We have a high security application and we want to allow users to enter URLs that other users will see.
This introduces a high risk of XSS hacks - a user could potentially enter javascript that another user ends up executing. Since we hold sensitive data it's essential that this never happens.
What are the best practices in dealing with this? Is any security whitelist or escape pattern alone good enough?
Any advice on dealing with redirections ("this link goes outside our site" message on a warning page before following the link, for instance)
Is there an argument for not supporting user entered links at all?
Clarification:
Basically our users want to input:
stackoverflow.com
And have it output to another user:
stackoverflow.com
What I really worry about is them using this in a XSS hack. I.e. they input:
alert('hacked!');
So other users get this link:
stackoverflow.com
My example is just to explain the risk - I'm well aware that javascript and URLs are different things, but by letting them input the latter they may be able to execute the former.
You'd be amazed how many sites you can break with this trick - HTML is even worse. If they know to deal with links do they also know to sanitise <iframe>, <img> and clever CSS references?
I'm working in a high security environment - a single XSS hack could result in very high losses for us. I'm happy that I could produce a Regex (or use one of the excellent suggestions so far) that could exclude everything that I could think of, but would that be enough?
If you think URLs can't contain code, think again!
https://owasp.org/www-community/xss-filter-evasion-cheatsheet
Read that, and weep.
Here's how we do it on Stack Overflow:
/// <summary>
/// returns "safe" URL, stripping anything outside normal charsets for URL
/// </summary>
public static string SanitizeUrl(string url)
{
return Regex.Replace(url, #"[^-A-Za-z0-9+&##/%?=~_|!:,.;\(\)]", "");
}
The process of rendering a link "safe" should go through three or four steps:
Unescape/re-encode the string you've been given (RSnake has documented a number of tricks at http://ha.ckers.org/xss.html that use escaping and UTF encodings).
Clean the link up: Regexes are a good start - make sure to truncate the string or throw it away if it contains a " (or whatever you use to close the attributes in your output); If you're doing the links only as references to other information you can also force the protocol at the end of this process - if the portion before the first colon is not 'http' or 'https' then append 'http://' to the start. This allows you to create usable links from incomplete input as a user would type into a browser and gives you a last shot at tripping up whatever mischief someone has tried to sneak in.
Check that the result is a well formed URL (protocol://host.domain[:port][/path][/[file]][?queryField=queryValue][#anchor]).
Possibly check the result against a site blacklist or try to fetch it through some sort of malware checker.
If security is a priority I would hope that the users would forgive a bit of paranoia in this process, even if it does end up throwing away some safe links.
Use a library, such as OWASP-ESAPI API:
PHP - http://code.google.com/p/owasp-esapi-php/
Java - http://code.google.com/p/owasp-esapi-java/
.NET - http://code.google.com/p/owasp-esapi-dotnet/
Python - http://code.google.com/p/owasp-esapi-python/
Read the following:
https://www.golemtechnologies.com/articles/prevent-xss#how-to-prevent-cross-site-scripting
https://www.owasp.org/
http://www.secbytes.com/blog/?p=253
For example:
$url = "http://stackoverflow.com"; // e.g., $_GET["user-homepage"];
$esapi = new ESAPI( "/etc/php5/esapi/ESAPI.xml" ); // Modified copy of ESAPI.xml
$sanitizer = ESAPI::getSanitizer();
$sanitized_url = $sanitizer->getSanitizedURL( "user-homepage", $url );
Another example is to use a built-in function. PHP's filter_var function is an example:
$url = "http://stackoverflow.com"; // e.g., $_GET["user-homepage"];
$sanitized_url = filter_var($url, FILTER_SANITIZE_URL);
Using filter_var allows javascript calls, and filters out schemes that are neither http nor https. Using the OWASP ESAPI Sanitizer is probably the best option.
