Official definition

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

There are many variants of cross-site scripting, characterized by a variety of terms or involving different attack topologies. However, they all indicate the same fundamental weakness: improper neutralization of dangerous input between the adversary and a victim.

The Same Origin Policy states that browsers should limit the resources accessible to scripts running on a given web site, or "origin", to the resources associated with that web site on the client-side, and not the client-side resources of any other sites or "origins". The goal is to prevent one site from being able to modify or read the contents of an unrelated site. Since the World Wide Web involves interactions between many sites, this policy is important for browsers to enforce.

When referring to XSS, the Domain of a website is roughly equivalent to the resources associated with that website on the client-side of the connection. That is, the domain can be thought of as all resources the browser is storing for the user's interactions with this particular site.

Detailed description

Cross-site scripting, commonly abbreviated as XSS, is a family of weaknesses with different delivery paths and attack topologies, but the same underlying failure: dangerous input is not properly neutralized between an adversary and a victim. It can occur when input is reflected directly from an HTTP request, stored in a trusted data store and later rendered, or inserted by a client-side application through the DOM. The browser's same-origin protections are intended to separate a site's client-side resources from those of other origins, but injected script can execute in the context of the affected site and gain access to resources available to that origin.

Characteristics

This is a base, simple, stable weakness introduced during implementation and is not specific to a programming language. Common forms include reflected or non-persistent XSS, stored or persistent XSS, and DOM-based XSS. Untrusted data may enter through parameters, cookies, network input, environment variables, reverse DNS results, query results, request headers, URL components, email, files, filenames, databases, external systems, or indirect API calls. XSS is common in web-based technology, but the record also identifies web servers and AI/ML technology as applicable platforms.

Common consequences

XSS can compromise confidentiality, integrity, availability, and access control. A script may read application data such as session information in cookies, perform actions on behalf of a victim, bypass protection mechanisms, expose confidential information, redirect users, alter page presentation, disclose end-user files, install Trojan horse programs, or run malicious code in combination with other flaws. Impact can range from annoyance to complete account compromise, with greater risk when the victim has administrative privileges; in some circumstances, arbitrary code may run on the victim's computer.

Impact	Scope	Explanation
Bypass Protection Mechanism, Read Application Data	Access Control, Confidentiality	The most common attack performed with cross-site scripting involves the disclosure of private information stored in user cookies, such as session information. Typically, a malicious user will craft a client-side script, which -- when parsed by a web browser -- performs some activity on behalf of the victim to an attacker-controlled system (such as sending all site cookies to a given E-mail address). This could be especially dangerous to the site if the victim has administrator privileges to manage that site. This script will be loaded and run by each user visiting the web site. Since the site requesting to run the script has access to the cookies in question, the malicious script does also.
Execute Unauthorized Code or Commands	Integrity, Confidentiality, Availability	In some circumstances it may be possible to run arbitrary code on a victim's computer when cross-site scripting is combined with other flaws, for example, "drive-by hacking."
Execute Unauthorized Code or Commands, Bypass Protection Mechanism, Read Application Data	Confidentiality, Integrity, Availability, Access Control	The consequence of an XSS attack is the same regardless of whether it is stored or reflected. The difference is in how the payload arrives at the server. XSS can cause a variety of problems for the end user that range in severity from an annoyance to complete account compromise. Some cross-site scripting vulnerabilities can be exploited to manipulate or steal cookies, create requests that can be mistaken for those of a valid user, compromise confidential information, or execute malicious code on the end user systems for a variety of nefarious purposes. Other damaging attacks include the disclosure of end user files, installation of Trojan horse programs, redirecting the user to some other page or site, running "Active X" controls (under Microsoft Internet Explorer) from sites that a user perceives as trustworthy, and modifying presentation of content.

