Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Stable Base
Structure: Simple
Description
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.
Common Consequences 3
Scope: Access ControlConfidentiality
Impact: Bypass Protection MechanismRead Application Data
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.
Scope: IntegrityConfidentialityAvailability
Impact: Execute Unauthorized Code or Commands
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."
Scope: ConfidentialityIntegrityAvailabilityAccess Control
Impact: Execute Unauthorized Code or CommandsBypass Protection MechanismRead Application Data
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.
Detection Methods 2
Automated Static AnalysisModerate
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.Black BoxModerate
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.Potential Mitigations 12
Phase: Architecture and Design
Strategy: Libraries or Frameworks
Use 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.
Phase: ImplementationArchitecture 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")
- 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.
Phase: Architecture and DesignImplementation
Strategy: Attack Surface Reduction
Understand 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.
Effectiveness: Limited
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 Client-Side Enforcement of Server-Side Security. 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: Architecture and Design
Strategy: Parameterization
If 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.
Phase: Implementation
Strategy: Output Encoding
Use 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.
Phase: Implementation
With Struts, write all data from form beans with the bean's filter attribute set to true.
Phase: Implementation
Strategy: Attack Surface Reduction
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.
Effectiveness: Defense in Depth
Phase: Implementation
Strategy: Input Validation
Assume 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.
Phase: Architecture and Design
Strategy: Enforcement by Conversion
When 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.
Phase: Operation
Strategy: Firewall
Use 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].
Effectiveness: Moderate
Phase: OperationImplementation
Strategy: Environment Hardening
When 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 Improper Neutralization of Directives in Dynamically Evaluated Code ('Eval Injection'), Variable Extraction Error, and similar issues.
Demonstrative Examples 5
The following code displays a welcome message on a web page based on the HTTP GET username parameter (covers a Reflected XSS (Type 1) scenario).
Code Example:
Bad
PHP
phpBecause the parameter can be arbitrary, the url of the page could be modified so $username contains scripting syntax, such as
Code Example:
Attack
bashThis results in a harmless alert dialog popping up. 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.
More realistically, the attacker can embed a fake login box on the page, tricking the user into sending the user's password to the attacker:
Code Example:
Attack
bashIf a user clicks on this link then Welcome.php will generate the following HTML and send it to the user's browser:
Code Example:
Result
bashThe trustworthy domain of the URL may falsely assure the user that it is OK to follow the link. However, an astute user may notice the suspicious text appended to the URL. An attacker may further obfuscate the URL (the following example links are broken into multiple lines for readability):
Code Example:
Attack
bashThe same attack string could also be obfuscated as:
Code Example:
Attack
bashBoth of these attack links will result in the fake login box appearing on the page, and users are more likely to ignore indecipherable text at the end of URLs.
The following code displays 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.
Code Example:
Bad
JSP
jspThe following ASP.NET code segment reads an employee ID number from an HTTP request and displays it to the user.
Code Example:
Bad
ASP.NET
asp.netThe 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.
The following code displays 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.
Code Example:
Bad
JSP
jspThe following ASP.NET code segment queries a database for an employee with a given employee ID and prints the name corresponding with the ID.
Code Example:
Bad
ASP.NET
asp.netThis 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.
The following code consists of two separate pages in a web application, one devoted to creating user accounts and another devoted to listing active users currently logged in. It also displays a Stored XSS (Type 2) scenario.
CreateUser.php
Code Example:
Bad
PHP
phpThe code is careful to avoid a SQL injection attack (Improper Neutralization of Special Elements used in an SQL Command ('SQL Injection')) but does not stop valid HTML from being stored in the database. This can be exploited later when ListUsers.php retrieves the information:
ListUsers.php
Code Example:
Bad
PHP
php//Print list of users to page* echo '
Currently Active Users:';
while ($row = mysql_fetch_assoc($results)) {
```
echo '
';
'.$row['fullname'].'
';
}
echo 'The attacker can set their name to be arbitrary HTML, which will then be displayed to all visitors of the Active Users page. This HTML can, for example, be a password stealing Login message.
The following code is a simplistic message board that saves messages in HTML format and appends them to a file. When a new user arrives in the room, it makes an announcement:
Code Example:
Bad
PHP
php//save HTML-formatted message to file; implementation details are irrelevant for this example.* saveMessage($announceStr);
An attacker may be able to perform an HTML injection (Type 2 XSS) attack by setting a cookie to a value like:
Code Example:
Attack
bashThe raw contents of the message file would look like:
Code Example:
Result
bashFor each person who visits the message page, their browser would execute the script, generating a pop-up window that says "Hacked". More malicious attacks are possible; see the rest of this entry.
Observed Examples 20
CVE-2024-49038XSS in AI assistant
CVE-2024-54142Plugin that enables AI features allows input with html entities, leading to XSS
CVE-2021-25926Python Library Manager did not sufficiently neutralize a user-supplied search term, allowing reflected XSS.
CVE-2021-25963Python-based e-commerce platform did not escape returned content on error pages, allowing for reflected Cross-Site Scripting attacks.
CVE-2021-1879Universal XSS in mobile operating system, as exploited in the wild per CISA KEV.
CVE-2020-3580Chain: improper input validation (Improper Input Validation) in firewall product leads to XSS (Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')), as exploited in the wild per CISA KEV.
CVE-2014-8958Admin GUI allows XSS through cookie.
CVE-2017-9764Web stats program allows XSS through crafted HTTP header.
CVE-2014-5198Web log analysis product allows XSS through crafted HTTP Referer header.
