Compound Element ID: 692 (Compound Element Base: Chain)
Status: Draft
Description
Description Summary
The product uses a blacklist-based protection mechanism to defend against XSS attacks, but the blacklist is incomplete, allowing XSS variants to succeed.
Applicable Platforms
Languages
C
C++
All
Common Consequences
Scope
Effect
Confidentiality
Integrity
Availability
Technical Impact: Execute unauthorized code or
commands
While XSS might seem simple to prevent, web browsers vary so widely in how
they parse web pages, that a blacklist cannot keep track of all the
variations. The "XSS Cheat Sheet" (see references) contains a large number
of attacks that are intended to bypass incomplete blacklists.
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Definition in a New Window
Weakness ID: 79 (Weakness Base)
Status: Usable
Description
Description Summary
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.
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
XSS
CSS:
"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
Languages
Language-independent
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
Scope
Effect
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
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 [R.79.6] 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 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.
Because the parameter can be arbitrary, the url of the page could be
modified so $username contains scripting syntax, such as
(Attack)
http://trustedSite.example.com/welcome.php?username=<Script
Language="Javascript">alert("You've been
attacked!");</Script>
This results in a harmless alert dialogue 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 his password to the
attacker:
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):
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.
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.
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;
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.
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)
$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:';
The attacker can set his 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.
Chain: library file is not protected against a direct request (CWE-425), leading to reflected XSS.
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 [R.79.16] for more details on the types of encoding and escaping that are needed.
Phases: Architecture and Design; Implementation
Strategy: Identify and Reduce Attack Surface
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
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
For every web page that is generated, use and specify a 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 cause the web browser to treat certain sequences as special, opening up the client to subtle XSS attacks. 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
Strategy: Identify and Reduce Attack Surface
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 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: 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
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
If you are using PHP, configure your application so that it does not use register_globals. During implementation, develop your 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.
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.
Weakness Ordinalities
Ordinality
Description
Resultant
(where
the weakness is typically related to the presence of some other
weaknesses)
[R.79.1] [REF-15] Jeremiah Grossman, Robert "RSnake" Hansen, Petko "pdp" D. Petkov, Anton Rager
and Seth Fogie. "XSS Attacks". Syngress. 2007.
[R.79.2] [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.
[R.79.3] [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.
"\" not in blacklist for web server, allowing path
traversal attacks when the server is run in Windows and other OSes.
Potential Mitigations
Ensure black list covers all inappropriate content outlined in the
Common Weakness Enumeration.
Combine use of black list with appropriate use of white lists.
Do not rely exclusively on blacklist validation to detect malicious
input or to encode output. There are too many variants to encode a
character; you're likely to miss some variants.
Weakness Ordinalities
Ordinality
Description
Primary
(where
the weakness exists independent of other weaknesses)
An incomplete blacklist frequently produces resultant weaknesses.
Some incomplete blacklist issues might arise from multiple interpretation
errors, e.g. a blacklist for dangerous shell metacharacters might not
include a metacharacter that only has meaning in one particular shell, not
all of them; or a blacklist for XSS manipulations might ignore an unusual
construct that's supported by one web browser, but not others.
Description
