I'm newish to PHP but I hear XSS exploits are bad. I know what they are, but how do I protect my sites?
To prevent from XSS attacks, you just have to check and validate properly all user inputted data that you plan on using and dont allow html or javascript code to be inserted from that form.
Or you can you Use htmlspecialchars() to convert HTML characters into HTML entities. So characters like <> that mark the beginning/end of a tag are turned into html entities and you can use strip_tags() to only allow some tags as the function does not strip out harmful attributes like the onclick or onload.
Escape all user data (data in the database from user) with htmlentities() function.
For HTML data (for example from WYSIWYG editors), use HTML Purifier to clean the data before saving it to the database.
strip_tags() if you want to have no tags at all. Meaning anything like <somthinghere>
htmlspecialchars() would covert them to html so the browser will only show and not try to run.
If you want to allow good html i would use something like htmLawed or htmlpurifier
The bad news
Unfortunately, preventing XSS in PHP is a non-trivial undertaking.
Unlike SQL injection, which you can mitigate with prepared statements and carefully selected white-lists, there is no provably secure way to separate the information you are trying to pass to your HTML document from the rest of the document structure.
The good news
However, you can mitigate known attack vectors by being particularly cautious with your escaping (and keeping your software up-to-date).
The most important rule to keep in mind: Always escape on output, never on input. You can safely cache your escaped output if you're concerned about performance, but always store and operate on the unescaped data.
XSS Mitigation Strategies
In order of preference:
If you are using a templating engine (e.g. Twig, Smarty, Blade), check that it offers context-sensitive escaping. I know from experience that Twig does. {{ var|e('html_attr') }}
If you want to allow HTML, use HTML Purifier. Even if you think you only accept Markdown or ReStructuredText, you still want to purify the HTML these markup languages output.
Otherwise, use htmlentities($var, ENT_QUOTES | ENT_HTML5, $charset) and make sure the rest of your document uses the same character set as $charset. In most cases, 'UTF-8' is the desired character set.
Why shouldn't I filter on input?
Attempting to filter XSS on input is premature optimization, which can lead to unexpected vulnerabilities in other places.
For example, a recent WordPress XSS vulnerability employed MySQL column truncation to break their escaping strategy and allow the prematurely escaped payload to be stored unsafely. Don't repeat their mistake.
Related
I'm quite surprised I haven't been able to find out what characters I need to strip from a message in order to keep my application safe.
I've got a php app, and most of the inputs are numerical, but I'm adding the ability for users to attache messages, so I need to cleanse the message and strip any characters that could be a threat.
My initial reaction was if I did
$message=addslashes(preg_replace('/[^a-zA-Z0-9\-,& $%\(\)##!\'\"?.]/','',$_POST['message']));
I'd be safe, but I haven't been able to find anything which states what characters can be damaging, and what characters would be safe.
I would say that you don't have to strip any characters from your input, at least generally speaking.
Instead, you must escape your data :
when sending it to your database
see mysql_real_escape_string, mysqli_real_escape_string, PDO::quote
or Prepared statements : MySQLi ; PDO
when sending it to the HTML output
see htmlspecialchars
Still, if you allow users to input HTML, you should take a look at HTMLPurifier, to make sure they are not able to inject any malicious HTML code into your web-pages :
HTML Purifier is a standards-compliant
HTML filter library written in PHP.
HTML Purifier will not only remove all
malicious code (better known as
XSS) with a thoroughly audited, secure yet permissive whitelist, it
will also make sure your documents are
standards compliant
This is where HTML Purifier comes in handy.
Instead of sanitizing your data just use Prepared Statements for database interaction. PDOs eliminate the need of hand santizing all of your input yourself.
PHP Manual
Note: I take care of SQL injection and output escaping elsewhere - this question is about input filtering only, thanks.
I'm in the middle of refactoring my user input filtering functions. Before passing the GET/POST parameter to a type-specific filter with filter_var() I do the following:
check the parameter encoding with mb_detect_encoding()
convert to UTF-8 with iconv() (with //IGNORE) if it's not ASCII or UTF-8
clean white-spaces with a function found on GnuCitizen.org
pass the result thru strip_tags() - no tags allowed at all, Markdown only
Now the question: does it still make sense to pass the parameter to a filter like htmLawed or HTML Purifier, or can I think of the input as safe? It seems to me that these two differ mostly on the granularity of allowed HTML elements and attributes (which I'm not interested into, as I remove everything), but htmLawed docs have a section about 'dangerous characters' that suggests there might be a reason to use it. In this case, what would be a sane configuration for it?
There are many different approaches to XSS that are secure. The only why to know if your approach holds water is to test though exploitation. I recommend using a Free XSS vulnerability Scanner*, or the open source wapiti.
To be honest I'll never use strip_tags() becuase you don't always need html tags to execute javascript! I like htmlspecialchars($var,ENT_QUOTES); .
