I have a folder on my server on which I have changed the permissions to 777 (read, write and execute all) to allow users to upload their pictures.
So I want to know, what are the security risks involved in this?
I have implemented code to restrict what file formats can be uploaded, but what would happen if someone was to find the location of the directory, can this pose any threat to my server?
Can they start uploading any files they desire?
Thanks.
When users are uploading files to your server through a web form and some PHP script, the disk access on the server happens with the user id the web server is running under (usually nobody, www-data, apache, _httpd or even root).
Note here, that this single user id is used, regardless of which user uploads the file.
As long as there are no local users accessing the system by other means (ssh, for example), setting the upload directories permissions to 0777 would make not much of a difference -- appart from somebody exploiting a security vulnerability somewhere else in your system there's no one those permissions apply to anyway, and such an attacker would probably just use /tmp.
It is always good practice to set only those permissions on a file or directory that are actually needed. In this case that means probably something like:
drwxrws--- 5 www-data www-data 4096 Nov 17 16:44 upload/
I'm assuming that other local users besides the web server will want to access those files, like the sysadmin or a web designer. Add those users to the group your web server runs under and they don't need sudo or root privileges to access that directory. Also, the +s means that new files and directories in upload/ will automatically be owned by the same group.
As to your last question: just because an attacker knows where the directory is, doesn't mean he can magically make files appear there. There still has to be some sort of service running that accepts files and stores them there... so no, setting the permissions to 0777 doesn't directly make it any less safe.
Still, there are several more dimensions to "safety" and "security" that you cannot address with file permissions in this whole setup:
uploaders can still overwrite each others files because they all work with the same user id
somebody can upload a malicious PHP script to the upload directory and run it from there, possibly exploit other vulnerabilities on your system and gain root access
somebody can use your server to distribute child porn
somebody could run a phishing site from your server after uploading a lookalike of paypal.com
...and there are probably more. Some of those problems you may have addressed in your upload script, but then again, understanding of unix file permissions and where they apply comes usually waaaay at the beginning when learning about security issues, which shows that you are probably not ready yet to tackle all of the possible problems.
Have your code looked at by somebody!
By what means are these users uploading their pictures? If it's over the web, then you only need to give the web server or the CGI script user access to the folder.
The biggest danger here is that users can overwrite other users files, or delete other users files. Nobody without access to this folder will be able to write to it (unless you have some kind of guest/anonymous user).
If you need a directory that everyone can create files in, what you want is to mimic the permissions of the /tmp directory.
$ chown root:root dir; chmod 777 dir; chmod +t dir;
This way any user can create a file, but they cannot delete files owned by other users.
Contrary to what others have said, the executable bit on a directory in unix systems means you can make that directory your current directory (cd to it). It has nothing to do with executing (execution of a directory is meaningless). If you remove the executable bit, nobody will be able to 'cd' to it.
If nothing else, I would remove the executable permissions for all users (if not owner and group as well). With this enabled, someone could upload a file that looks like a picture but is really an executable, which might cause no end of damage.
Possibly remove the read and write permissions for all users as well and restrict it to just owner and group, unless you need anonymous access.
You do not want the executable bit on. As far as *nix goes, the executable bit means you can actually run the file. So, for example, php scripts can be uploaded as type JPEG, and then someone can run that script if they know the location and it's within the web directory.
Related
On my system (Manjaro Linux) I have the public_html folder which I can access in my browser through localhost/~username, served by the pre-installed httpd service.
What would be the best configuration regarding file/folder permissions and ownership so that...
I (aka 'username') am allowed to edit all files and subfolders
the httpd service is allowed to execute/read all necessary files/folders
the httpd service is allowed to write/upload files in certain directories
I have a reasonable level of security (this is on my local pc, not a live server)
The httpd service is run by the http user, I think this is default for Arch-Linux and maybe others.
I think most files and folders default to permissions 755 which almost works great, except that the server (and php subsequently) can't write to directories in that case.
My solution so far is to set permissions to 757 for some folders if my web-app needs to be able to write, but I think in terms of security this is a bad idea.
I plan to create a web interface to configure a part of my system, including some files owned by root. I will be a NodeJS server and I know that running it as root is not a good idea.
Any suggestions about how to perform that without performance and security issues?
Thank you.
