Hi an XSS attack is treated as an attack from the client's machine. But is there any way to make an XSS attack over the server ?
I want to know is there any way to execute a code on the server using the client side interface like in the case of SQL Injection, but here it is not the Database Server but a Simple Web Server or an Application Server.
Sometimes, it's also possible to use XSS as a vector to trigger and leverage Cross-Site Request Forgery (CSRF) attacks.
Having an XSS on a website is like having control on the javascript a user will execute when visiting it. If an administrator stumbles upon your XSS code (either by sending a malicious link or by means of a stored XSS), then you might get him or her to execute requests or actions on the webserver that a normal user usually wouldn't have access to. If you know the webpage layout well enough, you can request webpages on the visitor's behalf (backends, user lists, etc.), and have the results sent (exfiltrated) anywhere on the Internet.
You can also use more advanced attack frameworks such as BeEF to attempt to exploit vulnerabilities in your visitor's browser. If the visitor in question is a website administrator, this might yield interesting information to further attack the webserver.
XSS per se won't allow you to execute code on the server, but it's a great vector to leverage other vulnerabilities present on the web application.
Vulnerabilities like XSS or SQL injection are specific instances of a more general problem: Improperly concatenating attacker-controllable text into some other format (eg, SQL, HTML, or Javascript)
If your server runs any such format (eg, eval()), it can have similar vulnerabilities.
The answer to your question is not entirely straightforward.
Specifically, no you cannot attack a server using XSS by by injecting code through its interface.
However, there are ways to "inject" code into the server through its interface and have the server run it. The techniques vary widely and substantially, and are highly implementation dependent.
For example, there was a web application that allowed users to upload image files for display. The web application had code that "touched up" the image. There was a vulnerability in the touch up code. A malicious user uploaded a carefully prepared, malicious .jpg file that overflowed a buffer in the code and shoveled off a shell to the attacker's machine. In a case like this, the attack was conducted by "injecting" code into the web app through its interface.
As long as you never process user input (other than storing it in the DB and returning it to other users), then you should be pretty safe from this type of attack. Probably 99% of web apps need to be much more worried about XSS attacks from users to other users than from code injection attacks against themselves.
Related
There is something i wonder. I am giving an example of XSS. We say it is divided into 3 types. Blind, Reflected and stored. There is no one who does not know reflected and stored. We say that the attacker is not informed about the vulnerable with a blind at the beginning of it's name, but if there is no information, how does the attacker understand that there is any vulnerable?
Thank you from now.
In blind XSS the attacker typically doesn't know if his attack will succeed at first.
You can think of it like setting up a trap. You don't know if it will succeed, or if the victim has protection, you are blind.
Actually, blind XSS vulnerabilities are a variant of stored (persistent) XSS vulnerabilities. The attacker's input is saved by a web server and then executed as a malicious script in another application or in another part of the application.
For example, an attacker injects a malicious payload into a contact/feedback form and POST it (setting up a trap). Let's say the info sent is then served by another application or in another part of the app:
The admin is opening his admin panel/dashboard to view feedback from his users. When the administrator of the application is reviewing the feedback entries - the attacker’s payload will be loaded and executed.
The attacker was blind - he didn't know if the server side of that form sanitize the input, or if the "admin panel" of the application has any protection against JS execution.
Example of web applications and web pages where blind XSS attacks can occur:
Contact/Feedback pages
Log viewers
Exception handlers
Chat applications / forums
Customer ticket applications
Web Application Firewalls
Any application that requires user moderation
In the case of blind XSS, the payload can be executed after a long period of time when the administrator visits the vulnerable page. It can take hours, days, or even weeks until the payload is executed.
Therefore, this type of vulnerability is much harder to detect compared to other reflected XSS vulnerabilities where the input is reflected immediately.
This isn't a language specific question, but I am using PHP5.
I am working on a project that has some amount of PII. Legally we are required to secure this data from hacking attempts, and because of that I have been researching best practices for defending common attack types. Obviously all database calls are using parameterized queries, and all data provided by the user is sanitized to prevent injection. I have also implemented sessions and methods to prevent session hijacking.
When it comes to defending against XSS attacks on forms, best practice seems to be to include a hidden input with a form token, then after the post to check the tokens match. There are further ways to make this more secure.