Still another example is the code from WordPress:
http://core.trac.wordpress.org/browser/tags/3.5.1/wp-includes/formatting.php#L2561
Additionally, since there is no way of knowing where the URL links (i.e., it might be a valid URL, but the contents of the URL could be mischievous), Google has a safe browsing API you can call:
https://developers.google.com/safe-browsing/lookup_guide
Rolling your own regex for sanitation is problematic for several reasons:
Unless you are Jon Skeet, the code will have errors.
Existing APIs have many hours of review and testing behind them.
Existing URL-validation APIs consider internationalization.
Existing APIs will be kept up-to-date with emerging standards.
Other issues to consider:
What schemes do you permit (are file:/// and telnet:// acceptable)?
What restrictions do you want to place on the content of the URL (are malware URLs acceptable)?
Just HTMLEncode the links when you output them. Make sure you don't allow javascript: links. (It's best to have a whitelist of protocols that are accepted, e.g., http, https, and mailto.)
You don't specify the language of your application, I will then presume ASP.NET, and for this you can use the Microsoft Anti-Cross Site Scripting Library
It is very easy to use, all you need is an include and that is it :)
While you're on the topic, why not given a read on Design Guidelines for Secure Web Applications
If any other language.... if there is a library for ASP.NET, has to be available as well for other kind of language (PHP, Python, ROR, etc)
For Pythonistas, try Scrapy's w3lib.
OWASP ESAPI pre-dates Python 2.7 and is archived on the now-defunct Google Code.
How about not displaying them as a link? Just use the text.
Combined with a warning to proceed at your own risk may be enough.
addition - see also Should I sanitize HTML markup for a hosted CMS? for a discussion on sanitizing user input
There is a library for javascript that solves this problem
https://github.com/braintree/sanitize-url
Try it =)
In my project written in JavaScript I use this regex as white list:
url.match(/^((https?|ftp):\/\/|\.{0,2}\/)/)
the only limitation is that you need to put ./ in front for files in same directory but I think I can live with that.
Using Regular Expression to prevent XSS vulnerability is becoming complicated thus hard to maintain over time while it could leave some vulnerabilities behind. Having URL validation using regular expression is helpful in some scenarios but better not be mixed with vulnerability checks.
Solution probably is to use combination of an encoder like AntiXssEncoder.UrlEncode for encoding Query portion of the URL and QueryBuilder for the rest:
public sealed class AntiXssUrlEncoder
{
public string EncodeUri(Uri uri, bool isEncoded = false)
{
// Encode the Query portion of URL to prevent XSS attack if is not already encoded. Otherwise let UriBuilder take care code it.
var encodedQuery = isEncoded ? uri.Query.TrimStart('?') : AntiXssEncoder.UrlEncode(uri.Query.TrimStart('?'));
var encodedUri = new UriBuilder
{
Scheme = uri.Scheme,
Host = uri.Host,
Path = uri.AbsolutePath,
Query = encodedQuery.Trim(),
Fragment = uri.Fragment
};
if (uri.Port != 80 && uri.Port != 443)
{
encodedUri.Port = uri.Port;
}
return encodedUri.ToString();
}
public static string Encode(string uri)
{
var baseUri = new Uri(uri);
var antiXssUrlEncoder = new AntiXssUrlEncoder();
return antiXssUrlEncoder.EncodeUri(baseUri);
}
}
You may need to include white listing to exclude some characters from encoding. That could become helpful for particular sites.
HTML Encoding the page that render the URL is another thing you may need to consider too.
BTW. Please note that encoding URL may break Web Parameter Tampering so the encoded link may appear not working as expected.
Also, you need to be careful about double encoding
P.S. AntiXssEncoder.UrlEncode was better be named AntiXssEncoder.EncodeForUrl to be more descriptive. Basically, It encodes a string for URL not encode a given URL and return usable URL.
You could use a hex code to convert the entire URL and send it to your server. That way the client would not understand the content in the first glance. After reading the content, you could decode the content URL = ? and send it to the browser.
Allowing a URL and allowing JavaScript are 2 different things.
