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

Common consequences

• Access ControlConfidentiality

  Bypass Protection Mechanism; Read 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...

• 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, for example, drive-by hacking.

• ConfidentialityIntegrityAvailabilityAccess Control

  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...

Potential mitigations

• Architecture and DesignLibraries 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...

• Architecture and DesignImplementationAttack Surface ReductionEffectiveness: Limited

  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.

• 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 DesignParameterization

  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.

• ImplementationOutput 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...

• Implementation

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

• ImplementationAttack Surface ReductionEffectiveness: Defense in Depth

  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...

MITRE lists 4 more mitigations on the official entry.

Related weaknesses

CAPEC attack patterns that exploit it

• CAPEC-63Cross-Site Scripting (XSS)
• CAPEC-85AJAX Footprinting
• CAPEC-209XSS Using MIME Type Mismatch
• CAPEC-588DOM-Based XSS
• CAPEC-591Reflected XSS
• CAPEC-592Stored XSS