# Can I prevent XSS attacks with MySQL REPLACE function?

Can i preventing XSS attack by MySQL REPLACE function? Is this safe and acceptable method?

Example below:

INSERT INTO persons(Name, Surname)

• This won't get you pass level two of any online anti-XSS game/challenge... – billc.cn Nov 15 '16 at 11:27

This is a really cack-handed way to attempt protect your application. Never try to transform input like this; it is a very long way from being effective in its objective, it is not maintainable/extensible, it will end up trashing your data.

The golden rule is to validate input and escape output

Check content against a set of rules, reject it and flag a problem for non-compliant data. Whenever data moves from one domain to another (e.g. from PHP or python or PERL to mysql, or from your program to an html stream) then make sure that the representation of the data isolates it from the construct it is being injected into.

There are good libraries for escaping output for a variety of languages and a variety of targets (including SQL and HTML). Start by learning how to use them effectively.

• Thanks. I just trying in database. I must use validate input and escaped output. – Ali Nov 12 '16 at 22:52

No, you can't, and you shouldn't.

This is not an acceptable method at all. Filtering is extremely difficult and you will most certainly get it wrong if you try to implement it yourself. Filters will also need to be updated as the HTML standard is extended (HTML5 for example added a whole bunch of new event attributes which can be used to enter a JavaScript context).

The only proper defense against XSS is context-depending encoding when printing values. Ideally, you would use a templating engine to automatically HTML encode all output, and JavaScript encode manually when necessary.

A simple POC to bypass your filter would be:

<img src=x onerror=alert(1)>


There are many more tags and attributes which can lead to XSS. Additionally, you should also be worried about HTML and CSS injection.

If you want an additional filter when retrieving input - which should never be your only line of defense, but which is recommended as defense in depth - you need to adapt it to each input field. Names are difficult, so I wouldn't filter them, but when you know that you only expect eg alphanum, you could filter for that. You could also use some library like HTMLPurifier. But again, I would only recommend it as defense in depth or if you really need some HTML Tags which are parsed (which isn't the case for names; names may contain characters like ' or even " and <, but they certainly shouldn't be parsed as HTML by the browser).

• Thanks. I change my method. I use validate methods in PHP or other programming languages. I must also escaped output – Ali Nov 12 '16 at 22:54

This won't prevent reflected XSS. This is not filtering all possible scripts.

In short, this won't prevent XSS.

• Yes i test reflected XSS. But can i use this method in comment form like areas? – Ali Nov 12 '16 at 22:36