Risk mitigations

Use context-appropriate output handling. Prefer vetted libraries or frameworks that provide safe output encoding constructs. Determine the required encoding for each output context, including the HTML body, element attributes, URIs, JavaScript sections, and CSS or style properties; HTML entity encoding is appropriate only for the HTML body. Specify an encoding that the downstream component will handle consistently, such as ISO-8859-1, UTF-7, or UTF-8, rather than allowing browsers or other components to guess.

Constrain and validate inputs. Treat all input as malicious and use strict allowlists based on the expected type, length, values, syntax, consistency, and business rules. Validate and cleanse all request data, including hidden fields, cookies, headers, URLs, and other fields not expected to be redisplayed; do not rely exclusively on denylists. Understand every possible input source, and use structured mechanisms that keep data separate from code. Where acceptable filenames or URLs come from a limited known set, map fixed input values such as numeric IDs to those objects and reject other values. Duplicate client-side security checks on the server. For Struts, set the bean filter attribute to true for form-bean data, and in PHP do not use register_globals or an unsafe emulation of it.

Add defense in depth. Set session cookies to HttpOnly where supported, recognizing that this does not prevent all XSS and is not supported by every browser. An application firewall can provide a temporary or additional control when code cannot immediately be fixed, but it may miss input vectors, be bypassed, or reject or modify legitimate requests.

Libraries or Frameworks · Architecture and DesignUse a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482]. Examples of libraries and frameworks that make it easier to generate properly encoded output include Microsoft's Anti-XSS library, the OWASP ESAPI Encoding module, and Apache Wicket.
Implementation, Architecture and DesignUnderstand the context in which your data will be used and the encoding that will be expected. This is especially important when transmitting data between different components, or when generating outputs that can contain multiple encodings at the same time, such as web pages or multi-part mail messages. Study all expected communication protocols and data representations to determine the required encoding strategies. For any data that will be output to another web page, especially any data that was received from external inputs, use the appropriate encoding on all non-alphanumeric characters. Parts of the same output document may require different encodings, which will vary depending on whether the output is in the: - HTML body - Element attributes (such as src="XYZ") - URIs - JavaScript sections - Cascading Style Sheets and style property etc. Note that HTML Entity Encoding is only appropriate for the HTML body. Consult the XSS Prevention Cheat Sheet [REF-724] for more details on the types of encoding and escaping that are needed.
Attack Surface Reduction · Architecture and Design, Implementation · Effectiveness: LimitedUnderstand all the potential areas where untrusted inputs can enter your software: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.This technique has limited effectiveness, but can be helpful when it is possible to store client state and sensitive information on the server side instead of in cookies, headers, hidden form fields, etc.
Architecture and DesignFor any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
Parameterization · Architecture and DesignIf available, use structured mechanisms that automatically enforce the separation between data and code. These mechanisms may be able to provide the relevant quoting, encoding, and validation automatically, instead of relying on the developer to provide this capability at every point where output is generated.
Output Encoding · ImplementationUse and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component. The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks.
ImplementationWith Struts, write all data from form beans with the bean's filter attribute set to true.
Attack Surface Reduction · Implementation · Effectiveness: Defense in DepthTo help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XmlHttpRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set.
Input Validation · ImplementationAssume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. When dynamically constructing web pages, use stringent allowlists that limit the character set based on the expected value of the parameter in the request. All input should be validated and cleansed, not just parameters that the user is supposed to specify, but all data in the request, including hidden fields, cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS vulnerabilities is to validate only fields that are expected to be redisplayed by the site. It is common to see data from the request that is reflected by the application server or the application that the development team did not anticipate. Also, a field that is not currently reflected may be used by a future developer. Therefore, validating ALL parts of the HTTP request is recommended. Note that proper output encoding, escaping, and quoting is the most effective solution for preventing XSS, although input validation may provide some defense-in-depth. This is because it effectively limits what will appear in output. Input validation will not always prevent XSS, especially if you are required to support free-form text fields that could contain arbitrary characters. For example, in a chat application, the heart emoticon ("<3") would likely pass the validation step, since it is commonly used. However, it cannot be directly inserted into the web page because it contains the "<" character, which would need to be escaped or otherwise handled. In this case, stripping the "<" might reduce the risk of XSS, but it would produce incorrect behavior because the emoticon would not be recorded. This might seem to be a minor inconvenience, but it would be more important in a mathematical forum that wants to represent inequalities. Even if you make a mistake in your validation (such as forgetting one out of 100 input fields), appropriate encoding is still likely to protect you from injection-based attacks. As long as it is not done in isolation, input validation is still a useful technique, since it may significantly reduce your attack surface, allow you to detect some attacks, and provide other security benefits that proper encoding does not address. Ensure that you perform input validation at well-defined interfaces within the application. This will help protect the application even if a component is reused or moved elsewhere.
Enforcement by Conversion · Architecture and DesignWhen the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.
Firewall · Operation · Effectiveness: ModerateUse an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth [REF-1481].An application firewall might not cover all possible input vectors. In addition, attack techniques might be available to bypass the protection mechanism, such as using malformed inputs that can still be processed by the component that receives those inputs. Depending on functionality, an application firewall might inadvertently reject or modify legitimate requests. Finally, some manual effort may be required for customization.
Environment Hardening · Operation, ImplementationWhen using PHP, configure the application so that it does not use register_globals. During implementation, develop the application so that it does not rely on this feature, but be wary of implementing a register_globals emulation that is subject to weaknesses such as CWE-95, CWE-621, and similar issues.

