Visit Our UNIX and Linux User Community


Security issue and temp files


 
Thread Tools Search this Thread
Top Forums UNIX for Dummies Questions & Answers Security issue and temp files
# 1  
Old 10-01-2009
Security issue and temp files

Hello,

One of the senior network admins at work told me that I should not hard code temp files into my scripts. Rather I should use the mktemp commands in the script to create them on the fly.

His argument was that if a malicious user knew the name of my temp files in the script they could create a symbolic link based on that name to a more important file to overwrite it. So if my temp file was called temp1 the hacker could create a symbolic link to say /etc/passwd.

On my system I created a symbolic link called temp1 pointing to /etc/passwd and then I used touch to create a file called temp1 to see what would happen. But when I tried to create the file with touch, most likely because the symbolic link called temp 1 was already there.

Hence my question: Is that the sysadmin said is true in light of my experiment? And if not, why should I use mktemp instead of hard coding tempf files in my scripts?
# 2  
Old 10-01-2009
IMO? If a hacker is guessing filenames, he's not a very good hacker!

The only significant difference (apart from the name) I can see with whatever.$$ and mktemp is the permissions (600).

So perhaps the race condition the man page is referring to is the time it takes you to chmod a file?

He has a point, I think, but IMO not a very big one.

Last edited by Scott; 10-02-2009 at 04:01 AM..
# 3  
Old 10-01-2009
Quote:
Originally Posted by mojoman
On my system I created a symbolic link called temp1 pointing to /etc/passwd and then I used touch to create a file called temp1 to see what would happen. But when I tried to create the file with touch, most likely because the symbolic link called temp 1 was already there.
It's safe enough to touch /etc/passwd, but imagine what would happen if you'd overwrote it instead.

Using that predictable a name for your temp files has another problem though: What if you want to run two instances of your script at once? I usually do something like
Code:
TMPFILE="/tmp/$$-myprogram"

so each process has a different tempfile. That's only a half step away from using mktemp...
# 4  
Old 10-01-2009
Quote:
Originally Posted by Corona688
It's safe enough to touch /etc/passwd, but imagine what would happen if you'd overwrote it instead.

Using that predictable a name for your temp files has another problem though: What if you want to run two instances of your script at once? I usually do something like
Code:
TMPFILE="/tmp/$$-myprogram"

so each process has a different tempfile. That's only a half step away from using mktemp...
1) What does the $$ do?
2) Was the senior administrator write about his example concerning symbolic links? Like I said, when I tried it did not work (symbolic link will not overwrite /etc/passwd since it is already there) I was using a test system so no worries...had it overwritten /etc/passwd I would not be in trouble (smile).
# 5  
Old 10-01-2009
Quote:
Originally Posted by mojoman
1) What does the $$ do?
2) Was the senior administrator write about his example concerning symbolic links? Like I said, when I tried it did not work (symbolic link will not overwrite /etc/passwd since it is already there) I was using a test system so no worries...had it overwritten /etc/passwd I would not be in trouble (smile).
1. the $$ is the PID number.
so /tmp/program.$$ becomes /tmp/program.10263 or maybe /tmp/program.542

2. the symbolic link will only let you over write the passwd file if your program is running as root.
is it a risk? yes, but soon as you start writing shell script that run as root you open yourself up to many possable security holes.

his use of mktemp is the right way of doing it, I would recomend you change your script to use it if you can.
if nothing else you will get on his good side :-)
# 6  
Old 10-02-2009
Consider this: Your program uses a temporary file with a predictable name, to be used as a log. To this log, you write any input that isn't acceptable. A black-hat hacker now creates a symlink to /etc/passwd, and inputs this to your program:
Code:
toor:aaQSqAReePlq6:0:0::/tmp:/bin/sh

If everything goes well he has now access to a user called "toor" with an empty password and full administration rights. Not good, wouldn't you say?
# 7  
Old 10-07-2009
Quote:
Originally Posted by mojoman
(symbolic link will not overwrite /etc/passwd since it is already there)
Oh yes it will. A symlink, if it's not broken, acts just like the file it points to.

