect_success 'clone from hooks' '
 
 	test_create_repo r0 &&

base-commit: 2668e3608e47494f2f10ef2b6e69f08a84816bcb
-- 
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