Managing cron job for web applications
Written by Georgi Stefkoff
Background
Almost every developer or DevOps has been serving their Web content via some web server like nginx or apache. All of these servers comes with the default user www-data. This user is used for a security reasons that is different from the normal system users (like root or something pre-configured while installing the system), so the application will have a limited resources over the entire server. Of course, you can change the running user for the web server to something else, but this is not recommended if there is not another reason behind this.
If you close the application source code, with some user, let's say root user, you have to change the permission of the source code to be owned by the www-data user. This is done by the following command: chown -R www-data:www-data /var/www/html. In this way, only the www-data user will have the rights to operate with the content in /var/www/html.
Depending on the use-cases, you would want to give the following permissions to all of the files: 770 or 660 if there are no executable files in the source code. This will means that only the user and the rest of the users that are part of the same group (in our case is www-data ground) to operate with the files.
One useful permission options can be 774 that means that members of the group can read, write and execute the files, and the rest of the users can only read. But I do not recommend this, so stay with 770 or 660 to limit the access over the source code.
Once you setup the web server, and adjust the correct file permissions, in most of the cases, you will need to run some background jobs or cron job that will do some magic behind the scenes. Usually, this jobs will fix some broken database records or will trigger some background jobs.
So, where is the problem?
The problem
If you configured the cron jobs with the root users (or something else), according to the file permissions of the source code, you may end up with Permissions Denied error, while trying to execute some PHP script (for example). One example is, if you users is ubuntu and the source code file permission are 660 this means that only the www-data user or users under www-data group can only access these files.
You can easily fix this, bu adding your user to the www-data group by sudo usermod -aG www-data ubutntu.
In this way, ubuntu user will have access over the files that are under www-data group.
This is not the best approach, because we are giving access to another user over the source code. The eases way will be just to set the web server to work with user ubuntu and get rid of file permissions and so on. But this is highly not recommended.
Another big issue will be the following: If you set the cron jobs by the root users.
In this way, since root user is part of the sudoers it have access over all of the files, and no permissions can stop him.
If you execute the cron job with the root users, probably everything will went fine and most of the time, you will not notice any issues (you have to pray that there will be no security issues with this), but imagine the following case: root user executes the cron job, which its work is to generate some files. After the execution, these files will be owned by the root users, since the root users forked the process. Then the issue will come if the web server is trying to access these files. The system will denied the read request, because the web server user is www-data, but the files are created by root user, and your application will stop working properly.
One workaround if this issue is to change the file permissions of the source code after every cron job to www-data user, but this is just a waste of CPU resource.
The solution
The correct solution here is to execute these cron jobs, by the www-data user. you can do this by running the following command:
sudo crontab -e -u www-data
Note the sudo at the begging. Changing another user's cron jobs, will require a root privileges!
In this way, the www-data user will have their own background jobs and they will operate with the source code by themselves and no additional permissions adjustments will be required after the cron job.
Extra: fixing in production
One extra information for use, is a similar scenario: if you need to fix something directly on the production server, or you need just to change some code for debugging, you will want to have access to the source code with the correct user. If you are using the root user, there will be no issue to make changes over the files and save them.
NOTE: if you apply some changes on a file with a different user that have explicit access over the file, this will not change the owner of the file
But if you are using an user, different that root (or the want that is not part of the sudoers), you may end up with Permission denied error.
Another issue will be the same as above - if you need to create a new file, or execute a script that create a file(s), this can lead to the web server user to have no access over the newly created files, because the files will be owed by your user and not by the web server user (www-data in our case)
The correct solution is to first login as the web server user and then apply the changes. In order to login with the www-data user, you have to do the following: (NOTE: you need to have root access for this job)
If you are not the root user the call this first:
sudo su
(enter password)
In order to enter is super-user mode.
Then execute the following command:
su -l -s /bin/bash www-data
And you will be logged as the www-data user.
Here is what this means:
-loptions is said to execute the login logic of the user - setting the environment variables, etc.-sis to provide the shell - in our case is/bin/bash. You can also use/bin/shor whatever shell you are using.www-datais the user that use need to impersonate
After this, you will work with www-data user and will have the correct access over your source code, as the web server is working
Conclusion
Managing the correct access for the web server files is the key to avoid some cyber attacks over your server. Imagine that if you do not limit the resource for you web server over the file system, some peace of code, could give access to attacker to a files that are not part of the source code and they can even fine some database access, access to the server itself or some other sensitive information which is not supposed to be served to the end user
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