Cross-site scripting

From Wikipedia, the free encyclopedia

Cross-site scripting (XSS) is a type of computer security vulnerability typically found in Web applications. XSS enables attackers to inject client-side script into Web pages viewed by other users. A cross-site scripting vulnerability may be used by attackers to bypass access controls such as the same origin policy. Cross-site scripting carried out on websites accounted for roughly 84% of all security vulnerabilities documented by Symantec as of 2007.^[1] Their effect 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 mitigation implemented by the site's owner.

Contents

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• 1 Background
• 2 Types
  • 2.1 Non-persistent
  • 2.2 Persistent
  • 2.3 Server-side versus DOM-based vulnerabilities
• 3 Exploit examples
• 4 Reducing the threat
  • 4.1 Contextual output encoding/escaping of string input
  • 4.2 Safely validating untrusted HTML input
  • 4.3 Cookie security
  • 4.4 Disabling scripts
  • 4.5 Emerging defensive technologies
• 5 Scanning service
• 6 Related vulnerabilities
• 7 See also
• 8 References
• 9 External links

Background[edit]

Security on the web is based on a variety of mechanisms, including an underlying concept of trust known as the same origin policy. This essentially states that if content from one site (such ashttps://mybank.example.com) is granted permission to access resources on the system, then any content from that site will share these permissions, while content from another site (https://othersite.example.com) will have to be granted permissions separately.^{[citation needed]}

Cross-site scripting uses known vulnerabilities in web-based applications, their servers, or plug-in systems on which they rely. Exploiting one of these, they fold malicious content into the content being delivered from the compromised site. When the resulting combined content arrives at the client-side web browser, it has all been delivered from the trusted source, and thus operates under the permissions granted to that system. 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.^{[citation needed]}

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 ofJavaScript 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 ActiveX, Java, VBScript, Flash, or even HTML scripts), causing some confusion to newcomers to the field of information security.^[2]

XSS vulnerabilities have been reported and exploited since the 1990s. Prominent sites affected in the past include the social-networking sites Twitter,^[3] Facebook,^[4] MySpace, YouTube andOrkut.^[5][6] In recent years, cross-site scripting flaws surpassed buffer overflows to become the most common publicly reported security vulnerability,^[7] with some researchers in 2007 viewing as many as 68% of websites as likely open to XSS attacks.^[8]

Types[edit]

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).

Non-persistent[edit]

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 parse and display a page of results for and to that user, without properly sanitizing the request.^[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]

A reflected attack is typically delivered via email or a neutral web site. The bait is an innocent-looking URL, pointing to a trusted site but containing the XSS vector. If the trusted site is vulnerable to the vector, clicking the link can cause the victim's browser to execute the injected script.

Persistent[edit]

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]

For example, suppose there is a dating website where members scan the profiles of other members to see if they look interesting. For privacy reasons, this site hides everybody's real name and email. These are kept secret on the server. The only time a member's real name and email are in the browser is when the member is signed in, and they can't see anyone else's.

Suppose that Mallory, an attacker, joins the site and wants to figure out the real names of the people she sees on the site. To do so, she writes a script designed to run from other people's browsers when they visit her profile. The script then sends a quick message to her own server, which collects this information.

To do this, for the question "Describe your Ideal First Date", Mallory gives a short answer (to appear normal) but the text at the end of her answer is her script to steal names and emails. If the script is enclosed inside a