Detection methods

Use automated static analysis, including data-flow analysis, to identify flows from untrusted input to web output. This has only moderate effectiveness because complete accuracy and coverage are not feasible, especially across multiple components. Use black-box testing with the XSS Cheat Sheet or automated test-generation tools to exercise many attack variations; for stored XSS, testing must cover both injection into the data store and later delivery to another user, which may occur after a substantial delay. These approaches are useful but are not complete guarantees.

Method	Approach	Effectiveness
Automated Static Analysis	Use automated static analysis tools that target this type of weakness. Many modern techniques use data flow analysis to minimize the number of false positives. This is not a perfect solution, since 100% accuracy and coverage are not feasible, especially when multiple components are involved.	Moderate
Black Box	Use the XSS Cheat Sheet [REF-714] or automated test-generation tools to help launch a wide variety of attacks against your web application. The Cheat Sheet contains many subtle XSS variations that are specifically targeted against weak XSS defenses.With Stored XSS, the indirection caused by the data store can make it more difficult to find the problem. The tester must first inject the XSS string into the data store, then find the appropriate application functionality in which the XSS string is sent to other users of the application. These are two distinct steps in which the activation of the XSS can take place minutes, hours, or days after the XSS was originally injected into the data store.	Moderate

Representative vulnerabilities

The official record lists the following representative examples, not an exhaustive set: CVE-2024-49038 and CVE-2024-54142 involve XSS in AI-related functionality; CVE-2021-25926 and CVE-2021-25963 involve reflected XSS in Python software; and CVE-2021-1879 describes universal XSS in a mobile operating system. Other examples include XSS reached through a cookie (CVE-2014-8958), crafted HTTP headers such as Referer (CVE-2017-9764, CVE-2014-5198), PATH_INFO (CVE-2008-5770), an email message (CVE-2008-5734), error messages (CVE-2008-4730), wiki pages (CVE-2008-5249), guestbooks (CVE-2006-3568, CVE-2006-3211), and a security product (CVE-2008-0971). Chained examples include CVE-2020-3580, CVE-2008-5080, CVE-2006-4308, CVE-2007-5727, and CVE-2006-3295, where another weakness or protection failure leads to XSS.

Below are representative vulnerabilities related to this CWE, prioritized by severity.

Sources (20)

CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

What is CWE-79?

Data statistics

Vulnerabilities mapped to CWE-79

Vulnerabilities in CISA KEV for CWE-79