I have imagined one type of attack and haven't found a solution for it. What if a malicious site loads a hidden iframe pointed at my site (eg, view-member.php?id=1234) and because the victim user is logged into my site, their session continues in that iframe. What is stopping this malicious site from iterating through the IDs and ripping the data to get ahold of PII? Should I be creating a unique token for each page view, and checking that token when the page loads?
I am not 100% sure, but assuming my site is using HTTPS, the browser should warn the user and/or prevent the connection. Is that correct? Is that enough security?
In fact, everytime you present a form or any kind of interaction, you should include a randomized, verifiable piece of information that changes every time. This is not for preventing XSS but CSRF: https://en.wikipedia.org/wiki/Cross-site_request_forgery
The main problem is: An attacker can just send automated requests to your input-handling script without going through the "pain" of filling in your form manually (or even visit your page).
However, you won't prevent XSS attacks with this technique, as XSS attacks are mainly user input containing executable code (javascript) that is not filtered by the input validation. So to prevent XSS as well, you should always make sure not to deliver unfiltered user-generated content anywhere.
HTTPS won't help you in either case unless you use client-side certificates that allow access to your website only from trusted clients. HTTPS mainly acts as a transmission scrambler and identity verifier but does not prevent a bot from sending valid (but malicious) data to your form.
Hosting a website in an iFrame does not grant the attacker the permission to read cookies or information from the target page (that would be awful) as long as you follow the same-origin policy: https://en.wikipedia.org/wiki/Same-origin_policy
With this, only domains you whitelist will get access to information hosted on your page.
Like a lot of developers, I want to make JavaScript served up by Server "A" talk to a web service on Server "B" but am stymied by the current incarnation of same origin policy. The most secure means of overcoming this (that I can find) is a server script that sits on Server "A" and acts as a proxy between it and "B". But if I want to deploy this JavaScript in a variety of customer environments (RoR, PHP, Python, .NET, etc. etc.) and can't write proxy scripts for all of them, what do I do?
Use JSONP, some people say. Well, Doug Crockford pointed out on his website and in interviews that the script tag hack (used by JSONP) is an unsafe way to get around the same origin policy. There's no way for the script being served by "A" to verify that "B" is who they say they are and that the data it returns isn't malicious or will capture sensitive user data on that page (e.g. credit card numbers) and transmit it to dastardly people. That seems like a reasonable concern, but what if I just use the script tag hack by itself and communicate strictly in JSON? Is that safe? If not, why not? Would it be any more safe with HTTPS? Example scenarios would be appreciated.
Addendum: Support for IE6 is required. Third-party browser extensions are not an option. Let's stick with addressing the merits and risks of the script tag hack, please.
Currently browser venders are split on how cross domain javascript should work. A secure and easy to use optoin is Flash's Crossdomain.xml file. Most languages have a Cross Domain Proxies written for them, and they are open source.
A more nefarious solution would be to use xss how the Sammy Worm used to spread. XSS can be used to "read" a remote domain using xmlhttprequest. XSS isn't required if the other domains have added a <script src="https://YOUR_DOMAIN"></script>. A script tag like this allows you to evaluate your own JavaScript in the context of another domain, which is identical to XSS.
It is also important to note that even with the restrictions on the same origin policy you can get the browser to transmit requests to any domain, you just can't read the response. This is the basis of CSRF. You could write invisible image tags to the page dynamically to get the browser to fire off an unlimited number of GET requests. This use of image tags is how an attacker obtains documnet.cookie using XSS on another domain. CSRF POST exploits work by building a form and then calling .submit() on the form object.
To understand the Same Orgin Policy, CSRF and XSS better you must read the Google Browser Security Handbook.
Take a look at easyXDM, it's a clean javascript library that allows you to communicate across the domain boundary without any server side interaction. It even supports RPC out of the box.
It supports all 'modern' browser, as well as IE6 with transit times < 15ms.
A common usecase is to use it to expose an ajax endpoint, allowing you to do cross-domain ajax with little effort (check out the small sample on the front page).
What if I just use the script tag hack by itself and communicate strictly in JSON? Is that safe? If not, why not?
Lets say you have two servers - frontend.com and backend.com. frontend.com includes a <script> tag like this - <script src="http://backend.com/code.js"></script>.
when the browser evaluates code.js is considered a part of frontend.com and NOT a part of backend.com. So, if code.js contained XHR code to communicate with backend.com, it would fail.
Would it be any more safe with HTTPS? Example scenarios would be appreciated.
