Trusting user inputted strings has always been a problem in computing. Users will always find a way to break your application with some kind of weird character. Programmers have found clever ways to get around this, such as preparing SQL statements, escaping unknown characters, or just returning an error when coming across unknown text. However, with the rise of the internet and the availability of tools, hackers have gotten smarter at the way they attack inputs.
In the last month of so, Django found this out in their
django.utils.text.Truncator class. This class had two methods,
words() which would attempt to clean input.
Well, for some reason, users wanted a way to clean HTML with these methods, so Django added a
html keyword argument to the methods, which would attempt to clean the text as if it were HTML. However, due to a catastrophic backtracking vulnerability in a regular expression in those functions, malicious users could input complicated HTML that would take a long time to process. This would result in a DoS attack on the web server, and bring down services to other users. Uh-oh.
So, looking at the CVE, you can see the security community ranked it a 5, the highest rating. Needless to say, Django quickly patched the issue and launched a hot fix.
The moral of the story is that security vulnerabilities can happen to anyone, and you should know what the framework you are using is doing, instead of just blatantly trusting that it will work. Be aware of security in your everyday life.
— Kyle Kaniecki