Package Instance

Flarum notifications can leak restricted content
Using the notifications feature, one can read restricted/private content and bypass access checks that would be in place for such content.

The notification-sending component does not check that the subject of the notification can be seen by the receiver, and proceeds to send notifications through their different channels. The alerts do not leak data despite this as they are listed based on a visibility check, however, emails are still sent out.

This means that, for extensions which restrict access to posts, any actor can bypass the restriction by subscribing to the discussion if the [*Subscriptions*](https://extiverse.com/extension/flarum/subscriptions) extension is enabled.

### Impact
The attack allows the leaking of some posts in the forum database, including posts awaiting approval, posts in tags the user has no access to if they could subscribe to a discussion before it becomes private, and posts restricted by third-party extensions.

Other leaks could also happen for different notification subjects if some features allowed to receive specific types of notifications for restricted content.

All Flarum versions prior to v1.6.3 are affected.

### Patches
The vulnerability has been fixed and published as flarum/core v1.6.3. All communities running Flarum should upgrade as soon as possible to v1.6.3 using:

```
composer update --prefer-dist --no-dev -a -W
```
You can then confirm you run the latest version using:

```
composer show flarum/core
```

### Workarounds
Disable the Flarum Subscriptions extension or disable email notifications altogether.

**There is no other supported workaround for this issue for Flarum versions below 1.6.3.**

### For more information
For any questions or comments on this vulnerability please visit https://discuss.flarum.org/

For support questions create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing [security@flarum.org](mailto:security@flarum.org), and we will address it promptly.

Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
flarum is a forum software package for building communities. In versions prior to 1.7.0 an admin account which has already been compromised by an attacker may use a vulnerability in the `LESS` parser which can be exploited to read sensitive files on the server through the use of path traversal techniques. An attacker can achieve this by providing an absolute path to a sensitive file in the custom `LESS` setting, which the `LESS` parser will then read. For example, an attacker could use the following code to read the contents of the `/etc/passwd` file on a linux machine. The scope of what files is vulnerable will depend on the permissions given to the running flarum process. The vulnerability has been addressed in version `1.7`. Users should upgrade to this version to mitigate the vulnerability. Users unable to upgrade may mitigate the vulnerability by ensuring that their admin accounts are secured with strong passwords and follow other best practices for account security. Additionally, users can limit the exposure of sensitive files on the server by implementing appropriate file permissions and access controls at the operating system level.

Flarum post mentions can be used to read any post on the forum without access control
Using the mentions feature provided by the flarum/mentions extension, users can mention any post ID on the forum with the special `@"<username>"#p<id>` syntax.

The following behavior never changes no matter if the actor should be able to read the mentioned post or not:

A URL to the mentioned post is inserted into the actor post HTML, leaking its discussion ID and post number.

The `mentionsPosts` relationship included in the `POST /api/posts` and `PATCH /api/posts/<id>` JSON responses leaks the full JSON:API payload of all mentioned posts without any access control. This includes the content, date, number and attributes added by other extensions.

An attacker only needs the ability to create new posts on the forum to exploit the vulnerability. This works even if new posts require approval. If they have the ability to edit posts, the attack can be performed even more discreetly by using a single post to scan any size of database and hiding the attack post content afterward.

### Impact
The attack allows the leaking of all posts in the forum database, including posts awaiting approval, posts in tags the user has no access to, and private discussions created by other extensions like FriendsOfFlarum Byobu. This also includes non-comment posts like tag changes or renaming events. 

The discussion payload is not leaked but using the mention HTML payload it's possible to extract the discussion ID of all posts and combine all posts back together into their original discussions even if the discussion title remains unknown.

All Flarum versions prior to `v1.6.3` are affected.

### Patches
The vulnerability has been fixed and published as flarum/core v1.6.3. All communities running Flarum have to upgrade as soon as possible to v1.6.3 using:

```
composer update --prefer-dist --no-dev -a -W
```
You can then confirm you run the latest version using:

```
composer show flarum/core
```

### Workarounds
Disable the mentions extension.

### For more information
For any questions or comments on this vulnerability please visit https://discuss.flarum.org/

For support questions create a discussion at https://discuss.flarum.org/t/support.

A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing [security@flarum.org](mailto:security@flarum.org), and we will address it promptly.

Cross-Site Request Forgery (CSRF)
Flarum allows CSRF against all POST endpoints, as demonstrated by changing admin settings.

Improper Input Validation
`User/Command/ConfirmEmailHandler.php` in Flarum mishandles invalidation of user email tokens.