Related
Our Firefox addon issues queries to Google at the backend (main.js), then extracts some content through xpath. For this purpose, we use innerHTML to create a document instance for xpath parsing. But when we submit this addon to Mozilla, we got rejected because:
This add-on is creating DOM nodes from HTML strings containing potentially unsanitized data, by assigning to innerHTML, jQuery.html, or through similar means. Aside from being inefficient, this is a major security risk. For more information, see https://developer.mozilla.org/en/XUL_School/DOM_Building_and_HTML_Insertion
Following the link provided, we tried to replace innerHTML with nsIParserUtils.parseFragment(). However, the example code:
let { Cc, Ci } = require("chrome");
function parseHTML(doc, html, allowStyle, baseURI, isXML) {
let PARSER_UTILS = "#mozilla.org/parserutils;1";
...
The Cc, Ci utilities can only be used on main.js, while the function requires a document (doc) as the argument. But we could not find any examples about creating a document inside main.js, where we could not use document.implementation.createHTMLDocument("");. Because main.js is a background script, which does not have reference to the global built-in document.
I googled a lot, but still could not find any solutions. Could anybody kindly help?
You probably want to use nsIDOMParser instead, which is the same as the standard DOMParser accessible in window globals except that you can use it from privileged contexts without a window object.
Although that gives you a whole document with synthesized <html> and <body> elements if you don't provide your own. If you absolutely need a fragment you can use the html5 template element to extract a fragment via domparser:
let partialHTML = "foo <b>baz</b> bar"
let frag = parser.parseFromString(`<template>${partialHTML}</template>`, 'text/html').querySelector("template").content
I'm developing a web crawler in nodejs. I've created a unique list of the urls in the website crawle body. But some of them have extensions like jpg,mp3, mpeg ... I want to avoid crawling those who have extensions. Is there any simple way to do that?
Two options stick out.
1) Use path to check every URL
As stated in comments, you can use path.extname to check for a file extension. Thus, this:
var test = "http://example.com/images/banner.jpg"
path.extname(test); // '.jpg'
This would work, but this feels like you'll wind up having to create a list of file types you can crawl or you must avoid. That's work.
Side note -- be careful using path. Typically, url is your best tool for parsing links because path is aimed at files/directories, not urls. On some systems (Windows), using path to manipulate a url can result in drama because of the slashes involved. Fair warning!
2) Get the HEAD for each link & see if content-type is set to text/html
You may have reasons to avoid making more network calls. If so, this isn't an option. But if it is OK to make additional calls, you could grab the HEAD for each link and check the MIME type stored in content-type.
Something like this:
var headersOptions = {
method: "HEAD",
host: "http://example.com",
path: "/articles/content.html"
};
var req = http.request(headersOptions, function (res) {
// you will probably need to also do things like check
// HTTP status codes so you handle 404s, 301s, and so on
if (res.headers['content-type'].indexOf("text/html") > -1) {
// do something like queue the link up to be crawled
// or parse the link or put it in a database or whatever
}
});
req.end();
One benefit is that you only grab the HEAD, so even if the file is a gigantic video or something, it won't clog things up. You get the HEAD, see the content-type is a video or whatever, then move along because you aren't interested in that type.
Second, you don't have to keep track of file names because you're using a standard MIME type to differentiate html from other data formats.
I'd want to use a login page to access different firewalls, so I need to get information about the firewall I'm logging in.
In my controller I'd use
$this->container->get('security.context')->getToken()->getProviderKey()
but as an anonymous user I don't have access to getProviderKey method.
I could also parse
_security.xxx.target_path
to get xxx firewall but I'm looking for a more general solution if it exists at all.
Any idea?
As of symfony 3.2, you can now get the current firewall configuration using the following:
public function indexAction(Request $request)
{
$firewall = $this->container
->get('security.firewall.map')
->getFirewallConfig($request)
->getName();
}
Ref: http://symfony.com/blog/new-in-symfony-3-2-firewall-config-class-and-profiler
For Symfony 3.4 I wrote this to avoid referencing the non-public "security.firewall.map" service:
$firewallName = null;
if (($firewallContext = trim($request->attributes->get("_firewall_context", null))) && (false !== ($firewallContextNameSplit = strrpos($firewallContext, ".")))) {
$firewallName = substr($firewallContext, $firewallContextNameSplit + 1);
}
(Referencing "security.firewall.map" on 3.4 will throw an exception.)