If you just converted your <script src="https://backend.com/code.js> to https, it would NOT be any secure. If the rest of your page is http, then an attacker could easily man-in-the-middle the page and change that https to http - or worse, include his own javascript file.
If you convert the entire page and all its components to https, it would be more secure. But if you are paranoid enough to do that, you should also be paranoid NOT to depend on an external server for you data. If an attacker compromises backend.com, he has effectively got enough leverage on frontend.com, frontend2.com and all of your websites.
In short, https is helpful, but it won't help you one bit if your backend server gets compromised.
So, what are my options?
Add a proxy server on each of your client applications. You don't need to write any code, your webserver can automatically do that for you. If you are using Apache, look up mod_rewrite
If your users are using the latest browsers, you could consider using Cross Origin Resource Sharing.
As The Rook pointed out, you could also use Flash + Crossdomain. Or you could use Silverlight and its equivalent of Crossdomain. Both technologies allow you to communicate with javascript - so you just need to write a utility function and then normal js code would work. I believe YUI already provides a flash wrapper for this - check YUI3 IO
What do you recommend?
My recommendation is to create a proxy server, and use https throughout your website.
Apologies to all who attempted to answer my question. It proceeded under a false assumption about how the script tag hack works. The assumption was that one could simply append a script tag to the DOM and that the contents of that appended script tag would not be restricted by the same origin policy.
If I'd bothered to test my assumption before posting the question, I would've known that it's the source attribute of the appended tag that's unrestricted. JSONP takes this a step further by establishing a protocol that wraps traditional JSON web service responses in a callback function.
Regardless of how the script tag hack is used, however, there is no way to screen the response for malicious code since browsers execute whatever JavaScript is returned. And neither IE, Firefox nor Webkit browsers check SSL certificates in this scenario. Doug Crockford is, so far as I can tell, correct. There is no safe way to do cross domain scripting as of JavaScript 1.8.5.
Watching SO come online has been quite an education for me. I'd like to make a checklist of various vunerabilities and exploits used against web sites, and what programming techniques can be used to defend against them.
What categories of vunerabilities?
crashing site
breaking into server
breaking into other people's logins
spam
sockpuppeting, meatpuppeting
etc...
What kind of defensive programming techniques?
etc...
From the Open Web Application Security Project:
The OWASP Top Ten vulnerabilities (pdf)
For a more painfully exhaustive list: Category:Vulnerability
The top ten are:
Cross-site scripting (XSS)
Injection flaws (SQL injection, script injection)
Malicious file execution
Insecure direct object reference
Cross-site request forgery (XSRF)
Information leakage and improper error handling
Broken authentication and session management
Insecure cryptographic storage
Insecure communications
Failure to restrict URL access
I second the OWASP info as being a valuable resource. The following may be of interest as well, notably the attack patterns:
CERT Top 10 Secure Coding Practices
Common Attack Pattern Enumeration and Classification
Attack Patterns
Secure Programming for Linux and Unix
A Taxonomy of Coding Errors that Affect Security
Secure Programming with Static Analysis Presentation
Obviously test every field for vulnerabilities:
SQL - escape strings (e.g. mysql_real_escape_string)
XSS
HTML being printed from input fields (a good sign of XSS usually)
Anything else thatis not the specific purpose that field was created for
Search for infinite loops (the only indirect thing (if a lot of people found it accidentally) that could kill a server really).
Some prevention techniques:
XSS
If you take any parameters/input from the user and ever plan on outputting it, whether in a log or a web page, sanitize it (strip/escape anything resembling HTML, quotes, javascript...) If you print the current URI of a page within itself, sanitize! Even printing PHP_SELF, for example, is unsafe. Sanitize! Reflective XSS comes mostly from unsanitized page parameters.
If you take any input from the user and save it or print it, warn them if anything dangerous/invalid is detected and have them re-input. an IDS is good for detection (such as PHPIDS.) Then sanitize before storage/printing. Then when you print something from storage/database, sanitize again!
Input -> IDS/sanitize -> store -> sanitize -> output
use a code scanner during development to help spot potentially vulnerable code.
XSRF
Never use GET request for
destructive functionality, i.e.
deleting a post. Instead, only
accept POST requests. GET makes it extra easy for hackery.
Checking the
referrer to make sure the request
came from your site does not
work. It's not hard to spoof the
referrer.
Use a random hash as a token that must be present and valid in every request, and that will expire after a while. Print the token in a hidden form field and check it on the server side when the form is posted. Bad guys would have to supply the correct token in order to forge a request, and if they managed to get the real token, it would need to be before it expired.
