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Most Dangerous Programming Errors, 25-21

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The Common Weakness Enumeration posted their Top 25 Most Dangerous Programming Errors last month. Most everything in the list is completely avoidable, but most new programmers, and especially those without real world experience (as opposed to trivial classroom projects), fall victim to at least some of them. A lot of them bit me in university and I still get nipped by some of them today.

Proper education is the first step, and the CWE have done a good job of describing their top 25, along with code samples of what not to do. I'd like to cover them here, not only for myself (since by explaining you understand more fully), but for new programmers as well.

When it comes to learning about safe and secure software, the sooner the better. CWE shows examples of breaking code, and they have mitigation techniques, but I couldn't find code examples of how to fix the problem, so I'll try to include some of those.

Disclaimer: I don't pretend to be a security expert. If you are working on a banking or other similar system, don't take my word! In fact, take all of this with a grain of salt, and think for yourself before blindly implementing things I think are pretty good. The code shown might not work exactly, and probably doesn't do much other error checking. Treat it like pseudo-code, and please don't copy and paste too much.

25. Race condition

In its simplest form, a race condition occurs when two or more processes try to use a single resource. Two users try to save a wiki page at the same time, as an example. The example shown is a bank transaction: read balance, check that the balance minus the withdrawal amount is greater than or equal to zero, and if it is, allow the transaction, and update the balance.

So what if you have $1000, and you want to withdraw $900? What if at the exact same time, in another process, you try to withdraw $10. You get the balance ($1000) and in both cases the withdrawal is allowed, but what if the balance gets updated twice: once to $100 and once to $990. You now have $910 in your hand, and you bank balance says $990. Woohoo!

Ways around it

Datastore operations

This is clearly a problem. To deal with things like this, locking and atomic operations are probably needed. If your datastore supports it, using the atomic operations it provides would most likely be best. MongoDB supports a few atomic operations and most relational database systems provide functionality to handle these types of situations.

Other locking techniques

If you absolutely must handle the logic in your application and not the datastore, use atomic operations provided by your environment. Locking a file on a Linux system (presumably any of the *NIX systems) is an atomic operation, and will block, or possibly fail. This style of locking could be used to ensure only one process is using the account at any given time. If you are working in one process with threads, using atomic update operations like compare-and-swap and other threadsafe atomic operations is the way to go.

The ruby example, fixed up:

24. Use of a Broken or Risky Cryptographic Algorithm

It should be pretty obvious what the problems are here. If the algorithm sucks, then it's easy to break, then all your data is out.

Ways around it

Don't write your own cryptography algorithm. Ever.

You're probably not smart enough. If you are, why are you reading my blog? Shouldn't you be hanging out with Bruce Schneier? Mathematicians spend years building an algorithm, then more years trying to break it. AES went through 5 years of evaluation along with others before the National Institute of Standards and Technology approved it. You think you can do better? Probably not.

Don't worry though, it's nothing to be ashamed of. Writing cryptography algorithms is hard. Leave it up to those smart math folks, and we can get on with our day writing the next big Facebook.

Don't use broken algorithms

Some algorithms are broken. If you are picking one to use, go find out what's good, and what's broken. MD5 (a hash function) is basically broken, so maybe you should use SHA1, or more preferably SHA256 or SHA512 instead if you need a hash function. Don't use DES as it's basically broken too. Right now, AES is the way to go.

Don't hope for security by obscurity

This goes back to the first point, but if you run your data through a couple XOR and SHIFT operations and think it's secure just because your potential attackers don't know what algorithm you used, think again. Your potential attackers are also probably smarter than you and will eat your application for breakfast, and crap out your supposedly secure data. AES is awesome because the the algorithm is out in the open and they still can't break it. That's the best kind of security.

23. URL Redirection to Untrusted Site ('Open Redirect')

This is blindly redirecting people to a site not your own. The CWE PHP example is pretty straightforward:

The problems here are more for your users than anything. If you blindly redirect them to sites and you don't really control where they are going, that's kind of a dick move, honestly. An exception (sort of) is a URL shortening site, like bit.ly, which is the whole point of the site.

Ways around it

The first two CWE mitigations are pretty solid, and I don't have much to add, so I'll summarize:

  1. Use a whitelist, input validation or spam checking service to reject bad inputs (URLs).
  2. Use a disclaimer page with a long timeout before redirecting, showing the destination URL, or force the user to manually click the URL.

The latter two are a bit more specific, so you can read up on those on their site if you want.

22. Allocation of Resources Without Limits or Throttling

This one can be fun. Blindly forking (or creating threads) without any concept of how many times you've forked (or how many threads are out there). Stuff like that. Another example given is check that a length is greater than zero…but that's it. What if the length is INT_MAX? Are you going to malloc that shit?

Ways around it

Have limits. Obey them.

If you can only handle 500 connections, make sure your app doesn't deal with more than that. If you are working with threads, use a thread pool to cap the max number of threads. Have a memory pool that you can reuse instead of blindly allocating memory every time.

Bad:

if (length > 0) { … }

Good:

if (length > 0 && length < MAX_LENGTH) { … }

Bad:

Good:

21. Incorrect Permission Assignment for Critical Resources

This problem is more towards the system admin side of things, but your software can do some things to help. If you app server is running as root, it can pretty much do anything. If your apache config is setup such that your .htaccess files can be served, they anybody can read them.

Ways around it

The first point the CWE makes regarding implementation is a good idea, but not something a lot of software does, as far as I know. Wordpress warns you (if I remember right) if your config file has weird permissions.

If you are reading a config file, it probably shouldn't be writable by everybody. Maybe you should check that, and raise an error if it is.

Stay tuned for errors 20-16!


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