Failure to Preserve Web Page Structure ('Cross-site Scripting')
Weakness ID: 79 (Weakness Base)Status: Usable
+ Description

Description Summary

The software does not sufficiently validate, filter, escape, and encode user-controllable input before it is placed in output that is used as a web page that is served to other users.

Extended Description

Cross-site scripting (XSS) vulnerabilities occur when:

1. Untrusted data enters a web application, typically from a web request.

2. The web application dynamically generates a web page that contains this untrusted data.

3. During page generation, the application does not prevent the data from containing content that is executable by a web browser, such as JavaScript, HTML tags, HTML attributes, mouse events, Flash, ActiveX, etc.

4. A victim visits the generated web page through a web browser, which contains malicious script that was injected using the untrusted data.

5. Since the script comes from a web page that was sent by the web server, the victim's web browser executes the malicious script in the context of the web server's domain.

6. This effectively violates the intention of the web browser's same-origin policy, which states that scripts in one domain should not be able to access resources or run code in a different domain.

There are three main kinds of XSS:

Type 1: Reflected XSS (or Non-Persistent)

The server reads data directly from the HTTP request and reflects it back in the HTTP response. Reflected XSS exploits occur when an attacker causes a victim to supply dangerous content to a vulnerable web application, which is then reflected back to the victim and executed by the web browser. The most common mechanism for delivering malicious content is to include it as a parameter in a URL that is posted publicly or e-mailed directly to the victim. URLs constructed in this manner constitute the core of many phishing schemes, whereby an attacker convinces a victim to visit a URL that refers to a vulnerable site. After the site reflects the attacker's content back to the victim, the content is executed by the victim's browser.

Type 2: Stored XSS (or Persistent)

The application stores dangerous data in a database, message forum, visitor log, or other trusted data store. At a later time, the dangerous data is subsequently read back into the application and included in dynamic content. From an attacker's perspective, the optimal place to inject malicious content is in an area that is displayed to either many users or particularly interesting users. Interesting users typically have elevated privileges in the application or interact with sensitive data that is valuable to the attacker. If one of these users executes malicious content, the attacker may be able to perform privileged operations on behalf of the user or gain access to sensitive data belonging to the user. For example, the attacker might inject XSS into a log message, which might not be handled properly when an administrator views the logs.

Type 0: DOM-Based XSS

In DOM-based XSS, the client performs the injection of XSS into the page; in the other types, the server performs the injection. DOM-based XSS generally involves server-controlled, trusted script that is sent to the client, such as Javascript that performs sanity checks on a form before the user submits it. If the server-supplied script processes user-supplied data and then injects it back into the web page (such as with dynamic HTML), then DOM-based XSS is possible.

Once the malicious script is injected, the attacker can perform a variety of malicious activities. The attacker could transfer private information, such as cookies that may include session information, from the victim's machine to the attacker. The attacker could send malicious requests to a web site on behalf of the victim, which could be especially dangerous to the site if the victim has administrator privileges to manage that site. Phishing attacks could be used to emulate trusted web sites and trick the victim into entering a password, allowing the attacker to compromise the victim's account on that web site. Finally, the script could exploit a vulnerability in the web browser itself possibly taking over the victim's machine, sometimes referred to as "drive-by hacking."

In many cases, the attack can be launched without the victim even being aware of it. Even with careful users, attackers frequently use a variety of methods to encode the malicious portion of the attack, such as URL encoding or Unicode, so the request looks less suspicious.

+ Alternate Terms

"CSS" was once used as the acronym for this problem, but this could cause confusion with "Cascading Style Sheets," so usage of this acronym has declined significantly.

+ Time of Introduction
  • Architecture and Design
  • Implementation
+ Applicable Platforms



Architectural Paradigms

Web-based: (Often)

Technology Classes

Web-Server: (Often)

Platform Notes

XSS flaws are very common in web applications since they require a great deal of developer discipline to avoid them.

