CWE

Common Weakness Enumeration

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CWE-80: Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)

 
Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)
Weakness ID: 80 (Weakness Variant)Status: Incomplete
+ Description

Description Summary

The software receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special characters such as "<", ">", and "&" that could be interpreted as web-scripting elements when they are sent to a downstream component that processes web pages.

Extended Description

This may allow such characters to be treated as control characters, which are executed client-side in the context of the user's session. Although this can be classified as an injection problem, the more pertinent issue is the improper conversion of such special characters to respective context-appropriate entities before displaying them to the user.

+ Time of Introduction
  • Implementation
+ Applicable Platforms

Languages

All

+ Common Consequences
ScopeEffect
Confidentiality
Integrity
Availability

Technical Impact: Read application data; Execute unauthorized code or commands

+ Likelihood of Exploit

High to Very High

+ Demonstrative Examples

Example 1

In the following example, a guestbook comment isn't properly encoded, filtered, or otherwise neutralized for script-related tags before being displayed in a client browser.

(Bad Code)
Example Language: JSP 
<% for (Iterator i = guestbook.iterator(); i.hasNext(); ) {
Entry e = (Entry) i.next(); %>
<p>Entry #<%= e.getId() %></p>
<p><%= e.getText() %></p>
<%
} %>
+ Observed Examples
ReferenceDescription
CVE-2002-0938XSS in parameter in a link.
CVE-2002-1495XSS in web-based email product via attachment filenames.
CVE-2003-1136HTML injection in posted message.
CVE-2004-2171XSS not quoted in error page.
+ Potential Mitigations

Phase: Implementation

Carefully check each input parameter against a rigorous positive specification (white list) defining the specific characters and format allowed. All input should be neutralized, 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. We often encounter 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.

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: 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

+ Weakness Ordinalities
OrdinalityDescription
Primary
(where the weakness exists independent of other weaknesses)
+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness BaseWeakness Base79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Development Concepts (primary)699
Research Concepts (primary)1000
ChildOfCategoryCategory896SFP Cluster: Tainted Input
Software Fault Pattern (SFP) Clusters (primary)888
MemberOfViewView630Weaknesses Examined by SAMATE
Weaknesses Examined by SAMATE (primary)630
+ Causal Nature

Explicit

+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERBasic XSS
+ White Box Definitions

A weakness where the code path has:

1. start statement that accepts input from HTML page

2. end statement that publishes a data item to HTML where

a. the input is part of the data item and

b. the input contains XSS syntax

+ Content History
Submissions
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modifications
Modification DateModifierOrganizationSource
2008-07-01Sean EidemillerCigitalExternal
added/updated demonstrative examples
2008-07-01Eric DalciCigitalExternal
updated Time_of_Introduction
2008-08-01KDM AnalyticsExternal
added/updated white box definitions
2008-09-08CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings, Weakness_Ordinalities
2008-10-14CWE Content TeamMITREInternal
updated Description
2009-05-27CWE Content TeamMITREInternal
updated Demonstrative_Examples, Description, Name
2009-07-17KDM AnalyticsExternal
Improved the White_Box_Definition
2009-07-27CWE Content TeamMITREInternal
updated White_Box_Definitions
2010-06-21CWE Content TeamMITREInternal
updated Demonstrative_Examples, Description, Name, Potential_Mitigations
2011-03-29CWE Content TeamMITREInternal
updated Description, Potential_Mitigations
2011-06-01CWE Content TeamMITREInternal
updated Common_Consequences
2012-05-11CWE Content TeamMITREInternal
updated Related_Attack_Patterns, Relationships
2012-10-30CWE Content TeamMITREInternal
updated Potential_Mitigations
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Basic XSS
2009-05-27Failure to Sanitize Script-Related HTML Tags in a Web Page (Basic XSS)
2010-06-21Improper Sanitization of Script-Related HTML Tags in a Web Page (Basic XSS)
Page Last Updated: February 18, 2014