Thursday, February 24, 2011

Storing Passwords in Databases

In the last few weeks/months there have been a couple high-profile computer breaches in the news. One was at Plenty of Fish, an online dating website. The second was at HBGary, a computer security company (ah, the irony).

Plenty of Fish made a major security mistake. They stored passwords in their database in plain text. This means, if you had access to their database (legitimately or through a SQL injection vulnerability) you could see anyone's password. And this is typically a big security no, no. Passwords should always be hashed before being stored in a database.

Hashing is a one-way encryption that prevents a password (or any character string) from being un-encrypted. So, when a user logs in, you hash their entered password and compare it against the hashed password in the database. If they match, the password is the same. It is simple AND safe.

Two particular hashes are quite popular. SHA1 and MD5. Although they are beginning to be a bit dated now and, if I remember correctly, MD5 has been shown to have some vulnerabilities. Some of the more recent hashes are SHA256 and SHA512. (Use the hash function to implement them in PHP.)

Over at HBGary, the "security" firm, they actually did use hashes to store passwords. They used the common MD5 hashing algorithm. The problem with their implementation, is that they added nothing to the password before hashing it. This is an issue because people have created massive online collections of auto-generated hashes from between 1 and 12 characters (typically). So, if you have a hash, and you're a hacker, you look up the hash in this table and (if it is a normal-sized password) it will likely be found.

The best way to make this look-up table irrelevant is to add a set of constant, known characters to the password before the hashing takes place. This technique, called adding "salt" to the password, will create an extra long password, that will never show up in a look-up table. Why? Because it would take more than a lifetime to compute look-up tables that long. Hashing is an expensive operation, in terms of CPU cycles, and the longer the original text, the longer it takes to compute.

If you are building a website, and you have passwords to store, remember: salt & hash. Mmmm, sounds tasty!

Friday, February 18, 2011

Mac crash / Mac blue screen flash during use (not at startup)

A few days ago my MacBook Pro (running OSX 10.6.6) starting having some weird issues. I would be working on the computer, and all of a sudden the screen would flash blue, and then a few seconds later show my blank desktop background, and then finally the dock would come back. All within a few seconds. All of my applications would be closed as if it had restarted.

This has happened about five times, all when I was using different programs. The first time it happened, it was after I dragged a file to trash; after that it happened while using iPhoto and it had frozen up on me; and another time while using Safari. Most recently, the blue screen flash occurred after waking the computer from sleep and trying to close the browser windows in Google Chrome.

I've researched a lot of Mac forums and apparently this is a common problem, and I saw posts from 2007 that were never resolved; I still haven't found a forum with a clear answer.

Anyways, I reviewed the console from the crash this morning, and saw that there was quite a bit of activity all occurring around the time of the blue screen flash, like "windowserver port death" and a whole lot of errors and warnings. This narrowed it down to a possible problem with the window server. I took it in to the Apple store nearby and spoke with a Mac Genius. I showed him the console with the errors, and he suspected it might be a permissions error causing the system to crash and then reboot (looks like it is logging out and then re-logging in at rapid pace).

The Mac Genius attached an external hard drive and rebooted from that and ran permissions repair. He recommended doing this every 3 months or so since it is easy to corrupt permissions. I had never done this before, and it took about 12 minutes. He also said he would recommend doing this at home by booting from the Install DVD I got with the computer. To do this, you have to press the 'option' key during startup in order to have the option to boot from the DVD. (I guess this is a change from pressing "C" with older models.)

Here's how to reboot and repair permissions from the OSX install DVD:

  • Insert install DVD, restart computer, and hold down the 'option' key during startup.
  • This will give the options of booting from the hard drive or the DVD – select DVD
  • The reboot could take up to 5 minutes or so.
  • When install screen comes up, go to "utilities" in the top menu bar and select "Disk Utility."
  • Click on the main hard drive (not the subfolder/partition), and click on "repair disk permission" button, allowing the process to complete. This could take a while, especially if never done before. (It could take up to 3 hours, but mine took 12 minutes.)
  • Quit Mac OSX installer from the menu bar and restart.
We were thinking this should clear up the problem, but if not, the second resort is to archive & reinstall OSX (archive will save all your data), and a third resort is to erase and install (after full backup of all data, of course). We'll see what happens! Hopefully it will work. That would be nice, since it is rather annoying to get the blue screen flash in the middle of doing something, and loosing any data you were working on.  

Update 5/17/11
Well, after many months, many disk permissions repairs and many more blue screen crashes, we decided to go back to the Genius Bar. It was evident that the problem was not fixed. When we returned they ran a test on the hard drive. It apparently failed the test to some degree because they replaced the hard drive. Perhaps it was a hardware problem all along? We'll see.

-- This is a guest post by my wife Susan

Tuesday, February 15, 2011


This can be a fun combo to work with. SELinux, or Security Enhanced Linux, is the life of any party. And Google searches about SELinux-related problems makes it pretty evident that very few people have taken the time to understand how this program works. I ran across numerous people simply suggesting that you turn off SELinux if it is getting in the way.

Well, I didn't buy into this wholesale approach to getting things to work. Besides, I want a secure system and if SELinux is going to help me in the long run, I want it enabled.

VSFTPD, or Very Secure FTP Daemon, is a pretty standard FTP server. You can install it on CentOS systems (or RHEL, for that matter) by running "yum install vsftpd" from the command line. Once you get it installed you can make changes to the configuration file at "/etc/vsftpd/vsftpd.conf" using a file editor. Also, to make sure it is always running in the background run "chkconfig vsftpd on" and "service vsftpd restart" from the command line.

Make sure the configuration file is set up properly. Only give FTP access to a limited number of users (never include root), disable anonymous access and make sure users can only access the files they need (I recommend chroot-ing them).

This is only half the battle. Now that VSFTPD is running, how do you allow users to access their home directories? SELinux usually will get in the way of this. Well, there is a SELinux setting for this called "ftp_home_dir" and this will allow users to access their home directory via FTP. To set this, from the command line run
setsebool -P ftp_home_dir 1

Be sure to also check file permissions and file ownership, if you run into problems. A file must be writable for everyone if they do not specifically own it.

If this fails to grant you FTP access where you need it, or your set-up is slightly different, you can always allow the FTP daemon full access to all files by running
setsebool -P allow_ftpd_full_access 1

This is granting a bit more power to the FTP daemon than is necessary, but it is much better than just disabling SELinux all-together.

By the way, here is a great intro to CentOS & SELinux.

Wednesday, February 9, 2011

Calculating Popularity

I am working on a new site that will have a sort of ranking system. It will be used to list a series of resources that visitors can rate and score. This scoring will then drive what resources are listed first and what resources are listed last.

Well, how in the world do you calculate ratings? I want to make sure new sites get a chance to rank high (so that old sites don't stay at the top forever). I also want to make sure that sites with a low number of votes are put at the top, just because all of the 2 people voted the max. And the list goes on, the more you think about it, the more there is to it.

Well, this blog entry does a great job of explaining different ranking algorithims and how they work. I give it a great score and a high rank. ;)