SQL injection
your ORM or db abstraction class should have sanitizing methods - use them, always. If you're not using an ORM or db abstraction class... you should be.
SQL injection
XSS (Cross Site Scripting) Attacks
Easy to oversee and easy to fix: the sanitizing of data received from the client side. Checking for things such as ';' can help in preventing malicious code being injected into your application.
G'day,
A good static analysis tool for security is FlawFinder written by David Wheeler. It does a good job looking for various security exploits,
However, it doesn't replace having a knowledgable someone read through your code. As David says on his web page, "A fool with a tool is still a fool!"
HTH.
cheers,
Rob
You can get good firefox addons to test multiple flaws and vulnerabilities like xss and sql injections from Security Compass. Too bad they doesn't work on firefox 3.0. I hope that those will be updated soon.
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I'm pretty green still when it comes to web programming, I've spent most of my time on client applications. So I'm curious about the common exploits I should fear/test for in my site.
I'm posting the OWASP Top 2007 abbreviated list here so people don't have to look through to another link and in case the source goes down.
Cross Site Scripting (XSS)
XSS flaws occur whenever an application takes user supplied data and sends it to a web browser without first validating or encoding that content. XSS allows attackers to execute script in the victim's browser which can hijack user sessions, deface web sites, possibly introduce worms, etc.
Injection Flaws
Injection flaws, particularly SQL injection, are common in web applications. Injection occurs when user-supplied data is sent to an interpreter as part of a command or query. The attacker's hostile data tricks the interpreter into executing unintended commands or changing data.
Malicious File Execution
Code vulnerable to remote file inclusion (RFI) allows attackers to include hostile code and data, resulting in devastating attacks, such as total server compromise. Malicious file execution attacks affect PHP, XML and any framework which accepts filenames or files from users.
Insecure Direct Object Reference
A direct object reference occurs when a developer exposes a reference to an internal implementation object, such as a file, directory, database record, or key, as a URL or form parameter. Attackers can manipulate those references to access other objects without authorization.
Cross Site Request Forgery (CSRF)
A CSRF attack forces a logged-on victim's browser to send a pre-authenticated request to a vulnerable web application, which then forces the victim's browser to perform a hostile action to the benefit of the attacker. CSRF can be as powerful as the web application that it attacks.
Information Leakage and Improper Error Handling
Applications can unintentionally leak information about their configuration, internal workings, or violate privacy through a variety of application problems. Attackers use this weakness to steal sensitive data, or conduct more serious attacks.
Broken Authentication and Session Management
Account credentials and session tokens are often not properly protected. Attackers compromise passwords, keys, or authentication tokens to assume other users' identities.
Insecure Cryptographic Storage
Web applications rarely use cryptographic functions properly to protect data and credentials. Attackers use weakly protected data to conduct identity theft and other crimes, such as credit card fraud.
Insecure Communications
Applications frequently fail to encrypt network traffic when it is necessary to protect sensitive communications.
Failure to Restrict URL Access
Frequently, an application only protects sensitive functionality by preventing the display of links or URLs to unauthorized users. Attackers can use this weakness to access and perform unauthorized operations by accessing those URLs directly.
The Open Web Application Security Project
-Adam
OWASP keeps a list of the Top 10 web attacks to watch our for, in addition to a ton of other useful security information for web development.
These three are the most important:
Cross Site Request Forgery
Cross Site Scripting
SQL injection
Everyone's going to say "SQL Injection", because it's the scariest-sounding vulnerability and the easiest one to get your head around. Cross-Site Scripting (XSS) is going to come in second place, because it's also easy to understand. "Poor input validation" isn't a vulnerability, but rather an evaluation of a security best practice.
Let's try this from a different perspective. Here are features that, when implemented in a web application, are likely to mess you up:
Dynamic SQL (for instance, UI query builders). By now, you probably know that the only reliably safe way to use SQL in a web app is to use parameterized queries, where you explicitly bind each parameter in the query to a variable. The places where I see web apps most frequently break this rule is when the malicious input isn't an obvious parameter (like a name), but rather a query attribute. An obvious example are the iTunes-like "Smart Playlist" query builders you see on search sites, where things like where-clause operators are passed directly to the backend. Another great rock to turn over are table column sorts, where you'll see things like DESC exposed in HTTP parameters.
