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> Symfony 5 > API Platform Part 2: Security
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33.

Locking down the CheeseListing.owner Field

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Inside CheeseListing, the owner property - that's the related User object that owns this CheeseListing - is required in the database. And, thanks to the cheese:write group, it's a writable field in our API.

In fact, even though I've forgotten to add an @Assert\NotBlank constraint to this property, an API client must send this field when creating a new CheeseListing. They can also change the owner by sending the field via a PUT request. We even added some fanciness where, when you create or edit a CheeseListing, you can modify the owner's username all at the same time. This works because that property is in the cheese:write group... oops. I forgot to change this group when we were refactoring - that's how it should look. The cheese:item:get group means this field will be embedded when we GET a single CheeseListing - that's the "item operation" - and cheese:write means it's writable when using any write operation for CheeseListing. That's some crazy stuff we set up on our previous tutorial.

Removing username Updatable Fanciness

But now, I want to simplify in two ways. First, I only want the owner property to be set when we create the CheeseListing, I don't want it to be changeable. And second, let's get rid of the fanciness of being able to edit the username property via a CheeseListing operation. For that second part, remove the cheese:write group from username. We can now also take off the @Assert\Valid() annotation. This caused the User to be validated during the CheeseListing operations... which was needed to make sure someone didn't set the username to an invalid value while updating a CheeseListing.

207 lines | src/Entity/CheeseListing.php
// ... lines 1 - 47
class CheeseListing
{
// ... lines 50 - 94
/**
// ... lines 96 - 97
* @Groups({"cheese:read", "cheese:write"})
// ... line 99
*/
private $owner;
// ... lines 102 - 205
}

Making owner only Settable on POST

Now, how can we make the owner property settable for the POST operation but not the PUT operation?

Open up AutoGroupResourceMetadataFactory. This monster automatically adds three serialization groups in all situations. We can use this last one to include a field only for a specific operation. Change cheese:write to cheese:collection:post. That follows the pattern: "short name", colon, collection, colon, then the operation name: post.

206 lines | src/Entity/CheeseListing.php
// ... lines 1 - 94
/**
// ... lines 96 - 97
* @Groups({"cheese:read", "cheese:collection:post"})
*/
private $owner;
// ... lines 101 - 206

Congratulations, the owner can no longer be changed.

Should Owner Be Set Automatically?

But... hold up. Isn't it kinda weird that we allow the API client to send the owner field at all? I mean... shouldn't we instead not make owner part of our API and then write some code to automatically set this to the currently-authenticated user?

Um, maybe. Automatically setting the owner property is kinda nice... and it would also make our API easier to use. We will talk about how to do this later. But I don't want to completely remove owner from my API. Why? Well, what if we created an admin interface where admin users could create cheese listings on behalf of other users. In that case, we would want the owner field to be part of our API.

But... hmm if we allow the owner field to be sent... we can't just allow API clients to create a CheeseListing and set the owner to whoever they want. Sure, maybe an admin user should be able to do this... but how can we prevent a normal user from setting the owner to someone else?

Two Ways to Protect a Writable Field

Backing up, if you need to control some behavior around the way a field is set based on the authenticated user, you have two options. First, you could prevent some users from writing to the field entirely. That's done by putting the property into a special serialization group then dynamically adding that group either in a context builder or in a custom normalizer. We've done that already with admin:read and admin:write.

Second, if the field should be writable by all users... but the data that's allowed to be set on the field depends on who is logged in, then the solution is validation.

Here's our goal: prevent an API client from POSTing an owner value that is different than their own IRI... with an exception added for admin users: they can set owner to anyone.

Let's codify this into a test first. Open CheeseListingResourceTest. Inside testCreateCheeseListing(), we're basically verifying that you do need to be logged in to use the operation. We get a 401 before we're authenticated... then after logging in, we get a 400 status code because access will be granted... but our empty data will fail validation.

Let's make this test more interesting! Create a new $authenticatedUser variable set to who we're logged in as. Then create an $otherUser variable set to... another user in the database.

75 lines | tests/Functional/CheeseListingResourceTest.php
// ... lines 1 - 9
class CheeseListingResourceTest extends CustomApiTestCase
{
// ... lines 12 - 13
public function testCreateCheeseListing()
{
// ... lines 16 - 21
$authenticatedUser = $this->createUserAndLogIn($client, 'cheeseplease@example.com', 'foo');
$otherUser = $this->createUser('otheruser@example.com', 'foo');
// ... lines 24 - 43
}
// ... lines 45 - 73
}

Here's the plan: I want to make another POST request to /api/cheeses with valid data... except that we'll set the owner field to this $otherUser... a user that we are not logged in as. Start by creating a $cheesyData variable set to an array with title, description and price. These are the three required fields other than owner.

75 lines | tests/Functional/CheeseListingResourceTest.php
// ... lines 1 - 13
public function testCreateCheeseListing()
{
// ... lines 16 - 29
$cheesyData = [
'title' => 'Mystery cheese... kinda green',
'description' => 'What mysteries does it hold?',
'price' => 5000
];
// ... lines 35 - 43
}
// ... lines 45 - 75

Now, copy the request and status code assertion from before, paste down here and set the json to $cheesyData plus the owner property set to /api/users/ and then $otherUser->getId().

75 lines | tests/Functional/CheeseListingResourceTest.php
// ... lines 1 - 13
public function testCreateCheeseListing()
{
// ... lines 16 - 34
$client->request('POST', '/api/cheeses', [
'json' => $cheesyData + ['owner' => '/api/users/'.$otherUser->getId()],
]);
// ... lines 38 - 43
}
// ... lines 45 - 75

In this case, the status code should still be 400 once we've coded all of this: passing the wrong owner will be a validation error. I'll add a little message to the assertion to make it obvious why it's failing:

75 lines | tests/Functional/CheeseListingResourceTest.php
// ... lines 1 - 13
public function testCreateCheeseListing()
{
// ... lines 16 - 37
$this->assertResponseStatusCodeSame(400, 'not passing the correct owner');
// ... lines 39 - 43
}
// ... lines 45 - 75

not passing the correct owner

I like it! We're logging in as cheeseplease@example.com... then we're trying to create a CheeseListing that's owned by a totally different user. This is the behavior we want to prevent.

While we're here, copy these two lines again and change $otherUser to $authenticatedUser. This should be allowed, so change the assertion to look for the happy 201 status code.

75 lines | tests/Functional/CheeseListingResourceTest.php
// ... lines 1 - 13
public function testCreateCheeseListing()
{
// ... lines 16 - 39
$client->request('POST', '/api/cheeses', [
'json' => $cheesyData + ['owner' => '/api/users/'.$authenticatedUser->getId()],
]);
$this->assertResponseStatusCodeSame(201);
}
// ... lines 45 - 75

You know the drill: once you've written a test, you get to celebrate by watching it fail! Copy the method name, flip over to your terminal and run:

php bin/phpunit --filter=testCreateCheeseListing

And... it fails!

Failed asserting response status code is 400 - got 201 Created.

So we are currently able to create cheese listings and set the owner as a different user. Cool! Next, let's prevent this with a custom validator.