Edit: This will not work in a custom exception controller function.
I was doing a little research on this myself recently so that I could send this information in an XACML request as part of the environment.
As far as I can tell from GitHub issues like this one:
https://github.com/symfony/symfony/issues/14435
There is currently no way to reliably get the information out of Symfony except the dirty compiler pass hack suggested on the linked issue. It does appear from the conversation on these issues, they are working on making this available, however, the status is still open, so we will have to be patient and wait for it to be provided.
#Adambean's answer is pretty elegant, but I'd write it as a one-liner:
$firewallName = array_slice(explode('.', trim($request->attributes->get('_firewall_context'))), -1)[0];
The difference is that $firewallName will always be a string (which may be empty).
Also, please note that this answer (like #Adambean's) doesn't work for a firewall with a dot in its name.
I would like to log all HTTP request in Jetty, which is well documented, but I can;t find any resources how can I mask some of the arguments.
E.g.:
json/users/detail?id=dsgrw543
should be logged as:
json/users/detail?id=********
or similar.
The main motivation is that I could give those logs for analytics, without worries that privacy of our users could be compromised. Ideally on-line, without using batch processing or other script.
Please note, that I use other authentication mechanism (cookies/all write methods are POST/etc.) and I can't change the existing URLs.
So far my only idea is to implement it as a class on top of NCSARequestLog:
http://download.eclipse.org/jetty/stable-7/apidocs/org/eclipse/jetty/server/NCSARequestLog.html
What are the better ways of doing that?
you may write a simple util like this, use it wherever you are logging
Enumeration<String> s = request.getParameterNames();
String q = "";
while(s.hasMoreElements()){
q += s.nextElement() + "=***&";
}
String url = request.getServletPath()+(q.length()>0?"?"+q:"");
or may be request.getRequestedURI() in place of getServletPath
I'm writing this in the forlorn hope that someone has already done something similar. I would have posted on drupal.org - but that site is about as user-friendly as a kick in the tomatoes.
I don't know about you, but when I develop I leave all my Drupal paths with open access, and then think about locking them down with access permissions at the end.
What would be be really useful is a module which parses all the paths available (by basically deconstructing the contents of the menu_router table) and then trying them (curl?) in turn whilst logged-in as a given user with a given set of roles.
The output would be a simple html page saying which paths are accessible and which are not.
I'm almost resigned to doing this myself, but if anyone knows of anything vaguely similar I'd be more than grateful to hear about it.
Cheers
UPDATE
Following a great idea from Yorirou, I knocked together a simple module to provide the output I was looking for.
You can get the code here: http://github.com/hymanroth/Path-Lockdown
My first attempt would be a function like this:
function check_paths($uid) {
global $user;
$origuser = $user;
$user = user_load($uid);
$paths = array();
foreach(array_keys(module_invoke_all('menu')) as $path) {
$result = menu_execute_active_handler($path);
if($result != MENU_ACCESS_DENIED && $result != MENU_NOT_FOUND) {
$paths[$path] = TRUE;
}
else {
$paths[$path] = FALSE;
}
}
$user = $origuser;
return $paths;
}
This is good for a first time, but it can't handle wildcard paths (% in the menu path). Loading all possible values can be an option, but it doesn't work in all cases. For instance, if you have %node for example, then you can use node_load, but if you have just %, then you have no idea what to load. Also, it is a common practice to omit the last argument, which is a variable, in order to correctly handle if no argument is given (eg. display all elements).
Also, it might be a good idea to integrate this solution with the Drupal's testing system.
I did a bit of research and wasn't able to find anything. Though I'm inclined to think there is a way to check path access through Drupal API as opposed to CURL - but please keep me updated on your progress / let me know if you would like help developing. This would a great addition to the Drupal modules.