I decided to create a specific script that will be owned by root with high restricted rights and allow a sudo on that script for a dedicated user without password so that could not log in (only root can do a su on it).
In the script i will perform wanted action (updte, upgrade, files copy, etc.)
Let's hope that scurity is good enought
I have a REST server whose purpose is to organize files generated by various users. To keep things simple, both the server and the users have access to a shared network filesystem.
The workflow is as follows: the user generates the file in a temp folder. He then notifies the server who then puts the file in a place of its own and stores some metadata in a database. The server should then own the files and take care of their deletion as needed.
My problem is the following: since the files can be quite big, I'd like to avoid a costly copy and instead simply move the files from the temp folder to their final destination. However, moving the files prevents the server from changing their ownership (see here for example).
Is there a way around this, without 1) copying the file, and 2)running the server as root?
EDIT: a couple precisions:
The file to be moved can be a directory with a hierarchy of files
It would be nice to have the server own the files in the final location to restrict access to other users.
If you create a separate user just to handle the chown, you can give that user the CAP_CHOWN capability, and you can have a single executable owned by that user that has the setuid bit set on it (so it executes as that user).
For security, this executable should do as little as possible, with as many checks as possible.
It should do the chown for the server user after the server user does the move. It should exist in a directory that is not writable by other users; it can do checks to insure that it is happy with all the attributes of the files it is asked to chown (current owner, location, etc.), it can have the server user hard-coded (so nobody else can use it), etc.
This will probably have to be a small C program, since most systems don't let you use setuid with scripts. You can find several small example programs on the web that do chown -- one is here
You should use a user group for all users and the server. Make the temp directory owned by that group and set it group-writable and sgid.
chown :groupname /path/to/temp
chmod g+s /path/to/temp
chmod 770 /path/to/temp
Then the server can adopt ownership of the file easily. Of course this means users can write other users' files, but I guess this is not a concern because they stay there a very short time?
I want to automate testing of my users' source code files by letting them upload c++,python, lisp, scala, etc. files to my linux machine where a service will find them in a folder and then compile/run them to verify that they are correct. This server contains no important information about any of my users, so there's no database or anything for someone to hack. But I'm no security expert so I'm still worried about a user somehow finding a way to run arbitrary commands with root privileges (basically I don't have any idea what sorts of things can go wrong). Is there a safe way to do this?
They will. If you give someone the power to compile, it is very hard not to escalate to root. You say that server is not important to you, but what if someone sends you an email from that server, or alters some script, to obtain some info on your home machine or another server you use?
At least you need to strongly separate you from them. I would suggest linux containers, https://linuxcontainers.org/ they are trendy these days. But be careful, this is the kind of service that is always dangerous, no matter how much you protect yourself.
Read more about chroot command in Linux.
This way you can provide every running user program with separate isolated container.
You should under no circumstances allow a user to run code on your server with root privileges. A user could then just run rm –rf / and it would delete everything on your server.
I suggest you make a new local user / group that has very limited permissions, e.g. can only access one folder. So when you run the code on your server, you run it in that folder, and the user can not access anything else. After the code has finished you delete the content of the folder. You should also test this vigorously to check that they really cant destroy / manipulate anything.
If you're running on FreeBSD you could also look at Jails, which is sort-of a way of virtualization and limiting a user / program to that sandbox.
I have user:nobody and group:nogroup set for apache in httpd.conf.
Since I also use my own user to manage files on ssh through Samba, I would like to have access to the www folder for read/write, and also allow apache to read these files.
Some folders should have apache's write permissions.
Should I leave apache as nobody|nogroup?
I was thinking I should set my own user under a group called say "webadmins" and set apache a new user called say "apache" under the same group. Then allow the group to read from all files, but only my user will have write files. Whenever apache would need a write permission inside a folder, I would manually change that. Is this a fair enough approach or am I missing something?
Thanks!
usually any daemon will need to access a number of ressources.
it is therefore good practice to run each daemon under a special user:group, rather than nobody:nogroup.
traditionally (e.g. on Debian systems) apache runs as www-data:www-data.
finally, user permissions take precedence over group permissions (which in turn take precedence over other permissions).
this means that a directory where the user does not have write perms but the user's group can write is effectively r/o for that user (but not for other members of the group)