Archive for the ‘security’ Category

md5sum, sha1sum

November 29, 2015 Leave a comment

You always forget how to use the commands md5sum and sha1sum.


bash$ ls -al
-rw-r--r--  1 jabba users 351 Nov 29 14:55 file.iso
bash$ md5sum file.iso
ca9779174955bbaaa6652bb0844ed64f  file.iso 
bash$ md5sum file.iso >na.md5
bash$ md5sum -c na.md5
file.iso: OK
bash$ sha1sum file.iso
76004a25fc2ba41cb805f010a0f66f41e023692d  file.iso
bash$ sha1sum file.iso >na.sha1
bash$ sha1sum -c na.sha1
file.iso: OK
bash$ python2
>>> len("ca9779174955bbaaa6652bb0844ed64f")
>>> len("ca9779174955bbaaa6652bb0844ed64f")*4
>>> len("76004a25fc2ba41cb805f010a0f66f41e023692d")
>>> len("76004a25fc2ba41cb805f010a0f66f41e023692d")*4

MD5 is 128-bit, SHA1 is 160-bit. sha1sum has some stronger variations: sha224sum, sha256sum, sha384sum and sha512sum.

Categories: bash, security Tags: ,

don’t use free proxy servers

January 4, 2015 Leave a comment

Why are free proxies free?

because it’s an easy way to infect thousands of users and collect their data

When you download a page via a proxy server, there is no guarantee that you get the original page… There is a high risk that the proxy server sent you back a modified page. Read the article above for more details.

Categories: network, security Tags: , ,

run a script as another user without password

October 8, 2013 Leave a comment

You can run a program/script/command as another user the following way (example):

sudo -u www-data /bin/date

That is: /bin/date is executed in the name of www-data and you get the output. However, it asks for your password.

Question: how to execute the command above without a password check?


Create the file /etc/sudoers.d/date_test :

jabba ALL=(www-data) NOPASSWD: /bin/date

Meaning: allow the user “jabba” to execute “/bin/date” in the name of “www-data” and ask no password.

You should read /etc/sudoers.d/README, it contains important pieces of information:

  • the file you create cannot contain ‘~‘ or ‘.
  • the file must have 0440 rights
  • the command at the end of the lines must have absolute path

Tip from here.

Categories: bash, security Tags: , ,

setfacl / getfacl

October 8, 2013 Leave a comment

With ACL (Access Control List) you can set exactly who can access your files and directories. With ACL you can set things like “nobody can read this file except user XY”, or “no one can write this directory except the user Z”.

With setfacl you can set the ACL rights. With getfacl you can ask the ACL rights of a file/folder.

Example #1
You have a pmwiki installation that runs on an Apache webserver. PmWiki has a directory called “wiki.d” that must be writable too, otherwise you cannot edit your wiki from a browser. Behind the scenes it’s Apache’s www-data user who wants to write in this directory.

A naive approach is to “chmod 777 pmwiki/wiki.d”. In this case anyone with a shell access to the server can modify the content of this folder.

A better way is to give the necessary grants to Apache’s www-data user:

setfacl -R -m u:www-data:rwx $HOME/public_html/pmwiki/wiki.d

Thanks to Jeszy for the tip.

Example #2
You have a web application that uses an SQLite database. Again, the www-data user would like to write into it. In addition, www-data must be able to write to the directory too that contains the database file.

$ cd /home/jabba/public_html/myapp
# say we have here an sqlite.db file
$ setfacl -m u:www-data:rw sqlite.db
$ setfacl -m u:www-data:rwx .

To grant rights to a group, use “g:groupid:rights” instead of “u:userid:rights“.

Categories: bash, security, ubuntu Tags: , ,

MD5 decrypter

October 2, 2013 Leave a comment

MD5 is a hash, not an encryption. From this hash value you cannot restore the original content. However, you can take a dictionary, hash every word in it with md5, then compare the original md5 value with them. If there is a match, your md5 is cracked. allows you to input an MD5 hash and search for its decrypted state in our database, basically, it’s a MD5 cracker / decryption tool… We have a total of just over 43.745 billion unique decrypted MD5 hashes since August 2007.” (source)

So, if you store your passwords in md5 format and someone has access to them, they are not safe at all… If an md5 hash is generated from a weak password, it can be cracked in an instant with the tool above.

OK, but… how should I store the passwords then?
See this post for a great tip: How to store and verify a password?

Categories: security, Uncategorized Tags: , , ,

Online Security

June 29, 2013 Leave a comment

Read this: The Best Browser Extensions that Protect Your Privacy @lifehacker.

I installed the following extensions:

Categories: firefox, security Tags: ,

Storing sensitive data in your Dropbox folder

January 7, 2012 1 comment

You want to store some sensitive data in your Dropbox folder, e.g. passwords. How to protect these data?

In your Dropbox folder create a Truecrypt volume and store your data in this encrypted virtual file system. For more info refer to this article.

I wanted to store some credentials that I wanted to access from several machines. In my Dropbox folder I created a 10 MB Truecrypt volume. I mounted it and put the sensitive data in it.

Categories: security Tags: ,