+ Common Consequences

The most common attack performed with cross-site scripting involves the disclosure of information stored in user cookies. Typically, a malicious user will craft a client-side script, which -- when parsed by a web browser -- performs some activity (such as sending all site cookies to a given E-mail address). 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.

Access Control

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.


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.

+ Likelihood of Exploit

High to Very High

+ Enabling Factors for Exploitation

Cross-site scripting attacks may occur anywhere that possibly malicious users are allowed to post unregulated material to a trusted web site for the consumption of other valid users, commonly on places such as bulletin-board web sites which provide web based mailing list-style functionality.

Stored XSS got its start with web sites that offered a "guestbook" to visitors. Attackers would include JavaScript in their guestbook entries, and all subsequent visitors to the guestbook page would execute the malicious code. As the examples demonstrate, XSS vulnerabilities are caused by code that includes unvalidated data in an HTTP response.

+ Detection Methods

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.

Effectiveness: Moderate

Black Box

Use the XSS Cheat Sheet [REF-14] 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.

Effectiveness: Moderate

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.

+ Demonstrative Examples

Example 1

This example covers a Reflected XSS (Type 1) scenario.

The following JSP code segment reads an employee ID, eid, from an HTTP request and displays it to the user.

(Bad Code)
Example Language: JSP 
<% String eid = request.getParameter("eid"); %>
Employee ID: <%= eid %>

The following ASP.NET code segment reads an employee ID number from an HTTP request and displays it to the user.

(Bad Code)
Example Language: ASP.NET 
protected System.Web.UI.WebControls.TextBox Login;
protected System.Web.UI.WebControls.Label EmployeeID;
EmployeeID.Text = Login.Text;
... (HTML follows) ...
<p><asp:label id="EmployeeID" runat="server" /></p>

The code in this example operates correctly if the Employee ID variable contains only standard alphanumeric text. If it has a value that includes meta-characters or source code, then the code will be executed by the web browser as it displays the HTTP response. Initially this might not appear to be much of a vulnerability. After all, why would someone enter a URL that causes malicious code to run on their own computer? The real danger is that an attacker will create the malicious URL, then use e-mail or social engineering tricks to lure victims into visiting a link to the URL. When victims click the link, they unwittingly reflect the malicious content through the vulnerable web application back to their own computers.

Example 2

This example covers a Stored XSS (Type 2) scenario.

The following JSP code segment queries a database for an employee with a given ID and prints the corresponding employee's name.

(Bad Code)
Example Language: JSP 
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("select * from emp where id="+eid);
if (rs != null) {;
String name = rs.getString("name");

Employee Name: <%= name %>

The following ASP.NET code segment queries a database for an employee with a given employee ID and prints the name corresponding with the ID.

(Bad Code)
Example Language: ASP.NET 
protected System.Web.UI.WebControls.Label EmployeeName;
string query = "select * from emp where id=" + eid;
sda = new SqlDataAdapter(query, conn);
string name = dt.Rows[0]["Name"];
EmployeeName.Text = name;

This code can appear less dangerous because the value of name is read from a database, whose contents are apparently managed by the application. However, if the value of name originates from user-supplied data, then the database can be a conduit for malicious content. Without proper input validation on all data stored in the database, an attacker can execute malicious commands in the user's web browser.

+ Observed Examples
CVE-2008-5080Chain: protection mechanism failure allows XSS
CVE-2006-4308Chain: only checks "javascript:" tag
CVE-2007-5727Chain: only removes SCRIPT tags, enabling XSS
CVE-2008-5770Reflected XSS using the PATH INFO in a URL
CVE-2008-4730Reflected XSS not properly handled when generating an error message
CVE-2008-5734Reflected XSS sent through email message.
CVE-2008-0971Stored XSS in a security product.
CVE-2008-5249Stored XSS using a wiki page.
CVE-2006-3568Stored XSS in a guestbook application.
CVE-2006-3211Stored XSS in a guestbook application using a javascript: URI in a bbcode img tag.
CVE-2006-3295Chain: library file is not protected against a direct request (CWE-425), leading to reflected XSS.
+ Potential Mitigations

Phase: Architecture and Design

Use languages, libraries, or frameworks that make it easier to generate properly encoded output.

