Improper Neutralization of Script in Attributes in a Web Page

Description

This vulnerability occurs when a web application fails to properly sanitize or block JavaScript URIs (like 'javascript:') within HTML tag attributes. Attackers can inject malicious code into attributes such as onmouseover, onload, onerror, or style, leading to cross-site scripting (XSS) attacks when the page renders.

Extended Description

This flaw is a specific type of Cross-Site Scripting (XSS) where the injection point is not within the main HTML content, but within an element's attribute values. Unlike standard reflected XSS, the malicious script is often triggered by a user interaction (like moving a mouse) or a page event (like an image failing to load), making it harder to detect during casual testing. Developers might overlook these attributes when implementing input validation or output encoding filters. To prevent this, you must treat all user-supplied data placed into HTML attributes as untrusted. Implement context-sensitive output encoding specifically for HTML attributes before inserting data. Use a reputable security library for this encoding, as different contexts (HTML, JavaScript, CSS) require different escaping rules. Additionally, consider implementing a Content Security Policy (CSP) as a robust defense-in-depth measure to block the execution of inline JavaScript.

Common Consequences 1

Scope: ConfidentialityIntegrityAvailability

Impact: Read Application DataExecute Unauthorized Code or Commands

Detection Methods 1

Automated Static AnalysisHigh

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Potential Mitigations 4

Phase: Implementation

Carefully check each input parameter against a rigorous positive specification (allowlist) 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 tag attributes, 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: 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

Observed Examples 8

CVE-2001-0520Bypass filtering of SCRIPT tags using onload in BODY, href in A, BUTTON, INPUT, and others.

CVE-2002-1493guestbook XSS in STYLE or IMG SRC attributes.

CVE-2002-1965Javascript in onerror attribute of IMG tag.

CVE-2002-1495XSS in web-based email product via onmouseover event.

CVE-2002-1681XSS via script in <P> tag.

CVE-2004-1935Onload, onmouseover, and other events in an e-mail attachment.

CVE-2005-0945Onmouseover and onload events in img, link, and mail tags.

CVE-2003-1136Javascript in onmouseover attribute in e-mail address or URL.

Applicable Platforms

Languages:

Not Language-Specific : Undetermined

Modes of Introduction

Implementation

Related Attack Patterns

Related Weaknesses

ChildOf:

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

Taxonomy Mapping

PLOVER
Software Fault Patterns