File upload. File upload messes people up because file pathnames look suspiciously like URL pathnames, and because web servers make it easy to implement the "download" part just by aiming URLs at directories on the filesystem. 7 out of 10 upload handlers we test allow attackers to access arbitrary files on the server, because the app developers assumed the same permissions were applied to the filesystem "open()" call as are applied to queries.
Password storage. If your application can mail me back my raw password when I lose it, you fail. There's a single safe reliable answer for password storage, which is bcrypt; if you're using PHP, you probably want PHPpass.
Random number generation. A classic attack on web apps: reset another user's password, and, because the app is using the system's "rand()" function, which is not crypto-strong, the password is predictable. This also applies anywhere you're doing cryptography. Which, by the way, you shouldn't be doing: if you're relying on crypto anywhere, you're very likely vulnerable.
Dynamic output. People put too much faith in input validation. Your chances of scrubbing user inputs of all possible metacharacters, especially in the real world, where metacharacters are necessary parts of user input, are low. A much better approach is to have a consistent regime of filtering database outputs and transforming them into HTML entities, like quot, gt, and lt. Rails will do this for you automatically.
Email. Plenty of applications implement some sort of outbound mail capability that enable an attacker to either create an anonymous account, or use no account at all, to send attacker-controlled email to arbitrary email addresses.
Beyond these features, the #1 mistake you are likely to make in your application is to expose a database row ID somewhere, so that user X can see data for user Y simply by changing a number from "5" to "6".
bool UserCredentialsOK(User user)
{
if (user.Name == "modesty")
return false;
else
// perform other checks
}
SQL INJECTION ATTACKS. They are easy to avoid but all too common.
NEVER EVER EVER EVER (did I mention "ever"?) trust user information passed to you from form elements. If your data is not vetted before being passed into other logical layers of your application, you might as well give the keys to your site to a stranger on the street.
You do not mention what platform you are on but if on ASP.NET get a start with good ol' Scott Guthrie and his article "Tip/Trick: Guard Against SQL Injection Attacks".
After that you need to consider what type of data you will permit users to submit into and eventually out of your database. If you permit HTML to be inserted and then later presented you are wide-open for Cross Site Scripting attacks (known as XSS).
Those are the two that come to mind for me, but our very own Jeff Atwood had a good article at Coding Horror with a review of the book "19 Deadly Sins of Software Security".
Most people here have mentioned SQL Injection and XSS, which is kind of correct, but don't be fooled - the most important things you need to worry about as a web developer is INPUT VALIDATION, which is where XSS and SQL Injection stem from.
For instance, if you have a form field that will only ever accept integers, make sure you're implementing something at both the client-side AND the server-side to sanitise the data.
Check and double check any input data especially if it's going to end up in an SQL query. I suggest building an escaper function and wrap it around anything going into a query. For instance:
$query = "SELECT field1, field2 FROM table1 WHERE field1 = '" . myescapefunc($userinput) . "'";
Likewise, if you're going to display any user-inputted information onto a webpage, make sure you've stripped any <script> tags or anything else that might result in Javascript execution (such as onLoad= onMouseOver= etc. attributes on tags).
This is also a short little presentation on security by one of wordpress's core developers.
Security in wordpress
it covers all of the basic security problems in web apps.
The most common are probably database injection attacks and cross-site scripting attacks; mainly because those are the easiest to accomplish (that's likely because those are the ones programmers are laziest about).
You can see even on this site that the most damaging things you'll be looking after involve code injection into your application, so XSS (Cross Site Scripting) and SQL injection (#Patrick's suggestions) are your biggest concerns.
Basically you're going to want to make sure that if your application allows for a user to inject any code whatsoever, it's regulated and tested to be sure that only things you're sure you want to allow (an html link, image, etc) are passed, and nothing else is executed.
SQL Injection. Cross Site Scripting.
Using stored procedures and/or parameterized queries will go a long way in protecting you from sql injection. Also do NOT have your web app access the database as sa or dbo - set a up a standard user account and set the permissions.
AS for XSS (cross site scripting) ASP.NET has some built in protections. The best thing is to filter input using validation controls and Regex.
I'm no expert, but from what I learned so far the golden rule is not to trust any user data (GET, POST, COOKIE). Common attack types and how to save yourself:
SQL Injection Attack: Use prepared queries
Cross Site Scripting: Send no user data to browser without filtering/escaping first. This also includes user data stored in database, which originally came from users.