CVE-2008-5080Chain: protection mechanism failure allows XSS
CVE-2006-4308Chain: incomplete denylist (Incomplete List of Disallowed Inputs) only checks "javascript:" tag, allowing XSS (Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) using other tags
CVE-2007-5727Chain: incomplete denylist (Incomplete List of Disallowed Inputs) only removes SCRIPT tags, enabling XSS (Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'))
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 (Direct Request ('Forced Browsing')), leading to reflected XSS (Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')).
References 23
XSS Attacks
Jeremiah Grossman, Robert "RSnake" Hansen, Petko "pdp" D. Petkov, Anton Rager, and Seth Fogie
Syngress
2007
ID: REF-709
24 Deadly Sins of Software Security
Michael Howard, David LeBlanc, and John Viega
McGraw-Hill
2010
ID: REF-44
24 Deadly Sins of Software Security
Michael Howard, David LeBlanc, and John Viega
McGraw-Hill
2010
ID: REF-44
Cross-site scripting
Wikipedia
26-08-2008
ID: REF-712
Writing Secure Code
Michael Howard and David LeBlanc
Microsoft Press
04-12-2002
ID: REF-7
Mitigating Cross-site Scripting With HTTP-only Cookies
Microsoft
https://learn.microsoft.com/en-us/previous-versions//ms533046(v=vs.85)?redirectedfrom=MSDN(2023-04-07)
ID: REF-715
Anti-XSS 3.0 Beta and CAT.NET Community Technology Preview now Live!
Mark Curphey, Microsoft
ID: REF-716
OWASP Enterprise Security API (ESAPI) Project
OWASP
ID: REF-45
XSS Defense HOWTO
Ivan Ristic
ID: REF-718
Web Application Firewall Evaluation Criteria
Web Application Security Consortium
http://projects.webappsec.org/w/page/13246985/Web%20Application%20Firewall%20Evaluation%20Criteria(2023-04-07)
ID: REF-720
Firefox Implements httpOnly And is Vulnerable to XMLHTTPRequest
RSnake
19-07-2007
ID: REF-721
XMLHttpRequest allows reading HTTPOnly cookies
Mozilla
ID: REF-722
XSS (Cross Site Scripting) Prevention Cheat Sheet
OWASP
ID: REF-724
DOM based XSS Prevention Cheat Sheet
OWASP
ID: REF-725
Top 25 series - Rank 1 - Cross Site Scripting
Jason Lam
SANS Software Security Institute
22-02-2010
ID: REF-726
The Art of Software Security Assessment
Mark Dowd, John McDonald, and Justin Schuh
Addison Wesley
2006
ID: REF-62
Samy (computer worm)
Wikipedia
ID: REF-956
Automated Source Code Security Measure (ASCSM)
Object Management Group (OMG)
01-2016
ID: REF-962
D3FEND: Application Layer Firewall
D3FEND
ID: REF-1481
D3FEND: D3-TL Trusted Library
D3FEND
ID: REF-1482
Likelihood of Exploit
High
Applicable Platforms
Languages:
Not Language-Specific : Undetermined
Technologies:
AI/ML : UndeterminedWeb Based : Often
Modes of Introduction
Implementation
Related Attack Patterns
Alternate Terms
XSS
A common abbreviation for Cross-Site Scripting.HTML Injection
Used as a synonym of stored (Type 2) XSS.Reflected XSS / Non-Persistent XSS / Type 1 XSS
Used when a server application reads data directly from the HTTP request and reflects it back in the HTTP response.Stored XSS / Persistent XSS / Type 2 XSS
Used when a server-side 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.DOM-Based XSS / Type 0 XSS
Used when a client-side application performs the injection of XSS into the page by manipulating the Domain Object Model (DOM).CSS
In the early years after initial discovery of XSS, "CSS" was a commonly-used acronym. However, this would cause confusion with "Cascading Style Sheets," so usage of this acronym has declined significantly.Related Weaknesses
Taxonomy Mapping
- PLOVER
- 7 Pernicious Kingdoms
- CLASP
- OWASP Top Ten 2007
- OWASP Top Ten 2004
- OWASP Top Ten 2004
- WASC
- Software Fault Patterns
- OMG ASCSM
Notes
Other
The attack methods for XSS can vary depending on the type of XSS and the attacker’s goal.
Reflected XSS exploits (Type 1)
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.
In a
Stored XSS exploit (Type 2)
, 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.
DOM-based XSS (Type 0)
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.
Other
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. 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.
Other
Cross-site scripting (XSS) vulnerabilities occur when:
1. Untrusted data enters a web application, typically from a web request.
1. The web application dynamically generates a web page that contains this untrusted data.
1. 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.
1. A victim visits the generated web page through a web browser, which contains malicious script that was injected using the untrusted data.
1. 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.
1. 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.
Relationship
There can be a close relationship between XSS and CSRF (Cross-Site Request Forgery (CSRF)). An attacker might use CSRF in order to trick the victim into submitting requests to the server in which the requests contain an XSS payload. A well-known example of this was the Samy worm on MySpace [REF-956]. The worm used XSS to insert malicious HTML sequences into a user's profile and add the attacker as a MySpace friend. MySpace friends of that victim would then execute the payload to modify their own profiles, causing the worm to propagate exponentially. Since the victims did not intentionally insert the malicious script themselves, CSRF was a root cause.
Applicable Platform
XSS flaws are very common in web applications, since they require a great deal of developer discipline to avoid them.