Examples include Microsoft's Anti-XSS library, the OWASP ESAPI Encoding module, and Apache Wicket.

Phases: Implementation; Architecture and Design

Understand 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.html")

  • 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-16] for more details on the types of encoding and escaping that are needed.

Phase: Architecture and Design

For 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.

Phase: Implementation

Use and specify a strong character encoding such as ISO-8859-1 or UTF-8. When an encoding is not specified, the web browser may choose a different encoding by guessing which encoding is actually being used by the web page. This can open you up to subtle XSS attacks related to that encoding. See CWE-116 for more mitigations related to encoding/escaping.

Phase: Implementation

With Struts, you should write all data from form beans with the bean's filter attribute set to true.

Phase: Implementation

To 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.

Phase: Implementation

Strategy: Input Validation

Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a whitelist 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. Do not rely exclusively on looking for malicious or malformed inputs (i.e., do not rely on a blacklist). However, blacklists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.

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 you are expecting colors such as "red" or "blue."

When dynamically constructing web pages, use stringent whitelists 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.

Phase: Operation

Use an application firewall that can detect attacks against this weakness. This might not catch all attacks, and it might require some effort for customization. However, 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.

+ Background Details

Same Origin Policy

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.


The Domain of a website when referring to XSS 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.

+ Weakness Ordinalities
(where the weakness is typically related to the presence of some other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)Named Chain(s) this relationship pertains toChain(s)
ChildOfWeakness ClassWeakness Class20Improper Input Validation
Seven Pernicious Kingdoms (primary)700
ChildOfWeakness ClassWeakness Class74Failure to Sanitize Data into a Different Plane ('Injection')
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory442Web Problems
Development Concepts699
ChildOfCategoryCategory712OWASP Top Ten 2007 Category A1 - Cross Site Scripting (XSS)
Weaknesses in OWASP Top Ten (2007) (primary)629
ChildOfCategoryCategory722OWASP Top Ten 2004 Category A1 - Unvalidated Input
Weaknesses in OWASP Top Ten (2004)711
ChildOfCategoryCategory725OWASP Top Ten 2004 Category A4 - Cross-Site Scripting (XSS) Flaws
Weaknesses in OWASP Top Ten (2004) (primary)711
ChildOfCategoryCategory7512009 Top 25 - Insecure Interaction Between Components
Weaknesses in the 2009 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)750
ChildOfCategoryCategory8012010 Top 25 - Insecure Interaction Between Components
Weaknesses in the 2010 CWE/SANS Top 25 Most Dangerous Programming Errors (primary)800
CanPrecedeWeakness BaseWeakness Base494Download of Code Without Integrity Check
Research Concepts1000
PeerOfCompound Element: CompositeCompound Element: Composite352Cross-Site Request Forgery (CSRF)
Research Concepts1000
ParentOfWeakness VariantWeakness Variant80Improper Sanitization of Script-Related HTML Tags in a Web Page (Basic XSS)
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness VariantWeakness Variant81Improper Sanitization of Script in an Error Message Web Page
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness VariantWeakness Variant83Failure to Sanitize Script in Attributes in a Web Page
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness VariantWeakness Variant84Failure to Resolve Encoded URI Schemes in a Web Page
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness VariantWeakness Variant85Doubled Character XSS Manipulations
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness VariantWeakness Variant86Failure to Sanitize Invalid Characters in Identifiers in Web Pages
Development Concepts (primary)699
Research Concepts (primary)1000
ParentOfWeakness VariantWeakness Variant87Failure to Sanitize Alternate XSS Syntax
Development Concepts (primary)699
Research Concepts (primary)1000
MemberOfViewView635Weaknesses Used by NVD
Weaknesses Used by NVD (primary)635
CanFollowWeakness BaseWeakness Base113Failure to Sanitize CRLF Sequences in HTTP Headers ('HTTP Response Splitting')
Research Concepts1000
CanFollowWeakness BaseWeakness Base184Incomplete Blacklist
Research Concepts1000
Incomplete Blacklist to Cross-Site Scripting692
+ Causal Nature