For instance this is vulnerable to xss:
print('link');
You don't need <> to execute javascript in this case because you can use
onmouseover, here is an example attack:
$_REQUEST[xss]='" onMouseOver="alert(/xss/)"';
The ENT_QUOTES will take care of the double quotes which will patch this XSS vulnerability.
*I am affiliated with this site/service.
i think what you're doing is safe, at least from my point of view no html code should get through your filter
I am creating a forum software using php and mysql backend, and want to know what is the most secure way to escape user input for forum posts.
I know about htmlentities() and strip_tags() and htmlspecialchars() and mysql_real_escape_string(), and even javascript's escape() but I don't know which to use and where.
What would be the safest way to process these three different types of input (by process, I mean get, save in a database, and display):
A title of a post (which will also be the basis of the URL permalink).
The content of a forum post limited to basic text input.
The content of a forum post which allows html.
I would appreciate an answer that tells me how many of these escape functions I need to use in combination and why.
Thanks!
When generating HTLM output (like you're doing to get data into the form's fields when someone is trying to edit a post, or if you need to re-display the form because the user forgot one field, for instance), you'd probably use htmlspecialchars() : it will escape <, >, ", ', and & -- depending on the options you give it.
strip_tags will remove tags if user has entered some -- and you generally don't want something the user typed to just disappear ;-)
At least, not for the "content" field :-)
Once you've got what the user did input in the form (ie, when the form has been submitted), you need to escape it before sending it to the DB.
That's where functions like mysqli_real_escape_string become useful : they escape data for SQL
You might also want to take a look at prepared statements, which might help you a bit ;-)
with mysqli - and with PDO
You should not use anything like addslashes : the escaping it does doesn't depend on the Database engine ; it is better/safer to use a function that fits the engine (MySQL, PostGreSQL, ...) you are working with : it'll know precisely what to escape, and how.
Finally, to display the data inside a page :
for fields that must not contain HTML, you should use htmlspecialchars() : if the user did input HTML tags, those will be displayed as-is, and not injected as HTML.
for fields that can contain HTML... This is a bit trickier : you will probably only want to allow a few tags, and strip_tags (which can do that) is not really up to the task (it will let attributes of the allowed tags)
You might want to take a look at a tool called HTMLPUrifier : it will allow you to specify which tags and attributes should be allowed -- and it generates valid HTML, which is always nice ^^
This might take some time to compute, and you probably don't want to re-generate that HTML each time is has to be displayed ; so you can think about storing it in the database (either only keeping that clean HTML, or keeping both it and the not-clean one, in two separate fields -- might be useful to allow people editing their posts ? )
Those are only a few pointers... hope they help you :-)
Don't hesitate to ask if you have more precise questions !
mysql_real_escape_string() escapes everything you need to put in a mysql database. But you should use prepared statements (in mysqli) instead, because they're cleaner and do any escaping automatically.
Anything else can be done with htmlspecialchars() to remove HTML from the input and urlencode() to put things in a format for URL's.
There are two completely different types of attack you have to defend against:
SQL injection: input that tries to manipulate your DB. mysql_real_escape_string() and addslashes() are meant to defend against this. The former is better, but parameterized queries are better still
Cross-Site scripting (XSS): input that, when displayed on your page, tries to execute JavaScript in a visitor's browser to do all kinds of things (like steal the user's account data). htmlspecialchars() is the definite way to defend against this.
Allowing "some HTML" while avoiding XSS attacks is very, very hard. This is because there are endless possibilities of smuggling JavaScript into HTML. If you decided to do this, the safe way is to use BBCode or Markdown, i.e. a limited set of non-HTML markup that you then convert to HTML, while removing all real HTML with htmlspecialchars(). Even then you have to be careful not to allow javascript: URLs in links. Actually allowing users to input HTML is something you should only do if it's absolutely crucial for your site. And then you should spend a lot of time making sure you understand HTML and JavaScript and CSS completely.
The answer to this post is a good answer
Basically, using the pdo interface to parameterize your queries is much safer and less error prone than escaping your inputs manually.
I have a tendency to escape all characters that would be problematic in page display, Javascript and SQL all at the same time. It leaves it readable on the web and in HTML eMail and at the same time removes any problems with the code.
A vb.NET Line Of Code Would Be:
SafeComment = Replace( _
Replace(Replace(Replace( _
Replace(Replace(Replace( _
Replace(Replace(Replace( _
Replace(Replace(Replace( _
HttpUtility.HtmlEncode(Trim(strInput)), _
":", ":"), "-", "-"), "|", "|"), _
"`", "`"), "(", "("), ")", ")"), _
"%", "%"), "^", "^"), """", """), _
"/", "/"), "*", "*"), "\", "\"), _
"'", "'")
First of all, general advice: don't escape variables literally when inserting in the database. There are plenty of solutions that let you use prepared statements with variable binding. The reason to not do this explicitly is because it is only a matter of time then before you forget it just once.
If you're inserting plain text in the database, don't try to clean it on insert, but instead clean it on display. That is to say, use htmlentities to encode it as HTML (and pass the correct charset argument). You want to encode on display because then you're no longer trusting that the database contents are correct, which isn't necessarily a given.
