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. An exploited cross-site scripting vulnerability can be used by attackers to bypass access controls such as the same origin policy. Cross-site scripting carried out on websites were roughly 80% of all security vulnerabilities documented by Symantec as of 2007.[1] Their impact may range from a petty nuisance to a significant security risk, depending on the sensitivity of the data handled by the vulnerable site, and the nature of any security mitigations implemented by the site's owner.
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Cross-site scripting holes are web application vulnerabilities that allow attackers to bypass client-side security mechanisms normally imposed on web content by modern browsers. By finding ways of injecting malicious scripts into web pages, an attacker can gain elevated access privileges to sensitive page content, session cookies, and a variety of other information maintained by the browser on behalf of the user. Cross-site scripting attacks are therefore a special case of code injection.
The expression "cross-site scripting" originally referred to the act of loading the attacked, third-party web application from an unrelated attack site, in a manner that executes a fragment of JavaScript prepared by the attacker in the security context of the targeted domain (a reflected or non-persistent XSS vulnerability). The definition gradually expanded to encompass other modes of code injection, including persistent and non-JavaScript vectors (including Java, ActiveX, VBScript, Flash, or even pure HTML), causing some confusion to newcomers to the field of information security.[2]
XSS vulnerabilities have been reported and exploited since the 1990s. Some prominent sites that have been affected in the past are the search engine Google , the email services of Google and Yahoo! , and the social networking sites Facebook,[3] MySpace, and Orkut.[4][5] The developers of MediaWiki have fixed at least 26 XSS holes in order to protect Wikipedia and other wiki users.[6] In recent years, cross-site scripting flaws surpassed buffer overflows to become the most common publicly-reported security vulnerability,[7] with some researchers claiming that as many as 68% of websites are likely open to XSS attacks.[8]
There is no single, standardized classification of cross-site scripting flaws, but most experts distinguish between at least two primary flavors of XSS: non-persistent and persistent. Some sources further divide these two groups into traditional (caused by server-side code flaws) and DOM-based (in client-side code).
| Example of non-persistent XSS |
|---|
| Non-persistent XSS vulnerabilities in Google could allow malicious sites to attack Google users who visit them while logged in.[9] |
The non-persistent (or reflected) cross-site scripting vulnerability is by far the most common type.[10] These holes show up when the data provided by a web client, most commonly in HTTP query parameters or in HTML form submissions, is used immediately by server-side scripts to generate a page of results for that user, without properly sanitizing the response.[11]
Because HTML documents have a flat, serial structure that mixes control statements, formatting, and the actual content, any non-validated user-supplied data included in the resulting page without proper HTML encoding, may lead to markup injection.[10][11] A classic example of a potential vector is a site search engine: if one searches for a string, the search string will typically be redisplayed verbatim on the result page to indicate what was searched for. If this response does not properly escape or reject HTML control characters, a cross-site scripting flaw will ensue.[12]
At first blush, this does not appear to be a serious problem: by submitting a malicious input to the web site, the user would only be able to compromise their own security context—that is, their own browser cookies, cache objects, and so forth. It is important to realize, however, that a third-party attacker may easily place hidden frames or deceptive links on unrelated sites and cause victims' browsers to navigate to URLs on the vulnerable site automatically—often completely in the background—and in such a case, the attacker can intrude into the security context that rightfully belonged to the victim.
| Example of persistent XSS |
|---|
| A persistent cross-zone scripting vulnerability coupled with a computer worm allowed execution of arbitrary code and listing of filesystem contents via a QuickTime movie on MySpace.[13] |
The persistent (or stored) XSS vulnerability is a more devastating variant of a cross-site scripting flaw: it occurs when the data provided by the attacker is saved by the server, and then permanently displayed on "normal" pages returned to other users in the course of regular browsing, without proper HTML escaping. A classic example of this is with online message boards where users are allowed to post HTML formatted messages for other users to read.[11]
Persistent XSS can be more significant than other types because an attacker's malicious script is rendered automatically, without the need to individually target victims or lure them to a third-party website. Particularly in the case of social networking sites, the code would be further designed to self-propagate across accounts, creating a type of a client-side worm.[14]
The methods of injection can vary a great deal; in some cases, the attacker may not even need to directly interact with the web functionality itself to exploit such a hole. Any data received by the web application (via email, system logs, etc.) that can be controlled by an attacker could become an injection vector.
| Example of DOM-based XSS |
|---|
| Before the bug was resolved, Bugzilla error pages were open to DOM-based XSS attack in which arbitrary HTML and scripts could be injected using forced error messages.[15] |
Traditionally cross-site scripting vulnerabilities would occur in server-side code responsible for preparing the HTML response to be served to the user. With the advent of web 2.0 applications a new class of XSS flaws emerged, DOM-based vulnerabilities. DOM-based vulnerabilities occur in the content processing stages performed by the client, typically in client-side JavaScript. The name refers to the standard model for representing HTML or XML contents which is called the Document Object Model (DOM). JavaScript programs manipulate the state of a web page and populate it with dynamically-computed data primarily by acting upon the DOM.
A typical example is a piece of JavaScript accessing and extracting data from the URL via the location.* DOM, or receiving raw non-HTML data from the server via XMLHttpRequest, and then using this information to write dynamic HTML without proper escaping, entirely on client side.
Attackers intending to exploit cross-site scripting vulnerabilities must approach each class of vulnerability differently. For each class, a specific attack vector is described here. The names below are technical terms, taken from the cast of characters commonly used in computer security.