+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERCross-site scripting (XSS)
7 Pernicious KingdomsCross-site Scripting
CLASPCross-site scripting
OWASP Top Ten 2007A1ExactCross Site Scripting (XSS)
OWASP Top Ten 2004A1CWE More SpecificUnvalidated Input
OWASP Top Ten 2004A4ExactCross-Site Scripting (XSS) Flaws
WASC8Cross-site Scripting
+ Related Attack Patterns
CAPEC-IDAttack Pattern Name
(CAPEC Version: 1.4)
85Client Network Footprinting (using AJAX/XSS)
86Embedding Script (XSS ) in HTTP Headers
32Embedding Scripts in HTTP Query Strings
19Embedding Scripts within Scripts
91XSS in IMG Tags
+ References
[REF-15] Jeremiah Grossman, Robert "RSnake" Hansen, Petko "pdp" D. Petkov, Anton Rager and Seth Fogie. "XSS Attacks". Syngress. 2007.
[REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 2: Web-Server Related Vulnerabilities (XSS, XSRF, and Response Splitting)." Page 31. McGraw-Hill. 2010.
[REF-17] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 3: Web-Client Related Vulnerabilities (XSS)." Page 63. McGraw-Hill. 2010.
"Cross-site scripting". Wikipedia. 2008-08-26. <>.
[REF-11] M. Howard and D. LeBlanc. "Writing Secure Code". Chapter 13, "Web-Specific Input Issues" Page 413. 2nd Edition. Microsoft. 2002.
[REF-14] RSnake. "XSS (Cross Site Scripting) Cheat Sheet". <>.
Microsoft. "Mitigating Cross-site Scripting With HTTP-only Cookies". <>.
Mark Curphey, Microsoft. "Anti-XSS 3.0 Beta and CAT.NET Community Technology Preview now Live!". <>.
"OWASP Enterprise Security API (ESAPI) Project". <>.
Ivan Ristic. "XSS Defense HOWTO". <>.
OWASP. "Web Application Firewall". <>.
Web Application Security Consortium. "Web Application Firewall Evaluation Criteria". <>.
RSnake. "Firefox Implements httpOnly And is Vulnerable to XMLHTTPRequest". 2007-07-19.
"XMLHttpRequest allows reading HTTPOnly cookies". Mozilla. <>.
"Apache Wicket". <>.
[REF-16] OWASP. "XSS (Cross Site Scripting) Prevention Cheat Sheet". <>.
+ Content History
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Time of Introduction
Suggested OWASP Top Ten 2004 mapping
2008-09-08CWE Content TeamMITREInternal
updated Alternate Terms, Applicable Platforms, Background Details, Common Consequences, Description, Relationships, Other Notes, References, Taxonomy Mappings, Weakness Ordinalities
2009-01-12CWE Content TeamMITREInternal
updated Alternate Terms, Applicable Platforms, Background Details, Common Consequences, Demonstrative Examples, Description, Detection Factors, Enabling Factors for Exploitation, Name, Observed Examples, Other Notes, Potential Mitigations, References, Relationships
2009-03-10CWE Content TeamMITREInternal
updated Potential Mitigations
2009-05-27CWE Content TeamMITREInternal
updated Name
2009-07-27CWE Content TeamMITREInternal
updated Description
2009-10-29CWE Content TeamMITREInternal
updated Observed Examples, Relationships
2009-12-28CWE Content TeamMITREInternal
updated Demonstrative Examples, Description, Detection Factors, Enabling Factors for Exploitation, Observed Examples
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Cross-site Scripting (XSS)
2009-01-12Failure to Sanitize Directives in a Web Page (aka 'Cross-site scripting' (XSS))
2009-05-27Failure to Preserve Web Page Structure (aka 'Cross-site Scripting')