If you're dealing with rich text (html), things get more complicated. Removing the "evil" bits from HTML without destroying the message is a difficult problem. Realistically speaking, you'll have to resort to a standardized solution, like HTMLPurifier. However, this is generally too slow to run on every page view, so you'll be forced to do this when writing to the database. You'll also have to ensure that the user can see their "cleaned up" html and correct the cleaned up version.
Definitely try to avoid "rolling your own" filter or encoding solution at any step. These problems are notoriously tricky, and you run a large risk of overlooking some minor detail that has big security implications.
I second Joeri, do not roll your own, go here to see some of the the many possible XSS attacks
http://ha.ckers.org/xss.html
htmlentities() -> turns text into html, converting characters to entities. If using UTF-8 encoding then use htmlspecialchars() instead as the other entities are not needed. This is the best defence against XSS. I use it on every variable I output regardless of type or origin unless I intend it to be html. There is only a tiny performance cost and it is easier than trying to work out what needs escaping and what doesn't.
strip_tags() - turns html into text by removing all html tags. Use this to ensure that there is nothing nasty in your input as a adjunct to escaping your output.
mysql_real_escape_string() - escapes a string for mysql and is your defence against SQL injections from little Bobby tables (better to use mysqli and prepare/bind as escaping is then done for you and you can avoid lots of messy string concatenations)
The advice given obve re avoiding HTML input unless it is essential and opting for BBCode or similar (make your own up if needs be) is very sound indeed.
my social networking site is w3c xhtml valid however users are able to post blog reports and stuff and at times enter in ampersand characters which in turn mess up my validation. How can I fix this and are there any other single characters that I need to look out for that could mess up my validation?
When displaying user produced content, run it through the htmlspecialchars() function.
As a matter of general principle it's a mistake to include user-submitted (or indeed any external) content into your page directly without validation or filtering. Besides causing validation errors it can also cause "broken pages" and large security holes (cross-site scripting attacks).
Whenever you get data from anywhere that isn't 100% trusted, you need to make it safe in some way. You can do this by doing some or all of:
Escaping textual data so that special characters are replaced by the HTML entities that represent them.
Stripping or filtering unsafe HTML tags.
Validating that HTML doesn't contain any unsafe or illegal constructs.
If your user input is meant to be interpreted as text then you're mostly looking at option 1; if you're letting the users use HTML then you're looking at options 2 and 3. A fourth option is to have the users use some more restrictive non-HTML markup such as Markdown or bbCode, translating between that markup and HTML using a library that (hopefully) doesn't allow the injection of security holes, page-breaking constructs, or other scary things.
It's a bad idea to allow users to enter HTML markup.
This enables all kinds of nasty things, most notably cross-site scripting (XSS) exploits and injection of hidden spam (hidden from you, not search engine bots).
You should:
Obliterate all HTML tags using htmlspecialchars() and only preserve newlines with nl2br(). You might allow some formatting by implementing your own safe markup that allows only very specific tags (things like phpBB or Wiki-like markup).
Use HTML Purifier to reliably eliminate all potentially-dangerous markup. PHP's strip_tags() function is fundamentally broken and allows dangerous code in attributes if you use whitelist argument.
I currently have a fairly robust server-side validation system in place, but I'm looking for some feedback to make sure I've covered all angles. Here is a brief outline of what I'm doing at the moment:
Ensure the input is not empty, or is too long
Escape query strings to prevent SQL injection
Using regular expressions to reject invalid characters (this depends on what's being submitted)
Encoding certain html tags, like <script> (all tags are encoded when stored in a database, with some being decoded when queried to render in the page)
Is there anything I'm missing? Code samples or regular expressions welcome.
You shouldn't need to "Escape" query strings to prevent SQL injection - you should be using prepared statements instead.
Ideally your input filtering will happen before any other processing, so you know it will always be used. Because otherwise you only need to miss one spot to be vulnerable to a problem.
Don't forget to encode HTML entities on output - to prevent XSS attacks.
You should encode every html tag, not only 'invalid' ones. This is a hot debate, but basically it boils down to there will always be some invalid HTML combination that you will forget to handle correctly (nested tags, mismatched tags some browsers interpret 'correctly' and so on). So the safest option in my opinion is to store everything as htmlentities and then, on output, print a validated HTML-safe-subset tree (as entities) from the content.
Run all server-side validation in a library dedicated to the task so that improvements in one area affect all of your application.
Additionally include work against known attacks, such as directory traversal and attempts to access the shell.
This Question/Answer has some good responses that you're looking for
(PHP-oriented, but then again you didn't specify language/platform and some of it applies beyond the php world):
What's the best method for sanitizing user input with PHP?
You might check out the Filter Extension for data filtering. It won't guarantee that you're completely airtight, but personally I feel a lot better using it because that code has a whole lot of eyeballs looking over it.
Also, consider prepared statements seconded. Escaping data in your SQL queries is a thing of the past.