| 词条 | Cross-site scripting |
| 释义 |
Cross-site scripting (XSS) is a type of computer security vulnerability typically found in web applications. XSS enables attackers to inject client-side scripts 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] In 2017, XSS is still considered a major threat vector.[2] XSS effects vary in range from petty nuisance to 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. BackgroundSecurity on the web depends 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 as https://mybank.example1.com) is granted permission to access resources (like cookies etc.) on a browser, then content from any URL with the same (1) URI scheme, (2) host name, and (3) port number will share these permissions. Content from URLs where any of these three attributes are different will have to be granted permissions separately.[3] Cross-site scripting attacks use known vulnerabilities in web-based applications, their servers, or the plug-in systems on which they rely. Exploiting one of these, attackers 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, to session cookies, and to a variety of other information maintained by the browser on behalf of the user. Cross-site scripting attacks are a case of code injection. Microsoft security-engineers introduced the term "cross-site scripting" in January 2000.[4] 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 (taking advantage of 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.[5]XSS vulnerabilities have been reported and exploited since the 1990s. Prominent sites affected in the past include the social-networking sites Twitter,[6] Facebook,[7]MySpace, YouTube and Orkut.[8][9] Cross-site scripting flaws have since surpassed buffer overflows to become the most common publicly reported security vulnerability,[10] with some researchers in 2007 estimating as many as 68% of websites are likely open to XSS attacks.[11]TypesThere is no single, standardized classification of cross-site scripting flaws, but most experts distinguish between at least two primary flavors of XSS flaws: 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 (reflected){{Quote box|width=30% |title=Example of a non-persistent XSS flaw |1=Non-persistent XSS vulnerabilities in Google could allow malicious sites to attack Google users who visit them while logged in.[12] }} The non-persistent (or reflected) cross-site scripting vulnerability is by far the most basic type of web vulnerability.[13] These holes show up when the data provided by a web client, most commonly in HTTP query parameters (e.g. HTML form submission), 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.[14] 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.[13][14] 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.[15] 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 (or stored){{Quote box| width = 30% | title = Example of a persistent XSS flaw | quote = 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.[16]| }} 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.[14] 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 <script> element, it won't be shown on the screen. Then suppose that Bob, a member of the dating site, reaches Mallory's profile, which has her answer to the First Date question. Her script is run automatically by the browser and steals a copy of Bob's real name and email directly from his own machine. Persistent XSS vulnerabilities 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 client-side worm.[17] 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, IM etc.) that can be controlled by an attacker could become an injection vector. Server-side versus DOM-based vulnerabilities{{Quote box|width=30% |title=Example of a DOM-based XSS flaw |Before the bug was resolved, Bugzilla error pages were open to DOM-based XSS attacks in which arbitrary HTML and scripts could be injected using forced error messages.[18] }} Historically XSS vulnerabilities were first found in applications that performed all data processing on the server side. User input (including an XSS vector) would be sent to the server, and then sent back to the user as a web page. The need for an improved user experience resulted in popularity of applications that had a majority of the presentation logic (maybe written in JavaScript) working on the client-side that pulled data, on-demand, from the server using AJAX. As the JavaScript code was also processing user input and rendering it in the web page content, a new sub-class of reflected XSS attacks started to appear that was called DOM-based cross-site scripting. In a DOM-based XSS attack, the malicious data does not touch the web server. Rather, it is being reflected by the JavaScript code, fully on the client side.[19] An example of a DOM-based XSS vulnerability is the bug found in 2011 in a number of JQuery plugins.[20] Prevention strategies for DOM-based XSS attacks include very similar measures to traditional XSS prevention strategies but implemented in JavaScript code and contained in web pages (i.e. input validation and escaping).[21] Some JavaScript frameworks have built-in countermeasures against this and other types of attack — for example Angular.js.[22] Self-XSSSelf-XSS is a form of XSS vulnerability which relies on Social Engineering in order to trick the victim into executing malicious JavaScript code into their browser. Although it is technically not a true XSS vulnerability due to the fact it relies on socially engineering a user into executing code rather than a flaw in the affected website allowing an attacker to do so, it still poses the same risks as a regular XSS vulnerability if properly executed.[23]Mutated XSS (mXSS)Mutated XSS happens when the attacker injects something that is seemingly safe, but rewritten and modified by the browser, while parsing the markup. This makes it extremely hard to detect or sanitize within the websites application logic. An example is rebalancing unclosed quotation marks or even adding quotation marks to unquoted parameters on parameters to CSS font-family. {{anchor|Exploit scenarios}}Exploit examplesAttackers 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 Alice-and-Bob cast of characters commonly used in computer security. The Browser Exploitation Framework could be used to attack the web site and the user's local environment. Non-persistent
Several things could have been done to mitigate this attack:
Persistent attack
Bob's website software should have stripped out the script tag or done something to make sure it didn't work, but the security bug is in the fact that he didn't. {{anchor|Mitigation|Reducing the threat}}Preventive measures{{manual|section|date=December 2014}}Contextual output encoding/escaping of string inputContextual output encoding/escaping could be used as the primary defense mechanism to stop XSS attacks. There are several escaping schemes that can be used depending on where the untrusted string needs to be placed within an HTML document including HTML entity encoding, JavaScript escaping, CSS escaping, and URL (or percent) encoding.[26] Most web applications that do not need to accept rich data can use escaping to largely eliminate the risk of XSS attacks in a fairly straightforward manner. Although widely recommended, performing HTML entity encoding only on the five XML significant characters is not always sufficient to prevent many forms of XSS attacks. As encoding is often difficult, security encoding libraries are usually easier to use.[26] Safely validating untrusted HTML inputMany operators of particular web applications (e.g. forums and webmail) allow users to utilize a limited subset of HTML markup. When accepting HTML input from users (say, <b>very</b> large), output encoding (such as <b>very</b> large) will not suffice since the user input needs to be rendered as HTML by the browser (so it shows as "very large", instead of "<b>very</b> large"). Stopping an XSS attack when accepting HTML input from users is much more complex in this situation. Untrusted HTML input must be run through an HTML sanitization engine to ensure that it does not contain XSS code. It should also be noted that many validations rely on parsing out (blacklisting) specific "at risk" HTML tags such as the following |