Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')ID: 79 | Date: (C)2012-05-14 (M)2022-10-10 | Type: weakness | Status: USABLE | Abstraction Type: Base |
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 DescriptionCross-site scripting (XSS) vulnerabilities occur when:1. Untrusted data enters a web application, typically from a web
request.2. The web application dynamically generates a web page that contains
this untrusted data.3. During page generation, the application does not prevent the data
from containing content that is executable by a web browser, such as
JavaScript, HTML tags, HTML attributes, mouse events, Flash, ActiveX,
etc.4. A victim visits the generated web page through a web browser, which
contains malicious script that was injected using the untrusted
data.5. Since the script comes from a web page that was sent by the web
server, the victim's web browser executes the malicious script in the
context of the web server's domain.6. This effectively violates the intention of the web browser's
same-origin policy, which states that scripts in one domain should not
be able to access resources or run code in a different domain.There are three main kinds of XSS:Type 1: Reflected XSS (or Non-Persistent)The server reads data directly from the HTTP request and reflects it
back in the HTTP response. Reflected XSS exploits occur when an attacker
causes a victim to supply dangerous content to a vulnerable web
application, which is then reflected back to the victim and executed by
the web browser. The most common mechanism for delivering malicious
content is to include it as a parameter in a URL that is posted publicly
or e-mailed directly to the victim. URLs constructed in this manner
constitute the core of many phishing schemes, whereby an attacker
convinces a victim to visit a URL that refers to a vulnerable site.
After the site reflects the attacker's content back to the victim, the
content is executed by the victim's browser.Type 2: Stored XSS (or Persistent)The application stores dangerous data in a database, message forum,
visitor log, or other trusted data store. At a later time, the dangerous
data is subsequently read back into the application and included in
dynamic content. From an attacker's perspective, the optimal place to
inject malicious content is in an area that is displayed to either many
users or particularly interesting users. Interesting users typically
have elevated privileges in the application or interact with sensitive
data that is valuable to the attacker. If one of these users executes
malicious content, the attacker may be able to perform privileged
operations on behalf of the user or gain access to sensitive data
belonging to the user. For example, the attacker might inject XSS into a
log message, which might not be handled properly when an administrator
views the logs.Type 0: DOM-Based XSSIn 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. Enabling Factors for ExploitationCross-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. Likelihood of Exploit: High to Very High Applicable PlatformsLanguage Class: Language-independentArchitectural Paradigm: Web-basedArchitectural Paradigm: OftenTechnology Class: OftenTechnology Class: Web-Server Time Of Introduction - Architecture and Design
- Implementation
Related Attack Patterns Common Consequences Scope | Technical Impact | Notes |
---|
Access_ControlConfidentiality | Bypass protection
mechanismRead 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. | IntegrityConfidentialityAvailability | 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. | ConfidentialityIntegrityAvailabilityAccess_Control | 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 Name | Description | Effectiveness | Notes |
---|
Automated Static Analysis | Use automated static analysis tools that target this type of weakness.
Many modern techniques use data flow analysis to minimize the number of
false positives. This is not a perfect solution, since 100% accuracy and
coverage are not feasible, especially when multiple components are
involved. | Moderate | | Black Box | Use the XSS Cheat Sheet [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. | Moderate | |
Potential Mitigations Phase | Strategy | Description | Effectiveness | Notes |
---|
Architecture and Design | 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. | | | 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: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. | | | Architecture and DesignImplementation | 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. | 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. | 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. | | | Architecture and Design | 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. | | | Implementation | 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. | | | Implementation | | With Struts, write all data from form beans with the bean's filter
attribute set to true. | | | Implementation | 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. | Defense in Depth | | Implementation | 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.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
(i.e., do not rely on a blacklist). A blacklist 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, blacklists 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 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. | | | Architecture and Design | 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. | | | Operation | 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. | 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. | OperationImplementation | 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. | | |
Relationships Related CWE | Type | View | Chain |
---|
CWE-79 ChildOf CWE-896 | Category | CWE-888 | |
Demonstrative Examples (Details) - 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.
- 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.
- This example also displays a Reflected XSS (Type 1)
scenario.
- This example covers a Stored XSS (Type 2) scenario.
Observed Examples - CVE-2008-5080 : Chain: protection mechanism failure allows XSS
- CVE-2006-4308 : Chain: only checks "javascript:" tag
- CVE-2007-5727 : Chain: only removes SCRIPT tags, enabling XSS
- CVE-2008-5770 : Reflected XSS using the PATH_INFO in a URL
- CVE-2008-4730 : Reflected XSS not properly handled when generating an error message
- CVE-2008-5734 : Reflected XSS sent through email message.
- CVE-2008-0971 : Stored XSS in a security product.
- CVE-2008-5249 : Stored XSS using a wiki page.
- CVE-2006-3568 : Stored XSS in a guestbook application.
- CVE-2006-3211 : Stored XSS in a guestbook application using a javascript: URI in a bbcode img tag.
- CVE-2006-3295 : Chain: library file is not protected against a direct request (CWE-425), leading to reflected XSS.
For more examples, refer to CVE relations in the bottom box. White Box Definitions None Black Box Definitions None Taxynomy Mappings Taxynomy | Id | Name | Fit |
---|
PLOVER | | Cross-site scripting (XSS) | | 7 Pernicious Kingdoms | | Cross-site Scripting | | CLASP | | Cross-site scripting | | OWASP Top Ten 2007 | A1 | Cross Site Scripting (XSS) | Exact | OWASP Top Ten 2004 | A1 | Unvalidated Input | CWE_More_Specific | OWASP Top Ten 2004 | A4 | Cross-Site Scripting (XSS) Flaws | Exact | WASC | 8 | Cross-site Scripting | |
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