Code:
$ mkdir a
$ cd a
$ echo qwertyuiop > b
$ ln -s b c
$ echo asdf > c
$ cat b
asdf
$

 

Previous Thread | Next Thread
Test Your Knowledge in Computers #8
Difficulty: Easy
macOS is based on the Unix operating system.
True or False?

10 More Discussions You Might Find Interesting

1. Shell Programming and Scripting

Removing large number of temp files

Hi All, I am having a situation now to delete a huge number of temp files created during run times approx. 16700+ files. We have never imagined that we will get this this much big list of files during run time. It worked fine for lesser no of files in the list. But when list is huge we are... (7 Replies)
Discussion started by: mad man
7 Replies

2. Shell Programming and Scripting

RMTF (ReMove Temp Files)

Heyas As some applications (sed,grep,vi, etc) create some tempfiles, i'd changed a script to this: (sadly i cant find the original post (code) anymore (which just removed 2 'diffrent kinds'), just similar ones - forgot that as i was new to all this) :) ~ $ cat $(which rmtf) #!/bin/sh #... (1 Reply)
Discussion started by: sea
1 Replies

3. Shell Programming and Scripting

Backup shell script created temp files .

Hi, I've a script which creates a temp flat file for storing all business dates received on a single day from diff control files sent by source system on that day. e.g on 12th april I receive txns for business day 8,9,10,11 april. I capture this business day and append to a flat file from... (1 Reply)
Discussion started by: manojg9
1 Replies

4. OS X (Apple)

Safari Temp Internet Files Location

I'm looking for help with finding where plugin data and other page resources are stored on the hard disk in safari 6.0. With the new update, the activity window has been merged into the develop menu under "show page resources" and one cannot access them directly. I tried running opensnoop to see... (3 Replies)
Discussion started by: sakurashinken
3 Replies

5. Shell Programming and Scripting

Script to temp create files more than inode limit

HI, I am from testing background. I have a scenario of a file generation, through cronjob, on a defined path. After I fill the data as 100 % utilized, my application is generating an empty file on the defined path. # df -kh Filesystem Size Used Avail Use% Mounted on... (3 Replies)
Discussion started by: atulbassi83
3 Replies

6. Shell Programming and Scripting

temp files

Hi there, As a regular unix user I am forever programming on the command line or writing scripts so that I first write a load of data to a file to read from. In the end I am always left with a bundle of .txt, .tmp which is what I usually call them. As a basic programmmer I was wondering is... (6 Replies)
Discussion started by: cyberfrog
6 Replies

7. UNIX for Dummies Questions & Answers

Temp mysql files jamming CPU resources

Hello Friends I am currently facing high CPU usage problem which is making my site extremely slow. Currently I am using a 8GB RAM with 8 cores but, the creation of temporary files is eating away a lot of the CPU resource making the site very slow. The normal CPU load average remains below 2-3... (5 Replies)
Discussion started by: egully
5 Replies

8. Shell Programming and Scripting

Joining 3 AWK scripts to avoid use "temp" files

Hi everyone, Looking for a suggestion to improve the below script in which Ive been working. The thing is I have 3 separated AWK scripts that I need to apply over the inputfile, and for scripts (2) and (3) I have to use a "temp" file as their inputfile (inputfile_temp and inputfile_temp1... (2 Replies)
Discussion started by: cgkmal
2 Replies

9. Shell Programming and Scripting

Script to Delete temp files and check file system

Hi all, new to the threads as well as Unix/Linux. I need to create a script that will delete any temporary files as well as check the files on the system for errors or corruption. This is what I have so far and I'm sure that I'm missing things or have the wrong commands. I'm not sure where to go... (3 Replies)
Discussion started by: Bwood1377
3 Replies

10. UNIX for Dummies Questions & Answers

Tidying up temp files on exit of script

Hi I believe there is a method to remove all temporary files when a KSH script terminates (either expectedly or unexpectedly). I think is some sort of subroutine you can create that runs when the script exits. Can anyone help me with this please? Many thanks Helen :confused: (2 Replies)
Discussion started by: Bab00shka
2 Replies

Featured Tech Videos