Saturday, September 22, 2007

Beyond XSS and SQL injections

What is common about HTML, XML and CSV files, SQL and LDAP queries, filenames and shell commands? All these things are based on text which is often generated by programs. And one commonly observed flaw in such programs is encoding rules are not being followed. These days many developers are aware about SQL injection and XSS problems as many books, online tutorials, blogs, coding standards, etc speak about them. Yet I'm not sure there is enough education so that developers use correct methods to protect their code from these problems. And besides this there is a lack of awareness that it is not just SQL and HTML. Definitely developers should think more broadly: if you generate programmatically any kind of text you must think about proper encoding of all data used in the generated text.

Talking about correct methods to secure code from text encoding related problems one my pet peeve is when people try to strip input data when they really should be thinking about protecting output. Nitesh Dhanjani covers this really well in his blog "Repeat After Me: Lack of Output Encoding Causes XSS Vulnerabilities". Quote:

The most common mistake committed by developers (and many security experts, I might add) is to treat XSS as an input validation problem. Therefore, I frequently come across situations where developers fix XSS problems by attempting to filter out meta-characters (<, >, /, “, ‘, etc). At times, if an exhaustive list of meta-characters is used, it does solve the problem, but it makes the application less friendly to the end user – a large set of characters are deemed forbidden. The correct approach to solving XSS problems is to ensure that every user supplied parameter is HTML Output Encoded
A good example of wrong approach is PHP's invention called magic quotes. I have mixed feelings about this thing. On one hand it was probably a good thing because so many web based software is developed by dilettantes so overall we are living in a slightly better world as magic quotes do somewhat limit damage from bad code. On the other hand it teaches bad habits while not fixing all problems in bad code. Also it causes everybody else to suffer. Good news is that they are getting rid of this abomination in PHP6.

Now let's take a look for some examples how not to generate text which I saw in real life. I'll skip HTML and SQL as this is well covered elsewhere and I'll take a look on other things I mentioned in the beginning of this article.

XML files: bad code which generates XML often shares similar problems as bad code which generates HTML - after all these two are closely related. But as XML is a more generic tool it is used in many domains other then web development where developers are not "blessed" with knowledge of XSS like problems. Moreover I noticed even web developers for some reason often consider XML to be something very different then HTML and suddenly forget they have to escape data. I'm especially amused when that many people are not aware that you cannot put arbitrary binary data in XML. You have to either encode it into text (base64 encoding is quite popular for this) or put it outside of the XML document.

CSV files: this format is still quite popular for exchange of tabular data between programs. Guess what? I've seen so many naive CSV producers and parsers that ignore reserved characters and which break later when these programs get real data. No, to write CSV file you cannot just do
print join ",", @columns
What if one of columns contains say "," (comma)?

LDAP queries: being text based query language it is a target of very similar problems as SQL. But while many developers are aware of SQL injection problem, not many are aware that you have exactly the same problem with LDAP queries too. Also it doesn't help that while nearly all SQL libraries provide tools to escape data in SQL queries it doesn't always seem to be the case for LDAP libraries. For example: PHP's LDAP extension - there is no API to escape data at all.

Using shell to execute commands: if you are running a command using system() in C, Perl, PHP or any other language and you are constructing the command from your data you again should treat this as a problem of proper encoding. The example below is from mozilla's source code
sprintf(cmd, "cp %s %s", orig_filename, dest_filename);
Guess what happens if any of these filenames were not escaped for characters which are special for shell?

While I'm at this I'd mention that it is probably a good idea to avoid APIs which use shell to execute commands at all. Simply because shell programming is too hard to get right.

What would help a lot if tools would support developers better when writing correct code which deals with text based APIs. Sometimes it is just lack of documentation on encoding rules. For example a month ago I was learning Facebook APIs. One of the provided APIs is API to execute so called FQL queries. This is an SQL like query language and naturally I'd expect FQL injections to be covered in documentation. They don't, it is not even documented how to escape string data in FQL queries! I played with different queries in FQL console and it seems like standard SQL-like method (i.e. using "\" (backslash)) does work as an escape character in strings but why do I have to find this on my own? It is also shame when libraries built around text APIs do not provides means to properly encode data for used text formats. I mentioned one such example above: PHP's LDAP extension provides no functions to escape data for LDAP queries. How hard is it to add this? If you are creating text based APIs or libraries around such APIs it is your duty to help developers who will be using them. So do document encoding rules and do provide tools to automatically encode data!

Thursday, September 6, 2007

Perl as replacement for shell scripting (Part I)

By shell scripting I mean bash as it is what most (all?) Linux distributions use. Bash can be used as a quite capable programming language. Bash allows programmer to build rather complex scripts by using other programs as building blocks. System comes with a number of such building blocks: find, grep, sed, awk and many others and unsurprisingly there is a lot you can do with them. But it is often a challenge to write robust shell scripts which work or at least fail gracefully for any kind of input. The main reason is that historically shell scripts could use one only data type - string*. Those building blocks, external programs you use in shell scripts have very restricted interface: there are program arguments which are strings, stream of strings as input, stream of strings as output and exit code.

Even a simple concept like a list have to be emulated. For example a list of file names often is passed as a string which contains these file names separated by whitespace. But what if one of these file names contains whitespace? You get a problem. To fix it you need to escape whitespace characters in the filename. And it is rather easy to miss places where you have to do escaping. A bit convolved example:

rm `ls`
This would delete all files in the current directory .. unless they have whitespace characters in their names. There are many similar cases where an unwary programmer can make a mistake in his(her) shell script. Passing data from one process to another often requires a lot of care and the simplest code is often wrong. Another problem is that you are very limited in how you can handle errors in shell scripts - you only have process's exit code to tell you if it finished successfully. And usually it is just a boolean value saying you if there was any error or not. Quote from the linked document:
However, many scripts use an exit 1 as a general bailout upon error. Since exit code 1 signifies so many possible errors, this probably would not be helpful in debugging.
If say mkdir fails your script cannot easily tell if it is because another directory with the same name already exists or you just don't have permissions for this operation.

So any solutions to this problem? As for myself any moment I see my shell script getting longer then three lines of code I rewrite whole thing into Perl. In Perl you don't need to use external programs as much as often as you need in bash. Therefore you are not limited to their restrictive interfaces of them (remember, only strings and exit codes for input and output); native Perl APIs can be much more expressive when they need to.

There is a price though. Perl code is not always as compact as similar shell code for some scripting tasks. This is because the shell scripting is optimized very well to handle interaction of processes and Perl is not as much. It is worth to mention that many things which come for granted in the shell scripting often require you using Perl modules including non standard CPAN Perl modules. It is not problem as such except that not all Perl programmers know where to look for things if they are not covered by perlfunc. This mainly a concern for newbie Perl programmers but it is still a real problem. Also using CPAN modules is not always an option.

Of course in your Perl program you can fail back to using same external programs you would use in a shell script but then you lose advantages of Perl over shell scripting. So .. don't do this if possible. As interesting example of this principle: Perl before version 5.6.0 would fail back to shell to execute operation glob. That was causing various problems for Perl developers: for example I saw Perl programs using glob to fail when run on one tightly secured web hosting server because binary Perl was calling was simply removed from the server for security reasons. In later versions of Perl the implementation of glob was changed: it is implemented purely in Perl now and doesn't use external programs.

To be continued in Part II: mapping between common shell operations and corresponding Perl modules.

[*] New versions of bash support arrays. I'd argue that usefulness of arrays in bash is limited as programs you call from shell scripts cannot use them to pass output data. You are still limited to string streams and exit codes. Not to mention that this is not very portable across different systems.