Non-persistent:
Persistent attack:
Framework:
A Browser Exploitation Framework could be used to attack the web site and the user's local environment.
Commonly referred to as HTML sanitization, one way to eliminate some XSS vulnerabilities is to validate and reject undesirable characters in input fields using a blacklist, whitelist or combination of both. Another way is to escape all untrusted data using a method appropriate for the output context.[17] There are several different escaping schemes that must be used depending on where the untrusted string needs to be placed—including HTML numeric entity encoding, JavaScript escaping, CSS escaping, and URL (or percent) encoding.[18] Most web applications that do not need to accept rich data can use escaping to largely eliminate the risk of XSS in a fairly straightforward manner.
It is worth noting that although it is widely recommended, simply performing HTML entity encoding on the five XML significant characters is not always sufficient to prevent many forms of XSS. Encoding can be tricky, and the use of a security encoding library is highly recommended.[18]
Many operators of particular web applications (e.g. forums and webmail) wish to allow users to utilize some of the features HTML provides, such as a limited subset of HTML markup. To achieve this while preventing cross-site scripting flaws, some web applications, such as social networking sites like MySpace and mainstream forum and blog software like WordPress and Movable Type, attempt to identify malicious HTML constructs, and remove or rewrite them to prevent attacks.[19] Due to the flexibility and complexity of HTML and related standards, the differences between browser implementations, and the continuous addition of new features, this proves to be a very complicated and risky task.[20]
Besides content filtering, other imperfect methods for cross-site scripting mitigation are also commonly used. One example is the use of additional security controls when handling cookie-based user authentication. Many web applications rely on session cookies for authentication between individual HTTP requests, and because client-side scripts generally have access to these cookies, simple XSS exploits can steal these cookies.[17] To mitigate this particular threat (though not the XSS problem in general), many web applications tie session cookies to the IP address of the user who originally logged in, and only permit that IP to use that cookie.[21] This is effective in most situations (if an attacker is only after the cookie), but obviously breaks down in situations where an attacker is behind the same NATed IP address or web proxy—or simply opts to tamper with the site or steal data through the injected script, instead of attempting to hijack the cookie for future use.[21]
Another mitigation present in IE (since version 6), Firefox (since version 2.0.0.5), Safari (since version 4) and Google Chrome, is a HttpOnly flag which allows a web server to set a cookie that is unavailable to client-side scripts. While beneficial, the feature does not fully prevent cookie theft nor can it prevent attacks within the browser.[22]
Finally, while Web 2.0 and Ajax designers favor the use of JavaScript,[23] some web applications are written to (sometimes optionally) operate completely without the need for client-side scripts.[24] This allows users, if they choose, to disable scripting in their browsers before using the application. In this way, even potentially malicious client-side scripts could be inserted unescaped on a page, and users would not be susceptible to XSS attacks.
Some browsers or browser plugins can be configured to disable client-side scripts on a per-domain basis. If scripting is allowed by default, then this approach is of limited value, since it blocks bad sites only after the user knows that they are bad, which is too late. Functionality that blocks all scripting and external inclusions by default and then allows the user to enable it on a per-domain basis is more effective. This has been possible for a long time in IE (since version 4) by setting up its so called "Security Zones",[25] and in Opera (since version 9) using its "Site Specific Preferences".[26] A solution for Firefox and other Gecko-based browsers is the open source NoScript add-on which, in addition to the ability to enable scripts on a per-domain basis, provides some anti-XSS protection even when scripts are enabled.[27]
The most significant problem with blocking all scripts on all websites by default is substantial reduction in functionality and responsiveness (client-side scripting can be much faster than server-side scripting because it does not need to connect to a remote server and the page or frame does not need to be reloaded).[28] Another problem with script blocking is that many users do not understand it, and do not know how to properly secure their browsers. Yet another drawback is that many sites do not work without client-side scripting, forcing users to disable protection for that site and opening their systems to vulnerabilities.[29] The Firefox NoScript extension enables users to allow scripts selectively from a given page while disallowing others on the same page. For example, scripts from example.com could be allowed, while scripts from advertisingagency.com that are attempting to run on the same page could be disallowed.[30]
Some[31] companies offer the service of periodic scans, essentially simulating the attack from they to the client's server with purpose to check if it would be successful. If the attack succeeds, the client receives the detailed information on how it was performed and has possibility to fix the issues before the real attack has been attempted by someone else. The active trust seal can be displayed on the site that passes the recent scan. The scanner may not find all possible vulnerabilities[32] but it may do detect some. After the client fixes them, the site is more secure than it was before ordering the service.
Several classes of vulnerabilities or attack techniques are related to XSS: cross-zone scripting exploits "zone" concepts in certain browsers and usually executes code with a greater privilege.[33] HTTP header injection can be used to create cross-site scripting conditions due to escaping problems on HTTP protocol level (in addition to enabling attacks such as HTTP response splitting).[34]
Cross-site request forgery (CSRF/XSRF) is almost the opposite of XSS, in that rather than exploiting the user's trust in a site, the attacker (and his malicious page) exploits the site's trust in the client software, submitting requests that the site believes represent conscious and intentional actions of authenticated users.[35]
Lastly, SQL injection exploits a vulnerability in the database layer of an application. When user input is incorrectly filtered any SQL statements can be executed by the application.[36][37]